H Gu

J Cui

P P Lavvas

D. -D Niu

X. -S Wu

J.-H Guo

F He

Y Wei

Dayside nitrogen and carbon escape on Titan: The role of exothermic chemistry

Keywords: planets and satellites, atmospheres -planets and satellites, individual (Titan)

Context. Atmospheric escape has an appreciable impact on the long-term climate evolution on terrestrial planets. Exothermic chemistry serves as an important mechanism driving atmospheric escape, and the role of such a mechanism is of great interest for Titan due to its extremely complicated atmospheric and ionospheric composition.

Aims. This study is devoted to a detailed investigation of neutral N and C escape on the dayside of Titan driven by exothermic neutral-neutral, ion-neutral, and dissociative recombination (DR) reactions, with the aid of the extensive measurements of Titan's upper atmospheric structure made by a number of instruments onboard Cassini, along with the improved understandings of the chemical network involved.

Methods

. A total number of 14 C-and N-containing species are investigated based on 146 exothermic chemical reactions that release hot neutrals with nascent energies above their respective local escape energies. For each species and each chemical channel, Article number, page 1 of 20 H. Gu et al.: Dayside nitrogen and carbon escape on Titan

the hot neutral production rate profile is calculated, which provides an estimate of the corresponding escape rate when combined with the appropriate escape probability profile obtained from a test particle Monte Carlo model.

Results

. Our calculations suggest a total N escape rate of 9.0 × 10 23 s -1 and a total C escape rate of 4.2 × 10 23 s -1 driven by exothermic chemistry and appropriate for the dayside of Titan. The former is primarily contributed by neutral-neutral reactions whereas the latter is dominated by ion-neutral reactions but the contributions from neutral-neutral and DR reactions cannot be ignored as well. Our calculations further reveal that the bulk of N escape occurs via hot N( 4 S) production from the collisional quenching of N( 2 D) by ambient N2, and C escape is mainly driven by hot CH3 and CH4 production via a number of important ion-neutral and neutral-neutral reactions.

Conclusions. When compared with previous investigations of other known escape mechanisms, we suggest that exothermic chemistry plays an insignificant role in N escape but likely contributes appreciably to non-thermal C escape on the dayside of Titan.

Introduction

Solar Extreme Ultraviolet (EUV) and X-ray photons deposit a substantial amount of energy in a dayside planetary upper atmosphere, causing dissociation and ionization of ambient neutrals and initializing a complicated chemical network including neutralneutral, ion-neutral, and dissociative recombination (DR) reactions (e.g. Fox et al. 2008).

The chemical products from these reactions may gain sufficient energy and escape to the interplanetary space (e.g. Johnson et al. 2008).

Titan, the largest satellite of Saturn, has a thick and permanent atmosphere composed of N 2 , CH 4 , and H 2 , along with various hydrocarbons, nitriles, and oxides as trace species (e.g. Waite et al. 2005;Niemann et al. 2005). Considerable neutral escape is thought to occur on Titan (e.g. [START_REF] Strobel | Titan's upper atmosphere/exosphere, escape processes, and rates[END_REF]. For neutral escape driven by exothermic chemistry, Cravens et al. (1997) predicted pre-Cassini escape rates of 2.5 × 10 25 s -1 for total N and 4 × 10 25 s -1 for total C, whereas De La Haye et al. (2007) estimated the post-Cassini escape rates to be 8.3 × 10 24 s -1 and 7.2 × 10 24 s -1 , respectively. In both studies, the ideal exobase approximation (e.g. Levine et al. 1978;Wallis 1978) was adopted to calculate the escape rates.

Other non-thermal or thermal escape mechanisms have also been explored on Titan (e.g. Johnson et al. 2008;Jiang et al. 2017). Shematovich et al. (2003) obtained a total N escape rate of 9.2 × 10 24 s -1 due to N 2 dissociation by solar EUV/X-ray photons as well as photoelectrons. Atmospheric sputtering was estimated to cause a total N escape rate of 10 24 -10 25 s -1 (Lammer & Bauer 1993;Shematovich et al. 2001Shematovich et al. , 2003;;Michael et al. Article number, page 2 of 20 2005) and a total C escape rate an order of magnitude lower (Gu et al. 2019). While the thermal evaporation of N-containing species is negligible on Titan, the same process likely drives strong C escape at a rate of ∼ 10 27 s -1 in the form of slow hydrodynamic escape of CH 4 (e.g. Yelle et al. 2008;Strobel 2009;Cui et al. 2012;Strobel 2012a,b), despite that such a conclusion is still under debate (e.g. Tucker & Johnson 2009;Bell et al. 2010Bell et al. , 2011;;Schaufelberger et al. 2012).

With the accumulation of extensive measurements of Titan's atmospheric neutral and ion densities (e.g. Cui et al. 2009a,b;Magee et al. 2009;Mandt et al. 2012), along with the improved understandings of Titan's atmospheric and ionospheric chemistry (e.g. Wilson, & Atreya 2004;Vuitton et al. 2006aVuitton et al. ,b, 2007Vuitton et al. , 2008;;Lavvas et al 2008a,b;Krasnopolsky 2014;Vuitton et al. 2019), it is now timely to perform a state-of-the-art evaluation of neutral escape on this interesting body as driven by exothermic chemistry. We calculate in Sections 2 and 3 the production rates of relevant hot neutrals and their escape probabilities.

The escape rates are then determined in Section 4 where we also discuss the relative contributions of various chemical channels. Finally, we discuss our results and end with concluding remarks in Section 5. If not stated explicitly, hot neutral species mentioned throughout the remaining of the paper always refer to those with nascent kinetic energies above the respective local escape energies.

Hot neutral production rates

Due to the relatively low gravity on Titan compared to other terrestrial planets, a large number of neutral species could gain sufficient kinetic energy from exothermic chemistry and escape. Here we consider 14 N-and C-containing species including N( 4 S), N( 2 D), 3 CH 2 , CH 3 , NH, CH 4 , NH 3 , C 2 H 2 , C 2 H 3 , HCN, C 2 H 4 , N 2 , C 2 H 5 , and C 2 H 6 in the order of increasing molecular mass up to 30 Da. The respective range of escape energy is from 0.32 eV for N( 4 S), N( 2 D), and 3 CH 2 to 0.68 eV for C 2 H 6 , all refereed to the exobase at an altitude of about 1500 km (Westlake et al. 2011;Cui et al. 2011). Other species lighter than 30 Da and all species heavier than 30 Da are not considered in the present investigation, either because their production rates are much lower or because they are more strongly bound by Titan's gravity.

In previous studies of neutral C and N escape driven by exothermic chemistry, the pre-Cassini investigation of Cravens et al. (1997) Cravens et al. (1997), 9 species including CH, C 2 H, CN, and 6 additional species heavier than 30 Da are not considered here because their contributions to total C or N escape are negligible. The H and H 2 escape rates due to exothermic chemistry were also evaluated by De La Haye et al. (2007), but these escape rates are far less than the thermal evaporation rates (e.g. Cui et al. 2008;Hedelt et al. 2010;Strobel 2010).

included CH, 3 CH 2 , CH 3 , CH 4 , C 2 H, C 2 H 2 , C 2 H 3 , C 2 H 4 , C 3 H 2 , C 3 H 3 , C 4 H 3 , C 4 H 4 , C 5 H 4 , C 5
The calculations of the hot neutral production rates are based on the combined list of exothermic chemical reactions presented in the literature (Cravens et al. 1997;De La Haye et al. 2007;Lavvas et al 2008a,b;Vuitton et al. 2007Vuitton et al. , 2019)). For each reaction, the kinetic energy release is evaluated from the enthalpy difference at room temperature between the reactants and products both assumed to be in their ground states (Baulch et al. 2005).

The energy partition between different products is taken to be inversely proportional to the molecular mass. Here we consider a subset of these reactions that produce candidate hot neutrals with kinetic energies exceeding the respective escape energies, as listed in Table A1 including 80 neutral-neutral reactions (of which 15 are three-body reactions), 31 ionneutral reactions, and 35 DR reactions. The atmospheric and ionospheric chemical network implemented here is far more complicated and detailed than those adopted in Cravens et al. (1997) and De La Haye et al. (2007).

For both neutral-neutral and ion-neutral reactions, rate coefficients appropriate for a fixed temperature of 150 K are adopted (e.g. [START_REF] Snowden | [END_REF]. The rate coefficient for a three-body reaction, k 3 , is expressed as

k 3 = k ∞ (k 0 [M]X + k R ) k ∞ + k 0 [M]X , (1)
where [M] is the density of the background neutral species assumed to be exclusively N 2 , k 0 and k ∞ are the termolecular and bimolecular rate constants, k R is an additional rate constant introduced to include radiative association (Vuitton et al. 2012), the nondimensional parameter, X, is given by X = F/(1 -F ) with F defined as

log F = log(F C ) 1 + [ log [Pr]+C N -0.14(log [Pr]+C) ] 2 .
In the above expression,

P r = k 0 [M]/k ∞ , N = 0.75-1.27 log F C , and C = -0.4-0.67 log F C
with F C being a fixed parameter for a specific reaction. For several three-body reactions with no available information on radiative association (R d 1, R k 1, and R m 1 in Table A1), the conventional Lindemann-Hinshelwood expression is used with k R = 0 and X = 1 in Equation 1 (e.g. Hörst et al. 2008). A further exception is the three-body reaction R k 2 in Table A1 for which a constant value of k 3 [M] ≈ 2 × 10 -15 cm 3 s -1 is used following Lavvas et al (2008a) independent of altitude. The kinetic energy release for a three-body reaction is obtained in the same way as a two-body reaction, with the contribution from radiative association ignored for simplicity (Vuitton et al. 2012). Such an approximation should not lead to significant uncertainty in the derived escape fluxes since three-body reactions are overall unimportant in hot N and C production near and above the exobase (see below for

Article number, page 4 of 20 details). Finally, as normal the DR rate coefficient is assumed to be inversely proportional to a certain power of the electron temperature (e.g. Viggiano et al. 2005), with the power index in the range of 0.39-1.2 (see Table A1). (Waite et al. 2005).

The background neutral atmosphere of Titan is displayed in Figure 1 on 10 December 2014, are included and the neutral densities are extracted following the procedure described in Cui et al. (2008Cui et al. ( , 2012)). The outbound density data are excluded to avoid possible contamination by the INMS wall effects (Cui et al. 2009a). The mixing ratio profiles for various neutral reactants necessary for determining the hot neutral production rates are displayed in Figure 2, adapted from the model results of Lavvas et al (2008a,b) up to an altitude of 1300 km. These profiles have been updated with the improved chemical network in Titan's atmosphere and ionosphere, and found to be in reasonable agreement with the latest Cassini INMS measurements. The species displayed in Figure 2

include 9 hydrocarbons (C 2 H 2 , C 2 H 4 , C 2 H 6 , C 3 H 2 , CH 3 CCH, CH 2 CCH 2 , C 3 H 6 , C 3 H 8 , C 4 H 10 ) in panel (a), 8 nitriles (NH 3 , HCN, CH 2 NH, CH 3 CN, C 2 H 3 CN, C 2 H 5 CN, HC 3 N, HC 5 N) in panel (b), and 25 radicals (H, C, CH, 3 CH 2 , 1 CH 2 , CH 3 , C 2 , C 2 H, C 2 H 3 , C 2 H 5 , C 3 H 3 , C 3 H 5 , C 3 H 7 , C 4 H, C 4 H 3 , C 4 H 5 , C 4 H 9 , C 6 H, C 6 H 5 , CN, C 2 N, N( 4 S), N( 2 D), NH, NH 2 )
in panels (c) and (d), respectively. The mixing ratio profiles displayed in the figure are extrapolated to higher altitudes assuming diffusive equilibrium. Unlike ions (see below), the densities of most neutral reactants involved here are not directly measured by the INMS instrument (especially radicals), and accordingly we choose to use directly the model results. Fig. 2. The mixing ratio profiles of various neutral species in Titan's dayside upper atmosphere including hydrocarbons (a), nitriles (b), and radicals (c and d), adapted from the model results of Lavvas et al (2008a,b) at 800-1300 km which have been updated with the improved chemical network in Titan's atmosphere and ionosphere. These mixing ratio profiles are extrapolated to higher altitudes assuming diffusive equilibrium (indicated by the dashed lines). Electron temperature (K)

Fig. 4. The electron density and temperature profiles based on the Cassini RPWS LP measurements (crosses) averaged over the dayside of Titan (Wahlund et al. 2005). The empirical electron density profile (solid blue) is obtained in a similar manner as the ion density profiles in Figure 3, whereas the empirical electron temperature profile (solid red) is obtained by using the functional form of Ergun et al. (2015).

to T107 are included. The identification of the ion species follows the scheme of for C 5 H + 3 , respectively, all in unit of Da. For those channels sampling more than one ion species, the percentage contributions as a function of altitude are taken from Vuitton et al. (2019). For each species in Figure 3, the raw INMS measurements (solid circles) show considerable variability, and a smooth empirical profile (solid line) based on the third-order polynomial fitting to logarithmic density is used instead.

The electron density and temperature profiles, as displayed in Figure 4, are based on the Cassini Radio and Plasma Wave Science (RPWS) Langmuir Probe (LP) measurements made over the dayside of Titan (Wahlund et al. 2005). Note that the electron density is not necessarily identical to the total ion density due to the presence of positive ions heavier than 100 Da not sampled by the INMS and also due to the presence of negative ions (e.g. Wahlund et al. 2009). In Figure 4, the empirical electron density profile is obtained in a similar manner as the ion density profiles, whereas the empirical electron temperature profile is obtained by using the functional form of Ergun et al. (2015).

The total hot neutral production rates are calculated via the neutral, ion, and electron density profiles in Figures 1234and are displayed in Figures 5 and6 for the 14 candidate escaping species quoted above. The contributions from all the 146 independent chemical channels listed in Table A1 are shown separately in Figures A1 andA2, where for clarification, the neutral-neutral, ion-neutral, and DR reactions are indicated by the solid, dashed, and dash-dotted lines, respectively. For each species, we identify the dominant produc-10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1

Production ate (cm -3 s -1 ) tion channels, which are addressed in detail below. Whenever possible, we compare our results to those of De La Haye et al. (2007) in terms of the relative importances of different channels in hot neutral production.

N( 4 S) and N( 2 D): The chemical production of hot N in the form of ground state N( 4 S) mainly occurs via the neutral-neutral reactions

N( 2 D) + N 2 → N( 4 S) + N 2 (R a 1), NH + CH 3 → N( 4 S) + CH 4 (R a 2), (2)
followed by the ion-neutral reaction

N + + CH 4 → N( 4 S) + CH + 4 (R a 3), (3)
of which R a 1 and R a 2 dominate above 950 km whereas R a 3 dominates at lower altitudes.

R a 1 essentially represents the collisional quenching of N( 2 D) to ground state N( 4 S). The production of hot N in the form of excited state N( 2 D) is of minor importance, mainly via two DR reactions

N + 2 + e → N( 4 S) + N( 2 D) (R a 5), N + 2 + e → 2 N( 2 D) (R a 6), (4)
which have comparable reaction rates at all altitudes of interest here.

NH and NH 3 : The dominant channels of hot NH production are two neutral-neutral reactions

N( 4 S) + C 2 H 5 → NH + C 2 H 4 (R c 3), N( 2 D) + NH 3 → NH + NH 2 (R c 4).
(5)

The production rate of hot NH 3 is substantially lower, mainly contributed by the DR reaction

CH 3 NH + 3 + e → NH 3 + CH 3 (R f 2), (6)
with a production rate at the exobase nearly 3 orders of magnitude smaller than the hot NH production rate at the same altitude. Note that R f 2 and R c 3 also produce hot CH 3 and C 2 H 4 in Titan's upper atmosphere (see below). R c 6 in Table A1 does not contribute to NH 3 escape because the respective kinetic energy of 0.34 eV is below the NH 3 escape energy of 0.39 eV near Titan's exobase.

HCN: HCN is an effective coolant of Titan's upper atmosphere (Yelle 1991) and its abundance was recently obtained by Cui et al. (2016) at 1000-1400 km based on the Cassini INMS measurements in the CSN mode. For hot HCN production, the most important channel below 1400 km is the ion-neutral reaction

HCNH + + C 3 H 3 N → HCN + C 3 H 4 N + (R h 9), (7)
which is due to the relatively high abundance of HCNH + in Titan's ionosphere (Cravens et al. 2005). Above 1400 km, hot HCN production is mainly contributed by the charge exchange reaction

HCN + + C 2 H 2 → HCN + C 2 H + 2 (R h 7). (8)
We also note that the ion-neutral reaction

CH + 3 + HC 3 N → HCN + c-C 3 H + 3 (R h 5), (9)
is likely the most important channel producing hot HCN below 800 km but this reaction should not contribute to HCN escape as the escape probability at such low altitudes is

Article number, page 9 of 20 vanishingly small (see Section 3). De La Haye et al. (2007) identified the DR reaction

HCNH + + e → HCN + H, (10)
and the neutral-neutral reaction

H 2 CN + H → HCN + H 2 , (11)
to be most important in hot HCN production above and below 1070 km, respectively, but these two reactions do not drive HCN escape since the corresponding nascent kinetic energies are below the local escape energy.

N 2 : The dominant channel for hot N 2 production is the collisional quenching reaction R j 2 (or R a 1 quoted above). Such a reaction is seriously reduced below 900 km and a number of ion-neutral reactions become more important with

N + + HCN → N 2 + CH + (R j 3), N + 2 + C 2 H 2 → N 2 + C 2 H + 2 (R j 4), (12)
providing the highest hot N 2 production rates near the lower boundary. The importance of R j 2 (or R a 1) was also reported by De La Haye et al. (2007). However, those authors identified the ion-neutral reaction

N 2 H + + CH 4 → CH + 5 + N 2 , (13)
to be more important above 1250 km while such a reaction is not considered here because the associated N 2 products have insufficiently energies to escape. Meanwhile, De La Haye et al. (2007) identified the collisional quenching reaction

1 CH 2 + N 2 → 3 CH 2 + N 2 , (14)
to be dominant near and below 1000 km but similarly, the kinetic energy release of this reaction is too low to drive N 2 escape on Titan.

3 CH 2 : Two reactions make the most important contributions to hot 3 CH 2 production, with the DR reaction

C 2 H + 5 + e → 3 CH 2 + CH 3 (R b 5), (15)
dominating above 970 km and the ion-neutral reaction

N + + C 2 H 4 → 3 CH 2 + HCNH + (R b 2), (16)
dominating at lower altitudes, respectively. R b 5 is also an important channel for the chemical loss of C 2 H + 5 , making roughly one third of its total chemical loss at a representative altitude of 1050 km (Vuitton et al. 2007).

CH 3 : The production of hot CH 3 in Titan's dayside upper atmosphere is very complicated and contributed by a large number of reactions. Below 1100 km, the dominant channel is the ion-neutral reaction

N + + CH 4 → CH 3 + NH + (R d 24), (17)
whereas at higher altitudes, two neutral-neutral reactions

N( 4 S) + C 2 H 5 → CH 3 + H 2 CN (R d 17), N( 2 D) + CH 4 → CH 3 + NH (R d 19), (18)
one ion-neutral reaction

HCN + + CH 4 → CH 3 + HCNH + (R d 25), (19)
and another DR reaction, which is R b 5 quoted above, become near equally important.

CH 4 : Hot CH 4 production occurs mainly via the ion-neutral reaction

CH + 5 + HCN → CH 4 + HCNH + (R e 13), (20)
followed by 3 additional ion-neutral reactions

CH + 5 + C 2 H 2 → CH 4 + C 2 H + 3 (R e 11), CH + 5 + C 2 H 4 → CH 4 + C 2 H + 5 (R e 12), C 2 H + 5 + C 2 H 4 → CH 4 + C 3 H + 5 (R e 15), (21)
as well as the neutral-neutral reaction

CH 3 + NH → CH 4 + N( 4 S) (R e 7), (22)
of which the last one also contributes to hot N( 4 S) production near Titan's exobase. According to Vuitton et al. (2007) essentially based on the pre-Cassini model results of Keller et al. (1992), and partly due to the extremely high DR coefficient used by those authors, more than a factor of 10 higher than our value (see Table A1). For the 4 ion-neutral reactions listed in Equations 20 and 21, the relative contributions from reactions R e 12 and R e 15 comparable to our estimates were obtained by De La Haye et al. (2007) whereas reactions R e 11 and R e 13 were not considered by those authors. The neutral-neutral reaction R e 7 was also not included in their study.

C 2 H 2 : Hot C 2 H 2 is mainly produced via the neutral-neutral reaction

CH 3 + C 2 H 3 → C 2 H 2 + CH 4 (R g 7), (23)
above 1150 km and via another neutral-neutral reaction

C 2 H + CH 2 CCH 2 → C 2 H 2 + C 3 H 3 (R g 11), (24)
at lower altitudes. R g 7 is also capable of driving CH 4 escape on Titan.

C 2 H 3 : The production of hot C 2 H 3 is dominated by the DR reaction

C 3 H + 5 + e → C 2 H 3 + 3 CH 2 (R i 5), (25)
at all altitudes of interest here, which is also the third most important channel for 3 CH 2 production above 1100 km. In addition, the neutral-neutral reaction

C 2 H 3 CN + C 2 → C 2 H 3 + C 3 N (R i 3), (26)
is an important channel producing hot C 2 H 3 at low altitudes and does not contribute appreciably to C 2 H 3 escape on Titan.

C 2 H 4 : Hot C 2 H 4 production in Titan's dayside upper atmosphere is mainly driven by two neutral-neutral reactions

CH + C 2 H 6 → C 2 H 4 + CH 3 (R k 3), CH 3 + C 2 H 5 → C 2 H 4 + CH 4 (R k 9), (27)
and two ion-neutral reactions

C 2 H + 5 + CH 3 CN → C 2 H 4 + CH 3 CNH + (R k 24), C 2 H + 5 + HC 3 N → C 2 H 4 + HC 3 NH + (R k 25). (28)
The relative importances of different reactions vary with altitude, with R k 9 dominating above 1100 km, R k 24 at 960-1100 km, and R k 3 below 960 km, respectively. R k 3 and R k 9 also contribute to hot CH 3 and CH 4 production, though of minor importance only.

Article number, page 12 of 20

C 2 H 5 : The production of hot C 2 H 5 occurs mainly via two DR reactions

C 3 H + 6 + e → C 2 H 5 + CH (R l 6), C 3 H + 8 + e → C 2 H 5 + CH 3 (R l 7), (29)
and one neutral-neutral reaction

C 2 H + C 2 H 6 → C 2 H 5 + C 2 H 2 (R l 4). (30)
However, hot C 2 H 5 production below 930 km is dominated by the three-body reaction

H + C 2 H 4 + M → C 2 H 5 + M (R l 1). (31)
Clearly, C 2 H 5 escape on Titan is primarily driven by the former 3 reactions. Note also that R l 7 makes only a minor contribution to hot CH 3 production. Our calculations highlight the impact of radiative association on hot neutral production in the tenuous part of Titan's upper atmosphere. For instance, the conventional Lindemann-Hinshelwood formulism that does not include radiative association predicts a C 2 H 5 production rate via Reaction R l 1 about 4 orders of magnitude too low near the exobase.

C 2 H 6 : Hot C 2 H 6 production at all altitudes is dominated by the three-body reaction

2 CH 3 + M → C 2 H 6 + M (R m 2), (32)
followed by the DR reaction

C 3 H + 7 + e → C 2 H 6 + CH (R m 8). (33)
In contrast, De La Haye et al. (2007) reported the latter reaction to be dominant at high altitudes as those authors did not include radiative association and therefore seriously underestimated hot C 2 H 6 production near and above the exobase.

Finally, it is noteworthy that in most cases, three-body reactions are only important in the relatively dense regions of Titan's upper atmosphere where escape becomes difficult (see Section 3). Accordingly, their contributions to total N escape (in the form of N 2 recoils) is ignored in Table A1 and throughout this paper. An exception is reaction R m 2 which dominates C 2 H 6 production at sufficiently high altitudes, but this reaction should not contribute substantially to N escape in the form of N 2 recoils because the respective production rate is well below the N 2 production rate via the collisional quenching reaction R j 2 (see above).

Article number, page 13 of 20

Hot neutral escape probabilities

For each species discussed in Section 2, the respective escape probability in Titan's dayside upper atmosphere is required to calculate rigorously the escape rate. The ideal exobase approximation was adopted by both Cravens et al. (1997) Fox, & Hać 2009, 2014, 2018), and is modified from our existing model of atmospheric sputtering on Titan (Gu et al. 2019). We also mention that analytic models capable of capturing the near exobase transition of escape probability in an approximate manner have also been proposed such as the single collision model of Cravens et al. (2017) and the multiple collision model of Cui et al. (2019).

The plane parallel background atmosphere used in this study is displayed in Figure 1, over the altitude range of 800-2000 km and containing N 2 , CH 4 , and H 2 . Below 800 km, the mean free path for collision is sufficiently short that the energy of a typical hot neutral is degraded rapidly to the local thermal energy over a length scale not exceeding 0.5 km, a situation consistent with local thermalization. At 2000 km, the collision probability drops to around 1%, implying that Titan's atmosphere above this altitude does not exert an appreciable influence on the derived escape probabilities. For the energy range encountered in this study, inelastic collision processes such as excitation and dissociation could be safely ignored. Similar to De La Haye et al. (2007), the collisions between hot neutrals and ambient neutrals are modeled under the hard sphere approximation for elastic collisions, with the appropriate hard sphere radii of relevant neutrals estimated from existing laboratory measurements of pure gas viscosity (e.g. Flynn & Thodos 1961;Fenghour et al. 1995;[START_REF] Rowley | Physical and thermodynamic properties of pure chemicals[END_REF]. Whenever no viscosity measurements have been made for a certain species, its hard sphere radius is either approximated by the known radius of a species in the same chemical group or taken from the American Chemical Society website on http://center.acs.org/periodic/tools/PT.html.

At a given altitude, a hot neutral particle is released in a random direction assuming isotropic production and with a prescribed nascent velocity depending on the chemical channel involved. The trajectory of this particle is followed under the influence of Titan's gravity and the position where it makes a further collision with ambient neutrals is determined in a stochastic manner with the aid of the known information of the collision cross section and the known structure of the background atmosphere. The collision partner, N 2 , CH 4 , or H 2 , is also decided stochastically based on the column density ratio between different ambient species over the path length of hot neutral free propagation. The post-collision velocities of both the hot neutral and the ambient neutral could then be favorably determined from the momentum and energy conservation laws, where the pre-collision velocity of the ambient neutral is assumed to be zero since its thermal energy [START_REF] Snowden | [END_REF] and wind-driven bulk kinetic energy (Müller-Wodarg et al. 2008) are both well below the kinetic energy release from exothermic chemistry. The post-collision velocity direction of the hot neutral is also chosen randomly under the assumption of isotropic scattering. The above procedure is repeated until one of the following conditions is satisfied: (1) When the hot neutral reaches the lower boundary or when its kinetic energy falls below the local escape energy via a cascade of collisions anywhere within the simulation box, it is no longer traced in our model calculations;

(2) When the hot neutral reaches the upper boundary, it is either assumed to be lost from the atmosphere or reflected downward elastically, depending on whether its kinetic energy is above or below the local escape energy.

The entire background atmosphere of Titan is divided into 17 altitude grids, each covering a depth of 50 km, to allow the full altitude profile of escape probability to be constructed. For each species, we consider a range of energy (see Table A2) that incorporates the range of nascent kinetic energy for various source reactions (see Section 2). For a unique combination of the nascent energy and the altitude of production, a total number of 100,000 hot neutrals are modeled independently to achieve statistically robust results

for each species and each reaction.

In Figure 7, we show the escape probability as a function of altitude for each candidate escaping species quoted in Section 2 and for a sequence of nascent kinetic energy quoted in the figure legend. Several interesting features are immediately seen in the figure. First, all profiles reveal the presence of a broad transition region with a depth of nearly 200 km around the ideal exobase at 1500 km (Westlake et al. 2011). The escape probability is vanishingly small at the lower end of the transition region and is around 0.5 at the upper end. The latter is consistent with the expected scenario that nearly all particles moving upward are able to escape (e.g. Cravens et al. 1997). The actual escape probability at the upper end might be modestly different from the ideal value of 0.5 due to non-negligible backscattering. Second, the escape probability increases with increasing energy at all altitudes, which is interpreted by the fact that a more energetic particle allows a greater number of collisions before its energy falls below the local escape energy (e.g. Cui et al. 2019). Third, the escape probability also varies from species to species due to the difference in collision cross section. As intuitively expected, the escape probabilities of small hot particles tend to be higher than those of large hot particles as small ones are less likely to collide with ambient neutrals (e.g. Fox, & Hać 2014).

Despite the variations with both energy and collision cross section, we find that the modeled altitude profile of the escape probability, ζ esc , could be reasonably described by a common functional form of

ζ esc = a 1 tanh( z -a 2 160 ) + a 3 , ( 34
)
where z is the altitude in km, a 1 , a 2 , and a 3 are free parameters to be constrained by 313 the Monte Carlo model results. As motivated by Figure 7, we adopt a common depth of 314 km for the transition region for all species and at all nascent energies. In the equation, a 2 denotes the central location of the transition region, whereas a 3 + a 1 represents the asymptotic escape probability at sufficiently high altitudes. Ideally, one may expect that the asymptotic escape probability at low altitudes, given by a 3 -a 1 according to Equation 34, should be zero, thus implying a 1 = a 3 . However, such a condition does not always lead to satisfactory fits to the modeled escape probability profiles. Accordingly, Equation 34cannot be extrapolated to arbitrarily low altitudes where the predicted escape probability could sometimes be negative. The best-fit profiles of escape probability are indicated by the dashed lines in Figure 7 for reference.

Since the location of the transition region appears to be energy independent according to Figure 7, we assume for simplicity a constant value of a 2 for any given species, as listed in Table A2. a 1 and a 3 are clearly energy dependent and described in this study by

a 1 = b 1 exp(-E) + b 2 (a)
,

a 1 = b 1 ln E + b 2 (b), a 3 = b 3 ln E + b 4 (c), ( 35
)
where E is the nascent energy of a hot particle in

Neutral escape rates driven by exothermic chemistry

The escape rate for a given species and a given chemical channel in Titan's dayside upper atmosphere, as listed in Table A1, is estimated via

Φ esc = 1600 km 800 km 2π(R T + z) 2 ζ esc (z)P hot (z)dz, (36)
where R T is Titan's solid body radius, P hot is the hot neutral production rate, and the other parameters are defined above. Combining the calculations for all the 14 species and all the 146 independent chemical channels, we are able to obtain the total C and N escape rates, as well as determine the relative contribution of each channel. For reference, we provide in Table A1 the escape rates of all neutral species via all chemical channels involved in this study, along with the respective fractional contributions to total C or N escape. We caution that a channel with a large column integrated hot neutral production rate does not necessarily contribute substantially to neutral escape since peak production may occur at low altitudes where the escape probability is small.

Table A1 reveals that for N escape, the most important channel is the collisional quenching reaction R a 1 which contributes to 80% of total N escape via exothermic chemistry, in the form of both hot N( 4 S) and hot N 2 (note that the N 2 escape rate listed in Table A1 for R j 2 is multiplied by 2 to represent the net N escape rate). The second most important channel is the neutral-neutral reaction R a 2 that produces hot N( 4 S) and accounts for 12% of total N escape. Two other neutral-neutral reactions, R c 3 and R c 5, contribute non-negligibly to 8% of total N escape by producing hot NH. It is interesting to note that the contribution from hot NH production to total N escape is higher than the contribution from hot N 2 production. This is an unexpected result that was not reported by Cravens et al. (1997) For all the three categories of exothermic chemistry considered here, we find that the intuitively expected trend that at any given altitude, the escape probability increases with the nascent kinetic energy and decreases with the size of the escaping particle. Calculations of the hot neutral production rate and the escape probability are combined to provide a reasonable estimate of the escape rate for each species and each channel.

Our calculations suggest a total N escape rate of 9.1 × 10 23 s -1 and a total C escape rate of 4.3 × 10 23 s -1 on the dayside of Titan, driven by exothermic chemistry. Total N escape is primarily contributed by neutral-neutral reactions, with a fractional contribution of less than 5% from ion-neutral and DR reactions. The situation for C escape is different in that all categories of reaction are non-negligible, with 60% from ion-neutral, 29% from neutral-neutral, and the remaining 11% from DR reactions, respectively. Meanwhile, the bulk of N escape is associated with hot N( 4 S) production and driven by the quenching of excited state N( 2 D) via collisions with atmospheric N 2 . Such a process, denoted as R a 1 in Table A1, accounts for 80% of dayside N escape on Titan. Hot NH production also contributes non-negligibly to N escape and plays an even more important role than hot N 2 production, a result that was not obtained in previous investigations (Cravens et al. 1997;De La Haye et al. 2007). Dayside C escape is mostly associated with hot CH 3 and CH 4 production, responsible for 27% and 63% of total C escape, respectively. Our calculations highlight the importance of one ion-neutral reaction: R e 13 that produces hot CH 4 from CH + 5 and HCN, which accounts for more than 30% of dayside C escape on Titan. While the importance of reaction R a 1 in driving N escape was also obtained by De La Haye et al. (2007), the situation for C escape is to be distinguished in that those authors identified CH + 5 DR as the most important chemical channel but we find it to be negligible. Such a difference is likely linked to the different choices of the DR coefficient and CH + 5 density. It is noteworthy that the De La Haye et al. ( 2007) results were obtained based on the early photochemical model calculations of Keller et al. (1992) and our current understandings of Titan's atmospheric and ionospheric chemistry are much more robust (e.g. Vuitton et al. 2019).

It is instructive to compare the total dayside N and C escape rates driven by exothermic chemistry to those by other viable mechanisms, especially the non-thermal ones (Johnson et al. 2008). Specifically, N 2 photodissociation is ignored in this study but existing works indicate that this process likely leads to a dayside averaged N escape rate much higher than the value reported here. For instance, the calculations of Shematovich et al. (2003) suggest an N escape rate of 9 × 10 24 s -1 driven by N 2 photodissociation, which is an order of magnitude higher than our estimate of the chemically driven N escape rate. Without

showing the details, we mention that the photochemical model implemented in Section 3 leads to a comparable dayside N escape rate of 8 × 10 24 s -1 via N( 4 S) production from N 2 photodissociation, where the escape probability profile appropriate for 0.82 eV is used based on the mean kinetic energy release to N( 4 S) weighted by the solar EUV/X-ray flux above the N 2 dissociation threshold, for a reference altitude of 1400 km. Photoelectron impact dissociation of N 2 , which was evaluated by Cravens et al. (1997), may increase further the above N escape rate. Similarly, we estimate the dayside C escape rate to be 2 × 10 24 s -1 via CH 3 production from CH 4 photodissociation, where a weighted mean kinetic energy of 0.46 eV is used to obtain the CH 3 escape probability profile. Clearly, C escape driven by exothermic chemistry is not negligibly small as compared to C escape driven by CH 4 photodissociation.

Atmospheric sputtering, as another viable escape mechanism, was modeled by a number of authors based on Monte Carlo calculations (e.g. Shematovich et al. 2001Shematovich et al. , 2003;;Michael et al. 2005). The sputter-induced total N escape rate, contributed by both N and N 2 , was predicted to be 4 × 10 25 s -1 by the one-dimensional calculations of Shematovich et al. (2003) and 2.5 × 10 25 s -1 by the three-dimensional calculations of Michael et al. (2005).

Recently, Gu et al. (2019) estimated the CH 4 -to-N 2 sputtering yield ratio in Titan's upper atmosphere to be 10%-20%, which should be close to the respective ratio in escape rate.

When compared with the N and C escape rates derived here, we may conclude that sputterinduced N and C escape is much stronger than chemically driven escape. However, we note that the sputter-induced escape rates quoted above are appropriate for the ramside of Titan which could be either the dayside or the nightside depending on the relative orientation between solar EUV/X-ray radiation and magnetospheric ion precipitation (Sittler et al. 2010, and references therein). This means that when Titan is in an orbital configuration with the dayside coincident with the wakeside, N and C escape driven by exothermic chemistry is likely much stronger than sputter-induced escape.

Clearly, all exothermic chemical reactions evaluated in this study, along with many other reactions ignored due to the non-escaping nature of their products, may provide an important heat source of Titan's upper atmosphere. For instance, the calculations of de La Haye et al. (2008) suggested that on both the dayside and nightside of the satellite, neutral heating via exothermic chemistry was dominated by ion-neutral and DR reactions above ∼ 1100 km, by two-body neutral-neutral reactions at ∼ 750 -1100 km, and by three-body neutral-neutral reactions at lower altitudes, respectively (see their Figure 10).

These authors estimated a peak heating rate in Titan's dayside upper atmosphere of ∼ 5 × 10 -9 ergs cm -3 s -1 near 950 km driven by exothermic chemistry, well above the peak heating rate of no more than 10 -10 ergs cm -3 s -1 near 1050 km driven by photoelectron and magnetospheric electron impact. Similarly, the DR reaction of O + 2 , the dominant species of the Venusian ionosphere (Taylor et al. 1980), was suggested to contribute significantly Appendix A: Supplementary information on the hot neutral production rates and the escape probabilities

For easy reference, we compile in this appendix the detailed information on the hot neutral production rates as well as the escape probabilities used for deriving the C and N escape fluxes. In Table A1, we list all the 146 independent exothermic chemical reactions investigated in the present study, as well as the kinetic energy release, the rate coefficient, the escape flux referred to the surface, the fractional contribution to total N or C escape, and the appropriate references, grouped by species. Note that some reactions are repetitively listed since they are able to produce more than one escaping species. The hot neutral production rates as a function of altitude for all channels are displayed in Figure A1 for Ncontaining species and in Figure A2 for C-containing species, also grouped by species. These figures provide necessary information on deciding the dominant channel for the production of each species considered here. esc is the corresponding fractional contribution to total C or total N escape. When not stated explicitly, all rate coefficients refer to a neutral temperature of 150 K. For three-body reactions, several parameters are provided, which could be used to obtain the appropriate rate coefficients with the aid of Equations 1 and 2 in Section 2.

No. In Table A2, we list the parameters (a 1 , a 2 , and a 3 ) used to describe the altitude dependence of escape probability for each species and for a range of selected nascent energy according to Equation 34 in Section 3. For an arbitrary nascent energy within the prescribed range, the escape probability profile could be determined with the aid of Equation 35 using a set of parameters (b 1 , b 2 , b 3 , and b 4 ). These parameters are listed in Table A3 for reference.

Reaction ∆E k Φesc Φesc/Φ (tot) esc Reference (eV) (cm 3 s -1 ) (s -1 ) N( 4 S)/N( 2 D) Ra1/Rj 2 N( 2 D) + N2 → N( 4 S) + N2 1.

  H 5 , N( 4 S), N( 2 D), NH, CN, HCN, and N 2 , whereas the early post-Cassini investigation of De La Haye et al. (2007) included 3 CH 2 , CH 3 , CH 4 , C 2 H 4 , C 2 H 5 , C 2 H 6 , N( 4 S), NH, N 2 , and HCN. All species considered by De La Haye et al. (2007) have been properly included in the present study as well. When compared to

  for the 3 most abundant species, N 2 , CH 4 , and H 2 , over the altitude range of 800-2000 km based on the dayside averaged Cassini Ion Neutral Mass Spectrometer (INMS) measurements in the closed source neutral (CSN) mode (Waite et al. 2005). All dayside INMS CSN data accumulated during 30 Cassini flybys with Titan, from TA on 26 October 2004 to T107

Fig. 3 .

 3 Fig.3. The density profiles of ion reactants involved in this study, based on the Cassini INMS measurements in the OSI mode (solid circles) according to the mass-to-charge ratio, M/Z(Mandt et al. 2012). Also shown are the smooth empirical profiles based on the third-order polynomial fittings to logarithmic density (solid lines).

159 2012 )

 2012 according to the mass-to-charge ratio, M/Z. All dayside INMS OSI data from TA

Fig. 6 .

 6 Fig. 6. Similar to Figure 5 but for all the C-containing species considered in this study.

  and De La Haye et al. (2007), essentially reflecting a sharp transition in escape probability from 0 to 0.5 at the exobase. Here to capture the realistic behavior of escaping neutrals over a broad transition region near the exobase, a more sophisticated test particle Monte Carlo model is constructed to obtain the escape probabilities of hot neutrals produced via each exothermic chemical channel. The model is analogous to previous models of atomic O escape on the dayside of Mars (e.g.

Fig. 8 .

 8 Fig.8. The C and N escape rates via the production of different N-and C-containing species, with the contributions from neutral-neutral, ion-neutral, and DR reactions indicated separately.

  Fig. A.1. The hot neutral production rates calculated via the neutral, ion, and electron density profiles in Figures 1-4, for N( 4 S) and N( 2 D) in panel (a), NH3 and NH in panel (b), HCN in panel (c), as well as N2 in panel (d), respectively, with the contributions from different chemical channels shown separately. For clarification, the neutral-neutral, ion-neutral, and DR reactions are indicated by the solid, dashed, and dash-dotted lines, respectively.

  Fig. 1. The background neutral atmosphere of Titan for the 3 most abundant species, N2, CH4, and H2, over the altitude range of 800-2000 km based on the dayside averaged Cassini INMS measurements in the CSN mode

		2000 1800					N 2 CH 4 H 2
	Altitude (km)	1200 1400 1600
		1000
		800	10 5	10 6	10 7 Number density (cm -3 ) 10 8 10 9 10 10	10 11	10 12

  , the former 4 reactions are important chemical loss channels of CH + 5 and C 2 H + 5 in Titan's ionosphere. A similar reaction, R e 14 in Table A1, plays a minor role due to the relatively low rate coefficient (McEwan & Anicich 2007). For com-

parison,

De La Haye et al. (2007)

identified the DR reaction R e 17 to be dominant but this reaction is unimportant according to our calculations. The difference is partly due to the overestimate of the CH + 5 abundance near Titan's exobase by

De La Haye et al. (2007)

,

  Escape probability profiles for various hot neutrals and covering the range of nascent kinetic energy encountered in this study. The crosses are adapted from test particle Monte Carlo calculations whereas the dashed lines indicate the best empirical fits (see text for details).

			(a) N( 4 S)/N( 2 D)				2000			(b) 3 CH2/CH3/CH4
								1900
								1800
	Altitude (km)					0.5eV 1.0eV 1.5eV 2.5eV	1600 1700 1500						0.5eV 1.0eV 1.5eV 2.5eV
						3.5eV	1400						3.5eV
						5.0eV 6.5eV	1300						5.0eV 6.5eV
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	1200	0.1	0.2	0.3	0.4	0.5	0.6	0.7
			(c) NH/NH3				2000			(d) C2H2
								1900
								1800
	Altitude (km)					0.5eV 1.0eV 1.5eV 2.5eV	1600 1700 1500						1.0eV 1.5eV 2.0eV
						3.5eV	1400						3.0eV
						5.0eV 6.5eV	1300						4.0eV 5.0eV
	0.1	0.2	0.3	0.4	0.5	0.6	0.7	1200	0.1	0.2	0.3	0.4	0.5	0.6	0.7
			(e) HCN				2000			(f) N2
								1900
								1800
	Altitude (km)					1.0eV 1.5eV 2.0eV	1600 1700 1500						1.0eV 1.5eV 2.0eV
						3.0eV	1400						3.0eV
						4.0eV 5.0eV	1300						4.0eV 5.0eV
	0.1	0.2		0.3	0.4	0.5	0.6	1200	0.1	0.2	0.3	0.4		0.5	0.6
			(g) C2H3/C2H4				2000			(h) C2H5/C2H6
								1900
								1800
	Altitude (km)					1.0eV 1.5eV 2.0eV	1600 1700 1500						1.0eV 1.5eV 2.0eV
						3.0eV	1400						3.0eV
						4.0eV 5.0eV	1300						4.0eV 5.0eV
	0.1	0.2	0.3 Escape probability 0.4	0.5	0.6	1200	0.1	0.2	0.3 Escape probability 0.4		0.5	0.6
	Fig. 7.

  eV, b 1 , b 2 , b 3 , and b 4 are free parameters to be constrained by the values listed in TableA2. Note that Equation 35a is used to describe the a 1 parameters of relatively light species including N( 4 S), N( 2 D), 3 CH 2 , CH 3 , CH 4 , NH, and NH 3 , whereas Equation 35b is used for heavier species considered in this study. Combining Equations 34 and 35, we are able to obtain the escape probability profile for any candidate escaping species and at any nascent energy.

  Mandt et al. 2012) as well as the improved understandings of the associated chemical network over the past decade (e.g.Wilson, & Atreya 2004; Vuitton et al. 2006a Vuitton et al. ,b, 2007Vuitton et al. , 2008;; Lavvas et al 2008a,b;Krasnopolsky 2014;Vuitton et al. 2019).A total number of 14 candidate escaping neutral species are considered in this study includingN( 4 S), N( 2 D), 3 CH 2 , CH 3 , NH, CH 4 , NH 3 , C 2 H 2 , C 2 H 3 , HCN, C 2 H 4 , N 2 , C 2 H 5 ,and C 2 H 6 in the order of increasing molecular mass up to 30 Da. A total number of 146 exothermic chemical reactions are evaluated, all of which are capable of producing hot neutrals with nascent kinetic energies above the respective local escape energies. These reactions fall into three categories, including 80 neutral-neutral reactions (of which 15 are three-body reactions), 31 ion-neutral reactions, and 35 DR reactions. The atmospheric and ionospheric chemical network implemented here is far more complicated than those ofCravens et al. (1997) andDe La Haye et al. (2007).

	392
	393	due to its extremely complicated atmospheric and ionospheric composition as revealed by
	394	the Cassini INMS measurements in both the CSN and OSI modes (e.g. Waite et al. 2005;
	395	Cravens et al. 2005). The total C and N escape rates on the dayside of Titan driven by
	396	exothermic chemistry were computed in the pre-Cassini investigation of Cravens et al.
		(1997) and the early post-Cassini investigation of De La Haye et al. (2007) (when the data

380

neutral-neutral reactions drive N escape nearly a factor of 7 stronger than C escape, ion-381 neutral reactions drive C escape nearly a factor of 20 stronger than N escape, whereas DR 382 reactions drive comparable C and N escape, respectively. We emphasize that to describe 383 properly total N escape, both ion-neutral and DR reactions could be ignored with an un-384 certainty no more than 5%, whereas to describe properly total C escape, all categories of 385 exothermic reaction have to be taken into account. 386 5. Discussions and concluding remarks 387 Atmospheric escape is a key process that controls the evolution of climate and habit-388 ability on terrestrial planets. One of the important mechanisms driving atmospheric escape 389 is exothermic chemistry that may produce hot neutrals sufficiently energetic to overcome 390 the gravitational potential of the central body (e.g. Johnson et al. 2008, and references 391 therein). This mechanism is especially interesting for Titan, the largest satellite of Saturn, 397 from a very limited number of close Titan flybys were available). The present investigation et al. 2009; Combining the state-of-the-art INMS measurements of neutral and ion densities, the RPWS LP measurements of electron density and temperature, as well as the photochemical model results of Lavvas et al (2008a,b) updated with the improved chemical network, we are able to calculate the hot neutral production rate as a function of altitude for each species and each chemical channel in Titan's dayside upper atmosphere. A test particle Monte Carlo model is further constructed to determine the corresponding profile of escape probability under the assumption of isotropic hard sphere approximation, which properly describes the realistic dynamical behavior of escaping neutrals over a broad transition region near Titan's exobase (e.g. Strobel, & Cui 2014). Our model results are consistent with

  Article number, page 26 of 20 H.Gu et al.: Dayside nitrogen and carbon escape on Titan TableA.1. Information on the exothermic reactions considered in this study that potentially produce escaping N( 4 S), N( 2 D), 3 CH2, CH3, NH, CH4, NH3, C2H2, C2H3, HCN, C2H4, N2, C2H5, and C2H6 in Titan's dayside upper atmosphere. ∆E is the kinetic energy release, k is the reaction coefficient, Te is the electron temperature, Φesc is the escape rate of a neutral species via a certain chemical channel, Φesc/Φ

	(tot)

  → C2H4 + 3 CH2 1.82 1.20 × 10 -7 (300/Te) 0.70 3.81 × 10 15 1.78 × 10 -8 18 R k 29/R d 34 C3H + 7 + e → C2H4 + CH3 2.90 3.80 × 10 -8 (300/Te) 0.68 1.56 × 10 20 7.27 × 10 -4 C2H6 + CH 1.17 3.80 × 10 -7 (300/Te) 0.68 2.25 × 10 20 1.05 × 10 -3 20 Rm9/R b 10 C3H + 8 + e → C2H6 + 3 CH2 2.55 1.60 × 10 -7 (300/Te) 0.70 6.65 × 10 18 3.10 × 10 -5 15 Rm10/R d 36 C3H + 9 + e → C2H6 + CH3 2.67 5.00 × 10 -7 (300/Te) 0.70 8.19 × 10 18 3.82 × 10 -5 15

			Table1-continued Table1-continued Table1-continued Table1-continued Table1-continued
	No. No. No. No. No.	Reaction Reaction Reaction Reaction Reaction	∆E ∆E ∆E ∆E ∆E	k k k k k	Φesc Φesc Φesc Φesc Φesc	Φesc/Φ (tot) esc Φesc/Φ (tot) esc Φesc/Φ (tot) esc Φesc/Φ (tot) esc Φesc/Φ (tot) esc	Reference Reference Reference Reference
			(eV) (eV) (eV) (eV)	(cm 3 s -1 ) (cm 3 s -1 ) (cm 3 s -1 ) (cm 3 s -1 )	(s -1 ) (s -1 ) (s -1 ) (s -1 )
	Rc7 Re2/Rg7 Rg18 R k 7/R d 11	NH + 2 + e → NH + H CH3 + C2H3 → CH4 + C2H2 C2H3 + C3H3 → C2H2 + CH3CCH 1.44 0.45 1.29 × 10 -7 (300/Te) 0.50 3.42 × 10 18 1.88 1.95 × 10 -10 3.44 × 10 21 8.00 × 10 -12 3.09 × 10 16 3 CH2 + C2H5 → C2H4 + CH3 1.22 3.00 × 10 -11 5.99 × 10 20	3.76 × 10 -6 8.03 × 10 -3 1.44 × 10 -7 2.79 × 10 -3	24, 25 45, 78 27, 44 37
	Rc8/Ra9 Re3/R k 9 Rg19 R k 8/R l 3	N2H + + e → N( 4 S) + NH CH3 + C2H5 → CH4 + C2H4 C2H3 + C3H5 → C2H2 + C3H6 3 CH2 + C3H7 → C2H4 + C2H5	1.04 2.47 × 10 -7 (300/Te) 0.84 1.57 × 10 20 1.92 6.87 × 10 -12 4.88 × 10 21 1.43 8.00 × 10 -12 1.17 × 10 17 1.62 3.00 × 10 -12 3.48 × 10 14	1.73 × 10 -4 0.011 5.44 × 10 -7 1.62 × 10 -9	7 15, 26 39 38
	Re4 Rg20 R k 9/Re3	CH3 + C3H5 → CH4 + CH2CCH2 C2H3 + C3H7 → C2H2 + C3H8 CH3 + C2H5 → C2H4 + CH4	1.49 1.73 1.10	CH3 1.56 × 10 -12 2.00 × 10 -12 6.87 × 10 -12	1.53 × 10 20 1.03 × 10 12 2.27 × 10 21	3.57 × 10 -4 4.79 × 10 -12 0.011	39 38 15, 26
	R d 1 Re5 Rg21/Rc2 R k 10/Rg9	H + 3 CH2 + M → CH3 + M CH3 + C3H6 → CH4 + C3H5 N( 4 S) + C2H3 → C2H2 + NH C2H + C2H5 → C2H4 + C2H2	2.98 0.53 0.71 2.03	k0 = 2.06 × 10 -29 4.85 × 10 -26 1.31 × 10 -11 3.00 × 10 -12	4.61 × 10 11 2.86 × 10 4 7.88 × 10 19 2.09 × 10 18	1.08 × 10 -12 6.67 × 10 -20 3.68 × 10 -4 9.74 × 10 -6	27, 28, 29 36 10, 21 37
	Re6 Rg22/R b 7 R k 11/Rg16	CH3 + C3H7 → CH4 + C3H6 C3H + 4 + e → C2H2 + 3 CH2 C2H3 + C2H3 → C2H4 + C2H2	k∞ = 1.76 × 10 -9 1.00 × 10 -11 2.09 1.77 × 10 -7 (300/Te) 0.67 1.37 × 10 19 2.19 3.90 × 10 16 1.60 3.50 × 10 -11 1.27 × 10 17	9.11 × 10 -8 6.40 × 10 -5 5.91 × 10 -7	15, 38 17 66
	R d 2 Re7/Ra2 Rg23/R d 32 R k 12	H2 + CH + M → CH3 + M CH3 + NH → CH4 + N( 4 S) C3H + 5 + e → C2H2 + CH3 C2H3 + C2H5 → 2 C2H4	2.90 0.59 2.56 3.00 × 10 -8 (300/Te) 0.70 1.71 × 10 20 k0 = 1.73 × 10 -30 1.98 × 10 18 1.00 × 10 -11 3.88 × 10 22 1.64 1.50 × 10 -11 2.40 × 10 17	4.61 × 10 -6 0.091 8.00 × 10 -4 2.10 × 10 -5	15, 30 20 15, 16 66
	Re8 R k 13	CH + 3 + C2H6 → CH4 + C2H + 5 C2H3 + C3H5 → C2H4 + CH2CCH2 1.39 1.22	k∞ = 1.13 × 10 -10 1.43 × 10 -9 HCN 4.00 × 10 -12	5.35 × 10 20 8.05 × 10 16	1.25 × 10 -3 3.76 × 10 -7	3 39
	Re9 R h 1 R k 14	CH + 4 + C2H2 → CH4 + C2H + 2 H + CN + M → HCN +M C2H3 + C3H7 → C2H4 + C3H6	0.67 2.74 1.97	k R = 6.00 × 10 -16 1.44 × 10 -9 k0 = 8.78 × 10 -31 2.00 × 10 -12	2.04 × 10 21 3.94 × 10 16 4.33 × 10 -8 /9.19 × 10 -8 4.77 × 10 -3 2.31 × 10 12 1.08 × 10 -11	3 40 38
	Re10 R k 15	CH + 4 + C2H4 → CH4 + C2H + 4 C2H3 + C4H3 → C2H4 + C4H2	1.28 2.07	Fc = 0.63 1.70 × 10 -9 k∞ = 2.44 × 10 -10 1.75 × 10 -12	4.17 × 10 21 1.15 × 10 15	9.73 × 10 -3 5.38 × 10 -9	3 27
	R d 3/R k 3 Re11 R k 16/Rm4	CH + C2H6 → CH3 + C2H4 CH + 5 + C2H2 → CH4 + C2H + 3 2 C2H5 → C2H4 + C2H6	2.30 0.57 1.44	3.09 × 10 -10 1.48 × 10 -9 k R = 1.00 × 10 -13 2.40 × 10 -12	9.04 × 10 20 2.50 × 10 22 1.24 × 10 19	2.11 × 10 -3 0.058 5.81 × 10 -5	31, 68 3 33
	R d 4 Re12 R k 17	CH + C3H8 → CH3 + C3H6 CH + 5 + C2H4 → CH4 + C2H + 5 C2H5 + C3H3 → C2H4 + CH3CCH 1.37 2.41 0.84	9.21 × 10 -11 1.50 × 10 -9 Fc = 0.40 6.68 × 10 -12	8.10 × 10 18 3.86 × 10 22 1.39 × 10 18	1.89 × 10 -6 0.090 6.48 × 10 -6	32, 70 3 37, 44
	R d 5 Re13 R h 2/R d 16 R k 18	CH + C4H10 → CH3 + C4H8 CH + 5 + HCN → CH4 + HCNH + N( 4 S) + C2H4 → HCN + CH3 C2H5 + C3H5 → C2H4 + C3H6	2.58 1.10 0.90 1.36	2.89 × 10 -10 5.80 × 10 -9 3.18 × 10 -16 6.68 × 10 -12	7.20 × 10 15 1.30 × 10 23 1.62 × 10 18 1.78 × 10 -6 /3.78 × 10 -5 1.68 × 10 -9 0.304 5.27 × 10 18 2.46 × 10 -5	32, 70 15 35 34
	R d 6/Rg4 Re14 R h 3 R k 19	1 CH2 + C2H3 → CH3 + C2H2 C2H + 5 + C2H2 → CH4 + c-C3H + 3 CN + C3H8 → HCN + C3H7 C2H5 + C3H7 → C2H4 + C3H8	3.66 0.96 0.67 1.67	3.00 × 10 -11 6.84 × 10 -11 1.18 × 10 -10 5.00 × 10 -11	3.23 × 10 18 1.50 × 10 21 9.91 × 10 15 1.09 × 10 -8 /2.31 × 10 -9 7.53 × 10 -6 3.51 × 10 -3 6.37 × 10 13 7.82 × 10 -9	37 3 46, 74 15, 38
	R d 7/R k 4 Re15 R h 4 R k 20	1 CH2 + C2H5 → CH3 + C2H4 C2H + 5 + C2H4 → CH4 + C3H + 5 CN + CH2NH → HCN + H2CN C2H5 + C4H3 → C2H4 + C4H4	2.56 0.63 0.76 2.13	1.50 × 10 -11 3.55 × 10 -10 2.80 × 10 -11 8.00 × 10 -13	5.09 × 10 19 7.83 × 10 21 3.30 × 10 17 3.62 × 10 -7 /7.70 × 10 -7 1.19 × 10 -4 0.018 4.50 × 10 8 2.10 × 10 -15	37 3 55, 75 27
	R d 8 Re16 R h 5 R k 21/Rc3 Ra2/Re7 R d 9 Re17 R h 6 R k 22 Ra3 R d 10/Rg6 Re18 R h 7 R k 23 Ra4 R d 11/R k 7 Re19 R h 8 R k 24 Ra5 R d 12 Re20/R b 6 R h 9 R k 25 Ra6 R d 13 Re21/R d 31 R k 26 Ra7 R d 14 Re22 Ri1 R k 27 Ra8 Ra9/Rc8 R d 15 R d 16/R h 2 Re23 R k 28/R b 9 Ra10 R d 17 R f 1 R d 18 Ri2/R k 5 R l 1 R b 1 R d 19/Rc5 Ri3 R d 20 Ri4 R d 21 R f 2/R d 37 Ri5/R b 8 R d 22 Ri6/R d 33 R l 2/R k 6 R b 2 R d 23 Rg1 Ri7/Rm11 R l 3/R k 8 R b 3 R d 24 R l 4/Rg11 R b 4 R d 25 Rj 1 R l 5 R b 5/R d 29 R d 26 R l 6 R b 6/Re20 R d 27 Rg2 R l 7/R d 35 R b 7/Rg22 R d 28 Rg3 R b 8/Ri5 R d 29/R b 5 Rg4/R d 6 Rj 2/Ra1 Rm1 R b 9/R k 28 R d 30 Rg5 Rj 3 R b 10/Rm9 R d 31/Re21 Rg6/R d 10 Rj 4 Rm2 R d 32/Rg23 Rg7/Re2 Rj 5 Rc1 R d 33/Ri6 Rg8 Rj 6 R d 34/R k 29 Rg9/R k 10 R d 35/R l 7 Rg10/R l 4 R k 1 Rm3/Rg17 R d 36/Rm10 Rg11 Rm4/R k 16	58 0.50 6.10 1.45 0.67 0.67 2.68 0.49 5.34 × 10 -8 (300/Te) 0.72 3.11 × 10 21 1.70 × 10 -14 6.43 × 10 23 2.46 × 10 -10 9.25 × 10 20 1.60 × 10 -10 7.20 × 10 18 1.43 × 10 -9 2.53 × 10 17 2.79 × 10 -7 /5.91 × 10 -7 0.708 2.16 × 10 -3 1.68 × 10 -5 7.10 × 10 -11 1.87 × 10 15 8.73 × 10 -9 1.00 × 10 -11 1.03 × 10 23 0.113 1.71 × 10 -11 3.13 × 10 14 7.29 × 10 -10 7.26 × 10 -3 1.83 5.00 × 10 -10 3.51 × 10 18 3.86 × 10 -6 /8.19 × 10 -6 0.89 2.09 × 10 -9 4.22 × 10 19 1.97 × 10 -4 1.08 5.00 × 10 -11 5.34 × 10 21 5.88 × 10 -3 3.41 3.00 × 10 -11 4.22 × 10 19 9.84 × 10 -5 1.92 5.60 × 10 -7 (300/Te) 1.20 3.03 × 10 20 7.07 × 10 -4 1.07 1.15 × 10 -9 1.37 × 10 19 1.51 × 10 -5 /3.20 × 10 -5 1.03 2.57 × 10 -9 5.40 × 10 20 2.52 × 10 -3 0.62 2.40 × 10 -9 8.68 × 10 20 9.56 × 10 -4 2.28 3.00 × 10 -11 1.31 × 10 21 3.06 × 10 -3 1.79 1.00 × 10 -9 (300/Te) 0.76 6.73 × 10 21 0.016 2.86 1.95 × 10 -10 1.07 × 10 18 1.18 × 10 -6 /2.50 × 10 -6 5 + CH3CN → C2H4 + CH4CN + 1.58 1 CH2 + C2H6 → CH3 + C2H5 N + + C2H6 → CH4 + HCNH + CH + 3 + HC3N → HCN + c-C3H + N( 4 S) + C2H5 → C2H4 + NH 3 NH + CH3 → N( 4 S) + CH4 1 CH2 + C3H7 → CH3 + C3H6 CH + 5 + e → CH4 + H CN + + CH4 → HCN + CH + C2H + 5 + NH3 → C2H4 + NH + 4 3 N + + CH4 → N( 4 S) + CH + 4 3 CH2 + C2H3 → CH3 + C2H2 C2H + 4 + e → CH4 + C HCN + + C2H2 → HCN + C2H + C2H + 5 + CH2NH → C2H4 + CH2NH + 2 2 N + + HCN → N( 4 S) + HCN + 3 CH2 + C2H5 → CH3 + C2H4 C2H + 5 + e → CH4 + CH CNC + + C2H4 → HCN + c-C3H + C2H + 3.80 × 10 -9 3.64 × 10 20 1.70 × 10 -3 3 N + 2 + e → N( 4 S) + N( 2 D) 3 CH2 + C3H7 → CH3 + C3H6 2.40 3.00 × 10 -12 6.89 × 10 14 1.61 × 10 -9 C2H + 6 + e → CH4 + 3 CH2 3.48 4.00 × 10 -8 (300/Te) 0.70 6.17 × 10 12 1.44 × 10 -11 HCNH + + C3H3N → HCN + C3H4N + 1.14 4.50 × 10 -9 2.02 × 10 18 2.22 × 10 -6 /4.70 × 10 -6 C2H + 3.55 × 10 -9 7.39 × 10 18 3.45 × 10 -5 5 + HC3N → C2H4 + HC3NH + 0.75 1.73 1.14 × 10 -7 (300/Te) 0.39 2.26 × 10 22 0.025 N + 2 + e → 2 N( 2 D) 0.53 1.06 × 10 -7 (300/Te) 0.39 5.94 × 10 21 C2H + CH4→ CH3 + C2H2 0.73 4.49 × 10 -13 1.62 × 10 21 3.79 × 10 -3 C2H + 7 + e → CH4 + CH3 3.94 5.00 × 10 -7 (300/Te) 0.70 2.25 × 10 20 5.25 × 10 -4 C2H3 C2H + 5.80 × 10 -9 2.35 × 10 19 1.10 × 10 -4 5 + C3H3N → C2H4 + C3H3NH + 1.51 6.54 × 10 -4 CN + + e → N( 4 S) + C 3.39 1.36 × 10 -8 (300/Te) 0.55 2.71 × 10 17 C2H + C2H5 → CH3 + C3H3 1.44 3.00 × 10 -11 2.06 × 10 19 4.81 × 10 -5 C3H + 4 + e → CH4 + C2 2.28 1.00 × 10 -8 (300/Te) 0.67 1.97 × 10 18 4.60 × 10 -6 H + C2H2 + M → C2H3 +M 0.77 k0 = 5.88 × 10 -30 3.79 × 10 13 1.77 × 10 -10 C2H + 6.00 × 10 -9 2.10 × 10 16 9.81 × 10 -8 5 + C3H5N → C2H4 + C3H5NH + 1.10 2.99 × 10 -7 CN + + e → N( 2 D) + C 2.11 3.26 × 10 -7 (300/Te) 0.55 2.88 × 10 18 3.17 × 10 -6 N2H + + e → N( 4 S) + NH 1.11 2.47 × 10 -7 (300/Te) 0.84 8.53 × 10 20 9.38 × 10 -4 C2H + CH3CCH → CH3 + C4H2 1.06 1.55 × 10 -10 2.57 × 10 19 5.99 × 10 -5 N( 4 S) + C2H4 → CH3 + HCN 1.62 3.18 × 10 -16 9.70 × 10 19 2.26 × 10 -4 C3H + 5 + e → CH4 + C2H 3.46 3.00 × 10 -8 (300/Te) 0.70 4.83 × 10 20 1.13 × 10 -3 NH3 k∞ = 3.74 × 10 -15 C3H + 6 + e 12 27, 71 10 3 22 1 38 50, 51 3 43 20 37 12 3 61 2, 10 37 13, 14 43 3 3 38 15, 16 56, 57 3 4, 5 34 15 15 4, 5 37 17 32, 77 15 6 6 72, 73 35 15, 16 k R = 2.16 × 10 -18 7 CNC + + e → N( 4 S) + C2 2.03 2.00 × 10 -8 (300/Te) 0.70 7.30 × 10 19 N( 4 S) + C2H5 → CH3 + H2CN 1.34 3.90 × 10 -11 2.37 × 10 22 0.055 22 NH2 + H + M → NH3 +M 1.78 k0 = 2.52 × 10 -28 6.11 × 10 15 6.73 × 10 -9 15, 54, 64 Fc = 0.18 C2H5 8.04 × 10 -5 7 N( 4 S) + CH3CCH → CH3 + CHCN 2.25 8.01 × 10 -16 1.89 × 10 17 4.40 × 10 -7 35 k∞ = 2.84 × 10 -10 1 CH2 + C3H5 → C2H3 + C2H4 1.25 6.67 × 10 -11 1.34 × 10 19 6.07 × 10 -5 39 H + C2H4 + M → C2H5 + M 0.77 k0 = 1.64 × 10 -28 2.85 × 10 16 1.33 × 10 -7 34, 77 3 CH2 H2 + C + M → 3 CH2 + M 2.26 k0 = 2.50 × 10 -28 4.60 × 10 18 N( 2 D) + CH4 → CH3 + NH 0.69 6.47 × 10 -14 2.77 × 10 22 0.065 1 k R = 2.82 × 10 -16 C2H3CN + C2 → C2H3 + C3N 1.62 4.4 × 10 -10 3.53 × 10 15 1.65 × 10 -8 k∞ = 4.85 × 10 -14 49, 59 1.07 × 10 -5 8, 9 CN + CH4 → CH3 + HCN 0.50 6.37 × 10 -14 5.78 × 10 19 1.35 × 10 -4 41, 69 Fc = 0.42 C3H + 4 + e → C2H3 + CH 2.09 2.95 × 10 -8 (300/Te) 0.67 4.51 × 10 18 2.10 × 10 -5 17 k R = 5.62 × 10 -17 k∞ = 1.41 × 10 -11 C2N + CH4 → CH3 + CHCN 0.97 6.00 × 10 -14 2.42 × 10 19 5.65 × 10 -5 60 CH3NH + 3 + e → NH3 + CH3 2.58 7.00 × 10 -7 (300/Te) 0.70 2.25 × 10 20 2.48 × 10 -4 15, 42 C3H + 5 + e → C2H3 + 3 CH2 0.99 3.00 × 10 -8 (300/Te) 0.70 8.31 × 10 19 3.37 × 10 -4 Fc = 0.20 15, 16 k R = 6.00 × 10 -16 CH3CN + C2H → CH3 + HC3N 2.09 1.06 × 10 -13 1.64 × 10 16 3.83 × 10 -8 26 C2H2 C3H + 2.05 1.20 × 10 -8 (300/Te) 0.70 4.70 × 10 14 1.92 × 10 -8 18 1 CH2 + C3H7 → C2H5 + C2H4 1.76 4.29 × 10 -11 2.40 × 10 14 1.12 × 10 -9 38 6 + e → C2H3 + CH3 Fc = 0.40 N + + C2H4 → 3 CH2 + HCNH + 4.13 1.58 × 10 -10 6.31 × 10 19 1.47 × 10 -4 C2H + 4 + C2H4 → CH3 + C3H + 5 0.40 7.03 × 10 -10 4.67 × 10 21 0.011 3 H + C2H + M → C2H2 +M 2.99 k0 = 9.00 × 10 -26 1.66 × 10 15 7.74 × 10 -9 37, 79 C4H + 3.69 8.70 × 10 -9 (300/Te) 0.59 8.15 × 10 18 3.81 × 10 -5 18, 48 3 CH2 + C3H7 → C2H5 + C2H4 1.56 3.00 × 10 -12 2.00 × 10 14 9.33 × 10 -10 38 9 + e → C2H3 + C2H6 10 C2H + 3 + e → 3 CH2 + CH 0.67 1.50 × 10 -8 (300/Te) 0.84 4.70 × 10 19 N + + CH4 → CH3 + NH + 1.43 5.00 × 10 -10 1.40 × 10 22 0.033 10, 26 k∞ = 1.85 × 10 -10 N2 C2H + C2H6 → C2H2 + C2H5 0.65 4.15 × 10 -11 1.68 × 10 19 1.51 × 10 -6 52 1.10 × 10 -4 11 C2H + 4 + e → 2 3 CH2 1.52 2.24 × 10 -8 (300/Te) 0.76 2.15 × 10 21 HCN + + CH4 → CH3 + HCNH + 1.90 1.14 × 10 -9 2.26 × 10 22 0.053 3 k R = 1.00 × 10 -13 2 N( 4 S) + M → N2 + M 4.91 k0 = 4.46 × 10 -32 4.63 × 10 19 1.02 × 10 -4 58 C2H + C3H7 → C2H5 + C3H3 1.22 2.00 × 10 -11 2.00 × 10 13 9.32 × 10 -11 38 5.02 × 10 -3 12 C2H + 5 + e → 3 CH2 + CH3 CH + 4 + e → CH3 + H 0.50 3.08 × 10 -7 (300/Te) 0.66 1.44 × 10 20 3.37 × 10 -4 25 Fc = 0.40 k∞ = 5.00 × 10 -16 C3H + 0.91 8.00 × 10 -8 (300/Te) 0.70 3.34 × 10 18 1.56 × 10 -5 18 6 + e → C2H5 + CH 2.02 2.08 × 10 -7 (300/Te) 1.20 1.51 × 10 22 0.035 13, 14 C2H + 6 + e → 3 CH2 + CH4 3.98 4.00 × 10 -8 (300/Te) 0.70 3.55 × 10 13 8.27 × 10 -12 C2H + 3 + e → CH3 + C 1.19 3.00 × 10 -9 (300/Te) 0.84 1.33 × 10 19 3.10 × 10 -5 11 H + C6H5 → C2H2 + C4H4 2.03 6.08 × 10 -12 1.65 × 10 15 7.68 × 10 -9 65 C3H + 8 + e → C2H5 + CH3 2.75 1.60 × 10 -7 (300/Te) 0.70 1.58 × 10 19 7.38 × 10 -5 18 k R = 5.00 × 10 -16 15, 16 C3H + 4 + e → 3 CH2 + C2H2 3.89 1.77 × 10 -7 (300/Te) 0.67 6.51 × 10 19 C2H + 4 + e → CH3 + CH 0.42 1.12 × 10 -8 (300/Te) 0.76 2.33 × 10 20 5.45 × 10 -5 12 1 CH2 + C2H → C2H2 + CH 1.03 3.00 × 10 -11 3.19 × 10 18 1.49 × 10 -5 37 Fc = 0.40 C2H6 1.52 × 10 -4 17 C3H + 5 + e → 3 CH2 + C2H3 1.90 3.00 × 10 -8 (300/Te) 0.70 2.75 × 10 20 C2H + 5 + e → CH3 + 3 CH2 1.89 2.08 × 10 -7 (300/Te) 1.20 1.43 × 10 22 0.034 13, 14 1 CH2 + C2H3 → C2H2 + CH3 2.11 3.00 × 10 -11 1.34 × 10 18 6.27 × 10 -6 37 N( 2 D) + N2 → N2 + N( 4 S) 0.79 1.70 × 10 -14 2.53 × 10 22 0.056 1 H + C2H5 + M → C2H6 + M 2.09 k0 = 2.38 × 10 -30 4.57 × 10 9 2.13 × 10 -14 62, 63 6.40 × 10 -4 15, 16 C3H + 6 + e → 3 CH2 + C2H4 3.64 1.20 × 10 -7 (300/Te) 0.70 1.05 × 10 16 C2H + 6 + e → 2 CH3 3.40 8.00 × 10 -8 (300/Te) 0.70 1.20 × 10 13 2.81 × 10 -11 15, 16 3 CH2 + C2H → C2H2 + CH 0.90 3.00 × 10 -11 4.11 × 10 19 1.92 × 10 -4 37 N + + HCN → N2 + CH + 1.28 1.29 × 10 -9 1.11 × 10 20 2.43 × 10 -4 3 k∞ = 1.66 × 10 -10 2.45 × 10 -8 18 C3H + 8 + e → 3 CH2 + C2H6 5.46 1.60 × 10 -7 (300/Te) 0.70 7.76×10 19 1.81 × 10 -4 C2H + 7 + e → CH3 + CH4 4.20 5.00 × 10 -7 (300/Te) 0.70 2.38 × 10 20 5.56 × 10 -4 15 3 CH2 + C2H3 → C2H2 + CH3 1.97 3.00 × 10 -11 1.78 × 10 19 8.30 × 10 -5 37 N + 2 + C2H2 → N2 + C2H + 2.02 9.40 × 10 -10 1.93 × 10 21 4.26 × 10 -3 10 2 CH3 + M → C2H6 + M 1.85 k0 = 2.04 × 10 -25 6.18 × 10 21 0.029 15 2 15 C3H + 5 + e → CH3 + C2H2 4.43 3.00 × 10 -8 (300/Te) 0.70 6.41 × 10 20 1.50 × 10 -3 15, 16 CH3 + C2H3 → C2H2 + CH4 1.16 1.95 × 10 -10 1.51 × 10 21 7.09 × 10 -3 45, 78 N + 0.99 3.90 × 10 -10 4.31 × 10 20 9.48 × 10 -4 3 k∞ = 7.45 × 10 -11 2 + HCN → N2 + HCN + NH H + N + M → NH + M 2.26 k0 = 5.00 × 10 -32 3.19 × 10 18 C3H + 6 + e → CH3 + C2H3 3.69 1.20 × 10 -8 (300/Te) 0.70 1.07 × 10 15 2.50 × 10 -9 18 C2H + C2H3 → 2 C2H2 2.13 1.60 × 10 -12 3.67 × 10 16 1.71 × 10 -7 37 N2H + + HCN → N2 + HCNH + 1.16 3.20 × 10 -9 4.48 × 10 20 9.86 × 10 -4 3 k R = 1.65 × 10 -13 3.52 × 10 -6 15, 19 C3H + 7 + e → CH3 + C2H4 5.41 3.80 × 10 -8 (300/Te) 0.68 3.03 × 10 20 7.06 × 10 -4 12 C2H + C2H5 → C2H2 + C2H4 2.19 3.00 × 10 -12 1.40 × 10 18 6.55 × 10 -6 37 Fc = 0.37 C2H4 k∞ = 5.00 × 10 -16 C3H + 8 + e → CH3 + C2H5 5.31 1.60 × 10 -7 (300/Te) 0.70 7.46 × 10 19 1.74 × 10 -4 18 C2H + C2H6 → C2H2 + C2H5 0.73 4.15 × 10 -11 1.37 × 10 19 6.37 × 10 -5 34, 52 H + C2H3 + M → C2H4 +M 2.40 k0 = 8.54 × 10 -27 1.51 × 10 12 7.08 × 10 -12 C2H3 + C2H5 → C2H6 + C2H2 1.53 8.00 × 10 -13 6.76 × 10 16 3.16 × 10 -7 37 47 k R = 5.00 × 10 -16 C3H + 9 + e → CH3 + C2H6 5.34 5.00 × 10 -7 (300/Te) 0.70 3.07 × 10 19 7.17 × 10 -5 15 C2H + CH2CCH2 → C2H2 + C3H3 1.16 2.58 × 10 -10 2.88 × 10 19 1.34 × 10 -4 60 k∞ = 1.76 × 10 -10 2 C2H5 → C2H6 + C2H4 1.34 2.40 × 10 -12 7.08 × 10 18 3.30 × 10 -5 33
	Rc2/Rg21 R d 37/R f 2 Rg12 Rm5 R k 2 Rc3/R k 21 Rg13 Rm6 R k 3/R d 3 Rc4 Re1 Rg14 Rm7 R k 4/R d 7 Rc5/R d 19 Rc6 Rg15 Rg16/R k 11 Rg17/Rm3 R k 5/Ri2 R k 6/R l 2 Rm8	Fc = 0.40 1.31 × 10 -11 2.92 7.00 × 10 -7 (300/Te) 0.70 2.60 × 10 20 1.23 1.94 × 10 20 2.00 1.20 × 10 -11 8.15 × 10 17 3.06 2.00 × 10 -15 4.37 × 10 19 1.72 5.00 × 10 -11 4.51 × 10 13 1.25 7.10 × 10 -11 4.17 × 10 22 CH4 2.61 1.00 × 10 -11 2.71 × 10 13 1.23 3.09 × 10 -10 4.13 × 10 20 1.74 8.00 × 10 -13 7.07 × 10 15 0.59 5.00 × 10 -11 1.50 × 10 20 2.87 k0 = 1.62 × 10 -28 1.36 × 10 18 0.93 6.10 × 10 -11 5.73 × 10 16 1.37 1.50 × 10 -11 2.41 × 10 19 0.82 5.00 × 10 -11 3.02 × 10 15 0.69 6.47 × 10 -14 2.77 × 10 22 0.38 7.05 × 10 -17 3.71 × 10 13 To be continued k∞ = 2.95 × 10 -10 k R = 1.80 × 10 -16 Fc = 0.56 To be continued 0.88 1.15 × 10 -10 1.51 × 10 13 1.72 3.50 × 10 -11 9.72 × 10 16 1.76 8.00 × 10 -13 8.96 × 10 16 To be continued 1.21 6.67 × 10 -11 1.30 × 10 19 1.82 4.29 × 10 -11 4.26 × 10 14 To be continued 7 + e → To be continued N( 4 S) + C2H3 → NH + C2H2 CH3NH + 3 + e → CH3 + NH3 C2H + C3H5 → C2H2 + CH2CCH2 C + CH4 + M → C2H4 + M C2H5 + C3H7 → C2H6 + C3H6 N( 4 S) + C2H5 → NH + C2H4 C2H + C3H7 → C2H2 + C3H6 CH + C2H6 → C2H4 + CH3 C2H5 + C4H3 → C2H6 + C4H2 N( 2 D) + NH3 → NH + NH2 H + CH3 + M → CH4 + M C2H + C3H8 → C2H2 + C3H7 1 CH2 + C2H5 → C2H4 + CH3 C2H + 3 + C3H8 → C2H6 + C3H + 5 N( 2 D) + CH4 → NH + CH3 2 NH2 → NH + NH3 C2H + C4H10 → C2H2 + C4H9 2 C2H3 → C2H2 + C2H4 C2H3 + C2H5 → C2H2 + C2H6 1 CH2 + C3H5 → C2H4 + C2H3 1 CH2 + C3H7 → C2H4 + C2H5 C3H +	2.13 × 10 -4 6.08 × 10 -4 3.80 × 10 -6 2.04 × 10 -4 2.10 × 10 -10 0.046 1.27 × 10 -11 1.93 × 10 -3 3.30 × 10 -8 1.65 × 10 -4 3.17 × 10 -6 2.67 × 10 -7 1.13 × 10 -4 1.41 × 10 -8 0.031 4.09 × 10 -11 7.07 × 10 -11 4.53 × 10 -7 4.18 × 10 -7 6.26 × 10 -5 1.99 × 10 -9	15, 42 39 67 15, 38 10, 21 38 31, 68 27 22 33, 77 34, 52 37 26 1 1 23, 76 53, 75 66 37 39 38

  Table A.2. Best-fit parameters (a1, a2, and a3) to describe the altitude dependence of escape probability for a given species at a given nascent energy (see Equation 34 for details). Table A.3. Best-fit parameters (b1, b2, b3, and b4) to describe the energy dependences of the parameters listed in Table A2 (a1, a2, and a3) (see Equation 35 for details). S)/N( 2 D) -0.0073 0.277 0.0709 0.234 3 CH 2 /CH 3 /CH 4 -0.129 0.313 0.0575 0.179 NH/NH 3 -0.134 0.313 0.0579 0.175 C 2 H 2

		Species		b 1	b 2	b 3	b 4
		N( 4 0.0383	0.236 0.0587 0.158
		HCN		0.0506	0.199 0.0608 0.153
		N 2		0.0368	0.232 0.0619 0.165
		C 2 H 3 /C 2 H 4		0.0259	0.245 0.0657 0.179
		C 2 H 5 /C 2 H 6		0.0399	0.222 0.0646 0.167
	Species	0.5	1.0	1.5	Energy (eV) 2.5	3.5	5.0	6.5	a 2
	N( 4 S)/N( 2 D)	a 1 0.272 0.274 0.275 0.276 0.276 0.277 0.277 1450 a 3 0.180 0.234 0.267 0.303 0.326 0.347 0.361
	3 CH 2 /CH 3 /CH 4	a 1 0.236 0.268 0.281 0.297 0.306 0.316 0.319 1500 a 3 0.135 0.181 0.206 0.235 0.254 0.271 0.283
	NH/NH 3	a 1 0.233 0.265 0.280 0.297 0.307 0.315 0.318 1500 a 3 0.130 0.177 0.202 0.231 0.250 0.268 0.279
					Energy (eV)
		1	1.5	2.0	3.0	4.0	5.0	-
	C 2 H 2	a 1 0.235 0.252 0.263 0.279 0.289 0.296 a 3 0.157 0.182 0.199 0.224 0.239 0.251	--	1530
	HCN	a 1 0.197 0.221 0.237 0.257 0.269 0.279 a 3 0.151 0.178 0.196 0.221 0.236 0.249	--	1600
	N 2	a 1 0.244 0.256 0.263 0.274 0.282 0.285 a 3 0.178 0.206 0.226 0.253 0.270 0.283	--	1520
	C 2 H 3 /C 2 H 4	a 1 0.230 0.247 0.258 0.273 0.282 0.290 a 3 0.164 0.191 0.208 0.234 0.251 0.263	--	1540
	C 2 H 5 /C 2 H 6	a 1 0.219 0.240 0.252 0.266 0.277 0.285 a 3 0.151 0.178 0.196 0.221 0.236 0.249	--	1560

Article number, page 3 of 20 H.Gu et al.: Dayside nitrogen and carbon escape on Titan

Article number, page 32 of 20

Acknowledgements. This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA17010201). JC and YW acknowledge supports from the National Science Foundation of China (NSFC) through grants 41525015, 41774186, and 41525016.

ORCID iDs

H. Gu https://orcid.org/0000-0002-9831-0618 J. Cui https://orcid.org/0000-0002-4721-8184 P. Lavvas https://orcid.org/0000-0002-5360-3660

is intended for an updated evaluation of the same issue by virtue of the extensive Cassini 399 INMS measurements of Titan's upper atmospheric structure (e.g. Cui et al. 2009a,b; Magee 10 -11 10 -9 10 -7 10 -5 10

10 -15 10 -13 10 -11 10 -9 10 -7 10 -5 10 -3 10