Reactions of NO 3 with Aromatic Aldehydes: Gas-Phase Kinetics and Insights into the Mechanism of the Reaction

: Rate coefficients for the reaction of NO 3 radicals with a series of aromatic aldehydes were measured in a 7300-liter simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde 30 (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were: k 1 = 2.6 ± 0.3, k 2 = 8.7 ± 0.8, k 3 = 4.9 ± 0.5, k 4 = 4.9 ± 0.4, k 5 = 15.1 ± 1.3, k 6 = 12.8 ±1.2 and k 7 = 6.2 ±0.6, respectively, in the units of 10 −15 cm 3 molecule −1 s −1 at 298±2 K. The rate coefficient k 13 for the reaction of the NO 3 radical with deuterated benzaldehyde (benzaldehyde-d 1 ) was found to be half that of k 1 . The end product of the reaction in an excess 35 of NO 2 was measured to be C 6 H 5 C(O)O 2 NO 2 . Theoretical calculations of aldehydic bond energies and reaction pathways indicate that NO 3 radical reacts primarily with aromatic aldehydes through the abstraction of an aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.


Introduction
Aromatic aldehydes are a family of organic compounds emitted into the atmosphere from anthropogenic and biogenic sources. For example, benzaldehyde (BA) has been detected in the incomplete combustion of fuels (Legreid et al., 2007). Methylated benzaldehydes (ortho, meta, 45 para-tolualdehydes) are present in wood smoke and biomass burning plumes (Koss et al., 2018).
Benzaldehyde also has extensive industrial usage in perfume, soap, food and drink production, and as solvent for oils and resins (OECD, 2002). BA and ortho/meta/para-tolualdehyde could also be formed from the atmospheric degradation of aromatic hydrocarbons, although their yields in the atmosphere are expected to be small (Calvert et al., 2002;Obermeyer et al., 2009). 50 It is believed that their reaction with the OH radical dominates the photochemical loss of aromatic aldehydes. Yet, oxidation of aromatic aldehydes via their reaction with the nitrate radical, NO3, may be important in NOx-rich locations at night since both the aromatic aldehydes and the nitrate radical can arise from anthropogenic emissions and biomass burning. Therefore, aromatic aldehydes can be degraded at night with consequences for ozone and secondary 55 organic aerosol (SOA) formation. The reactions of aromatic aldehydes with NO3 likely leads to acyl peroxy nitrates (APNs, often also referred to as PANs) in the NOx-rich regions with characteristically high NO3 abundances. The APNs would then transport nitrogen oxides and the aromatic moieties from the polluted to cleaner parts of the globe (Wayne, 2000) with consequences for ozone production and particle formation away from the polluted regions. 60 Therefore, quantifying the kinetics and understanding the mechanism of the reaction of NO3 with aromatic aldehydes is needed.
We report the rate coefficients at 298 K for the above reactions and we have determined the stable products formed in Reaction (1). We also have attempted to elucidate the mechanism 80 of these reactions via studies of isotopic substitution, quantum chemistry calculations, and an examination of the linear free energy relationship.

Experimental setup and procedures
The kinetics and products were studied in a 7300 liter indoor simulation chamber described 85 in detail previously (Zhou et al., 2017); therefore, it is described only briefly here. The chamber was made of Teflon foil. Two Teflon fans located inside the chamber rapidly mixed the contents of the chamber within about 30 s. Purified air was used as a bath gas and to flush the chamber to clean it in between experiments. Upon flushing, the levels of NO2 and O3 were less than 50 pptv (detection limit of our instruments). A proton transfer reaction time-of-flight mass 90 spectrometer (PTR-TOF-MS) and a Fourier transform infrared spectrometer (FT-IR, Nicolet 5700) coupled to a White-type multipass cell (143 m optical path length) were employed to monitor the organic compounds in the chamber. The white cell was located within the chamber.
An inlet situated in the center of the chamber fed the PTR-TOF-MS. The masses in PTR-TOF-MS and IR lines in the FTIR were used to measure the aldehydes that are listed in the SI (Table  95   S1). The PTR-TOF-MS and/or FTIR signals were measured as a function of manometrically determined hydrocarbons concentrations. Calibration plots for the quantification of the aromatic aldehydes and the reference gases were constructed using these measurements. They are shown in Figure S1 in the SI. The concentrations of NO3 and N2O5 were determined using a cavity ring down spectrometer (CRDS). The NO3 radical was detected using its strong 662 100 nm absorption. The sum of NO3 and N2O5 were detected simultaneously by thermally dissociating N2O5 in a second channel. The time resolution of CRDS was 1s, and detection limits for NO3 and N2O5 were approximately 0.5 pptv for a 5 s intergration (Brown et al., 2002;Dubé et al., 2006). NO3 radicals were generated from the thermal decomposition of N2O5 in the chamber. 105 We accounted for the dilution of the chamber necessitated by the addition of air to keep 115 the pressure constant while we continually withdrew the chamber's contents for analysis.

Rate coefficient measurements
Similarly, we also accounted for the depletion of the hydrocarbons due to the loss to the walls.
The two processes together are represented as a first order loss process with a rate coefficient kd: 120 aromatic aldehydes/references → wall loss/dilution ; kd Above, k1-7 and kref are the rate coefficient for the reaction of NO3 with studied aromatic aldehydes (BA, O-TA, M-TA, P-TA, 2,4-DMBA, 2,5-DMBA, and 3,5-DMBA) and the reference compound (methyl methacrylate, MMA), respectively. The dilution rate coefficient 125 was measured by adding a small amount of unreactive SF6 into the chamber and watching its temporal decay. We quantified the rate coefficient (ranging from 5.510 -7 s -1 to 1.610 -6 s -1 ) for the removal of the aldehydes and reference compound due to loss on the walls by monitoring their decay in the absence of the NO3 reactant The typical experimental procedure for relative rate measurements consisted of a sequence 130 of three steps: (1) SF6 was added and monitored for about 30 mins to measure the dilution rate coefficient; (2) VOCs (aromatic aldehydes, reference compounds) were introduced into the chamber and monitored for roughly 30 minutes to obtain kd; and (3) N2O5 was then continually introduced to the chamber using pure air as a carrier gas, and the consumption of VOCs was monitored using PTR-TOF-MS for 1-2 hours. The typical initial VOC concentrations were (0.9-135 4.5) × 10 12 molecule cm -3 (Table S2). Typical NO3 concentrations were (0.25-2.5) × 10 9 molecule cm -3 .
Assuming that the aromatic aldehydes and reference compounds were lost by reaction with NO3 radical and dilution/loss to the walls, it can be shown that: CRDS in an excess of each aromatic aldehyde (Zhou et al., 2017), and were simultaneously fit 150 to a reaction scheme (see below) to extract the reaction rate coefficient as described by Zhou et al., (2017). The typical experimental procedure consisted of the following steps: (1) N2O5 was introduced with pure air into the chamber and the temporal profiles of NO3 and N2O5 concentrations were measured to determine the rate coefficient for loss of NO3 and N2O5 to the walls, reactions with impurities, and dilution; (2) The aromatic aldehyde of interest was 155 introduced while continually measuring the temporal profiles of NO3 and N2O5; and (3) SF6 was introduced to determine the dilution rate when needed.
To obtain the rate coefficients of NO3 radical reaction with aromatic aldehydes, a box model was used to integrate the set of Reactions (1-11), and the obtained temporal profiles of NO3 and N2O5 concentrations were fit to the observed profiles using a non-linear least squares 160 algorithm.
NO3 → wall loss ; k1 0 (10) N2O5 → wall loss ; k1 1 (11) 165 NO3 + aromatic aldehydes → products ; k1 -7 (1-7) In the algorithm, the sum of squares of both the NO3 and N2O5 were minimized while varying the input parameters. In the absence of the aldehydes, the input parameter was just kd. The value of kd obtained was then held constant when subsequently fitting the second set of temporal 170 profiles where the rate coefficient for reactions (1)- (7) Table S3, the concentration of aromatic aldehyde was always 50-1500 times higher than that of NO3 in the chamber at all times, and thus NO3 loss was essentially 180 first order in its concentration.

Chemicals
The aromatic aldehydes were purchased from Sigma-Aldrich. The stated purities of these DMBA (≥ 90%), 2,5-DMBA (99%) and 3,5-DMBA (97%). Methyl methacrylate was bought from TCI. The chemical purity of benzaldehyde-α-d1 was 99% while its isotopic purity was 98%. All the aldehydes (in liquid form) were further purified by repeated freeze-pump-thaw cycles before use. Substantial concentrations of reactive impurities were not detected in these samples based on the PTR-TOF-MS/FTIR measurements. A mixture of NO2 and O3 was flowed 190 into a 1 liter bulb to generate N2O5 through Reactions 12 and 8.
The N2O5 crystals were collected in a cold trap (190 K) and purified by trap-to-trap distillation in a mixture of O2/O3. N2O5 was stored in a cold trap maintained 190 K. 195

Rate coefficients determination of NO3 reaction with a series of aromatic aldehydes
In the study, we measured the rates coefficients k1-k7 using a relative method and an absolute method. They are outlined separately below for ease of presentation. 200

Relative method
Methyl methacrylate, MMA, was used as the reference compound in this work because the rate coefficient at 298 K for its reaction with NO3 radical is well established (Zhou et al., 2017) to be kMMA=(2.98±0.35)×10 -15 cm 3 molecule -1 s -1 . The experimental conditions and associated 205 parameters are shown in Table S2.  The relative rate coefficients of the studied aromatic aldehydes, termed kRR, are shown in 225   Table S2. The estimated uncertainties for kRR were the sum of the precision of our measurements (noted above) and the quoted uncertainties in the rate coefficient for the reference reaction according to the expression:

Absolute method
The method used to analyze the temporal profiles of NO3 and N2O5 to obtain k1-k7 has been described by Zhou et al. (2017;. Figure 2 shows an example of the temporal profiles of NO3 and N2O5 for Reaction (1) from which the rate coefficient k1 was derived. Similar analyses yielded k2-k7 ( Figure S2-7). to be in the ranges (3.7-7.1)×10 -3 s -1 and (3.2-12.0)×10 -4 s -1 , respectively. As explained in Zhou 240 et al.,(2017), we cannot merely take the slopes of decay of NO3 with time to calculate the rate coefficients k1-k7 since NO3 and N2O5 are coupled through their equilibrium. The equilibration is maintained throughout the course of Reactions (1)-(7). To account for this situation, we fit the profiles of both NO3 and N2O5 to a reaction scheme, as described previously. Figure 2 show the NO3 and N2O5 temporal profiles on a linear scale 245 in the absence and in the presence of the reactant aldehyde. The two panels also show the fits of the data to the mechanism that included only Reactions (1)-(11). The fits are acceptable but with larger variations at longer reaction times. We have to consider the contributions to NO3 and N2O5 losses due to the reactions of NO3 with the products of the Reactions (1)-(7). Values of the rate coefficients derived from such fits included all the potential reactions that can 250 contribute to the removal of NO3, and they are shown in the SI (Table S4). When the secondary reactions with the products were included, the fits were better at the longer reaction times, and they are shown in Panel (d) Table S4 are taken to be those measured by the direct method. As expected, the rate coefficients calculated by including the contributions of the secondary reactions were slightly less than those without the secondary reaction contributions (see Figure S8). The differences were on the average about 5%.  The results of our measured values for k1-7 are summarized in Table S3. The quoted errors of the rate coefficient from each experiment are at the 95% confidence level based on the 275 precision of the fits; they were typically less than 7%. The weighted averages of results from multiple experiments were calculated, and then the influence of the small number of measurements were accounted for by using a Student t-distribution table. We added the estimated systematic uncertainties to the precision in quadrature, assuming that they are uncorrelated. Contributions to estimated systematic errors included: (1) The systematic errors 280 of -8/+11% and -9/+12%, respectively, in the measurements of NO3 and N2O5 (Zhou et al., 2019); (2) The uncertainty of around 10% in the rate coefficients used in the reaction schemes shown in Table S4; and (3) The estimated uncertainty of 7% in the concentration of aromatic aldehydes. This includes the uncertainties in the calibration and spectral analysis. All the noted uncertainties are at the 95% confidence level, assuming a Gaussian error distribution. 285

Panels (b) and (c) of
It is important to note that we need the absolute concentrations of NO3 and N2O5 even though the reaction was first order in NO3 due to the strong coupling between the concentrations of NO3 and N2O5 via an equilibrium. The presence of the aldehydes would influence the temporal profiles of both N2O5 and NO3. Here we are attributing the entire change to the reaction of NO3 with the aldehydes. The validity of this assumption is shown by the measured 290 rate coefficients being independent of the ratio of [NO3]/[N2O5]. This ratio was changed simply by changing [NO2] that shifts the equilibrium concentrations of NO3 and N2O5.

Comparison of Rate coefficients obtained from absolute and relative methods
The rate coefficients for the reactions of NO3 with 7 different aromatic aldehydes, k1-k7, 295 measured using the absolute and relative methods are summarized in Table 1. The rate coefficient values from the two methods are in good agreement with each other. The differences are less than 10%, except for k1, which differs by 18%. They, however, overlap within the combined errors of our measurements. Table S5, the final rate coefficients of 7 aromatic aldehydes reaction with 300 NO3 radical were derived from the weighted average of the absolute and the relative rate methods, using the equation discussed above (Eq. S2 and Eq. S3).  Table S5 summarizes the rate coefficients measured in this work with data from the literature for the reactions of the NO3 radical with aromatic aldehyde, BA, O-TA, M-TA, P-TA, 310 2,4-DMBA, 2,5-DMBA, and 3,5-DMBA. As shown in Table S5, the rate coefficient for BA has been reported by five studies. Three of them (Atkinson et al., 1984;Carter et al., 1981;Clifford et al., 2005) used the relative method with different reference compounds. Atkinson et al.,(1991) corrected the values from their earlier report, and they are used for the comparison. Bossmeyer et al.,(2006) measured both NO3 and BA using differential optical 315 absorption spectroscopy (DOAS) in their chamber. They measured the loss of BA in a known (measured continuously) NO3 concentration and fitted BA's measured temporal profile to obtain k1. Calvert et al.,(2011) recommended k7 to be 4.0 ×10 -15 cm 3 molecule -1 s -1 with a 30% uncertainty based on these studies. However, our value from absolute and relative methods using methyl methacrylate as reference (its rate coefficient has been determined using absolute 320 method in our previous study (Zhou et al., 2017) are in good agreement with Atkinson (1991) and Bossmeyer et al., (2006). Hence, we suggest that the weighted average based on these three studies, 2.6±0.3 ×10 -15 cm 3 molecule -1 s -1 at 298±2 K, is a reliable value. The rate coefficients for ortho/meta/para-tolualdehyde have only been studied by Clifford et al.,(2005) who reported them to be (9.8±0.4), (9.5±0.4) and (9.5±0.7) ×10 -15 cm 3 molecule -1 s -1 , respectively. In this 325 work, k1-k7 were determined by relative (MMA as the reference) and absolute methods to be: respectively. This work agrees best with the value of Clifford et al.,(2005) for orthotolualdehyde, but those of k3 and k4 are smaller than those of Clifford et al.,(2005). The reasons for these discrepancies are unclear, but the excellent agreement (<7% difference) between the 330 two techniques presented here give us confidence in our determinations. This work provides the first experimental determinations of the rate coefficients for NO3 reactions with 2,4-DMBA, 2,5-DMBA, and 3,5-DMBA, where the two complementary methods agree well. The recommended rate coefficients of the weighted average of relative method and absolute method are shown in Table S5.

Products investigation from the reaction of benzaldehyde with NO3
The stable products formed in Reaction (1) were investigated at 298±2 K and 760 torr in the same 7300 liter simulation chamber. Benzaldehyde, 0.9-1.4×10 13 molecule cm -3 (as shown in 340 Table S6) was introduced into the chamber and its removal was measured for 2 hours to obtain the wall loss rate coefficient. Then, roughly 1.1×10 12 -1.5×10 14 molecule cm -3 of N2O5 was introduced. Stable products formed in the chamber were identified and quantified (when possible) using the PTR-TOF-MS and FTIR.

345
Two stable products, C6H5C(O)O2NO2 (benzaldehyde-PAN; BAPAN) and C6H5ONO2, were detected and measured. The former was detected using both PTR-TOF-MS (m/z 184.024 and its fragment m/z 105.034) and FTIR (965-1005 cm -1 centered at 989 cm -1 ) and the latter using only PTR-TOF-MS. Since we do not have a sample of BAPAN, we could not quantify the yield of this product using PTR-TOF-MS. However, Caralp et al.,(1999) have reported the IR band 350 strengths for BAPAN. Using their reported band strength, we could quantify BAPAN to be 80±10% of the benzaldehyde that was removed via reaction, where the quoted uncertainty is the precision in the fit at the 2  level. When we account for the uncertainties in the absorption cross sections of BAPAN reported by Caralp et al., (1999) (~20%) and the uncertainties in the concentration of initial benzaldehyde concentration (~10%), and add the precision of the 355 measurements, we conclude that the yield of BAPAN is 80±22%. We assume that the uncertainties are uncorrelated and hence added them in quadrature. The obtained BAPAN amounts are shown in Figure 3 as the function of benzaldehyde consumption; the details are shown in Table S6 in the SI. We could not quantify C6H5ONO2 because of the lack of a standard.
Assuming that the ion-molecule reaction rate coefficients for proton transfer to BAPAN and 360 C6H5ONO2 are similar, we estimate that the yield of C6H5ONO2 is smaller than that of BAPAN.
These two products are expected if the reaction proceeds via H atom abstraction, most of the peroxy radical reacts with NO2 (Platz et al., 1998) as denoted by the reaction scheme A:

(A) 365
A fraction of the peroxy radicals react with itself (or other peroxy radicals) to make the phenoxy radical, which ultimately leads to a nitrate, according to the mechanism B: Unfortunately, we could not reduce the concentration of NO2 sufficiently to completely suppress pathway B. A small fraction of the C6H5C(O)O2 would also react with NO3 but would 370 still yield the C6H5C(O)O radicals. Also, any reactions of phenyl radical with NO2 can be neglected because of the large abundance of O2 that will quickly convert it to C6H5O2 radical.
Numerical modeling of the reaction sequence shown in the SI (Table S4) suggests that the yield of BAPAN is more than 95% under our experimental conditions. Based on these results, we suggest the yield of BAPAN in our reaction system is essentially 1 and that we detect the nitrate 375 because of the excellent sensitivity for its detection in our PTR-TOF-MS.

Kinetic Isotope Effect in the reaction
To further examine the mechanism of NO3 reactions with the aromatic aldehydes, we measured the rate coefficient for the reaction of NO3 radical with benzaldehyde-α-d1 (C6H5CDO): 385 As shown in Figure 4, k13 is half that of k1, i.e., k1/k13 is 1.92. A factor of 2 decrease in the rate coefficient going from benzaldehyde to benzaldehyde-α-d1 is consistent with a primary kinetic isotope effect (KIE), suggesting that abstraction of the aldehydic H atom occurs in the ratelimiting step of this reaction pathway (See calculated KIE below).

Reaction pathway
Linear free energy relationships comparing the reactivities of OH and NO3 with a series of organic compounds is shown in the SI ( Figure S9). Such correlations have been demonstrated in the past by Clifford et al.,(2005). We have added our measured values of k1-k7 to the plot. 400 This plot suggests that the linear free energy relationship is consistent with NO3 radicals adding to the seven aromatic aldehydes studied here. The reaction of OH radicals with aliphatic aldehydes is known to proceed through the formation of a pre-reaction complex, making the results obtained for NO3 radicals consistent with an addition, albeit a weak adduct, pathway. Figure 5 shows the rate coefficients for the reaction of the NO3 radical with the seven 405 aldehydes studied here, along with those with benzene and toluene (<3×10 -17 and <6.6×10 -17 cm 3 molecule -1 s -1 ) (Calvert et al., 2011). Benzene and toluene do not react with NO3 to measurable extents. This observation suggests that NO3 does not react with the aromatic ring to form a stable product. Since it is not sufficiently reactive to abstract an H atom from either the ring or the methyl group, the rate coefficient is very slow, if not zero. Therefore, the 410 observed reaction rate coefficients suggest that the reaction proceeds via an H-atom abstraction from -CHO group (Clifford et al., 2005;Wayne et al., 1991). The rate coefficient for the reaction of NO3 with benzaldehyde, (2.6±0.3)×10 -15 cm 3 molecule -1 s -1 , is similar to that with acetaldehyde (2.7±0.5)×10 -15 cm 3 molecule -1 s -1 , as shown in Figure 5. The mechanism for the https://doi.org/10.5194/acp-2021-228 Preprint. Discussion started: 22 April 2021 c Author(s) 2021. CC BY 4.0 License. reaction of NO3 with acetaldehyde is believed to be H-atom abstraction from the -CHO group 415 after the formation of a pre-reaction complex.

Figure 5
The measured rate coefficients for the reactions of NO3 radical with the seven aldehydes studied here. The results of previously reported rate coefficients are also shown. The 420 rate coefficients for acetaldehyde of k(CH3CHO)=(2.7±0.5)×10 -15 cm 3 molecule -1 s -1 is from IUPAC recommendations. The rate coefficients for benzene, k(benzene)<3×10 -17 cm 3 molecule -1 s -1 , and for toluene, k(toluene)6.6×10 -17 cm 3 molecule -1 s -1 are from Calvert et al., 2011. As shown in Figure 5, this work finds a higher rate coefficient for the reactions of NO3 425 with ortho/meta/para-tolualdehydes than that with benzaldehyde. This enhanced reactivity with ring substitution by electron-donating groups suggests an electrophilic role for NO3 in the reaction. Clifford et al. (2005) noted that the direct influence of the electron-donation to the ring by the CH3 group is effectively canceled out by the electron-withdrawing effect of the -CHO group as an explanation for their measured rate coefficients being the same for all the 430 tolualdehydes. The trends we observe for substitution of methyl groups in the tolualdehydes and the demethylated aldehydes would be consistent with electrophilic addition. Of course, the initial addition has to lead to the abstraction of the aldehydic H atom, as shown by the products and the kinetic isotope effect. Alternatively, the reactivity trend could simply be due to the C-H bond energy changes in the aldehydic group upon methyl substitution. 435 The C-H bond dissociation enthalpies (BDEs) of benzaldehyde and the three tolualdehydes have been obtained from Active Thermochemical Tables (ATcT). As opposed to traditional sequential thermochemistry, ATcT obtains enthalpies of formation by constructing, statistically analyzing, and solving a global thermochemical network (TN), which is formed by including experimental and theoretical determinations pertinent to the chemical species that are included 440 in the network, as explained in more details elsewhere (Ruscic et al., 2004;Ruscic et al., 2005). The results presented here are based on the most current ATcT TN (ver. 1.122x), obtained by further expanding prior Zaleski et al., 2021) versions (1.122r and 1.122v) by including species of interest to the present study, as well as other ongoing studies. The current TN incorporates ~2,350 species, interconnected by ~29,000 experimental and theoretical determinations. 445 Typically, the insertion of a new chemical species in the TN begins by linking the new species to the already existing species via a provisional skeleton of theoretical isodesmic reactions computed using a standard set of mid-level composite calculations carried in-house, and currently consisting of W1 (Martin and de Oliveira, 1999;Parthiban and Martin, 2001), CBS-APNO (Ochterski et al., 1996), G4 (Curtiss et al., 2007), G3X (Curtiss et al., 2000), and CBS-QB3 (Montgomery et al., 1999(Montgomery et al., , 450 2000. This is subsequently complemented by experimental and theoretical determinations from the literature, and, when possible, by additional state-of-the-art high-level composite calculations that can deliver sub-kJ mol -1 accuracies. As the number and accuracy of additional determinations grows, the dependence of the final result on the determinations spanning the initial skeleton diminishes and ultimately vanishes. given at 298.15 K and at 0 K in Table S7, together with the corresponding enthalpies of formation of the parents and the related radicals. While only the 298.15 K BDEs are discussed below, the corresponding 0 K BDEs (a.k.a. D0 values) are also given in the same table and are, as expected, approximately 6.3 kJ mol -1 (or ~2.5 RT) lower. The ATcT uncertainties provided in Table S7 correspond to 95% confidence intervals, following the standard in thermochemistry 470 (Ruscic, 2014;Ruscic and Bross, 2019), and were obtained by using the full ATcT covariance matrix. Consequently, when the enthalpy of formation of the radical is highly correlated to that of the parent, as happens to be true in benzaldehyde and tolualdehydes, the uncertainty of the resulting BDE is perceptibly lower than the uncertainty that would be obtained by manually propagating the uncertainties of the individual enthalpies of formation in quadrature, since the 475 latter summation assumes zero covariances.
Of particular relevance here is the fact that the aldehydic C-H BDEs in meta-and para- .09 ± 1.13 kJ mol -1 , lower by 3.85 ± 1.00 kJ mol -1 than that of metatolualdehyde, 3.76 ± 1.00 kJ mol -1 than that in para-tolualdehyde, and 4.01 ± 0.92 kJ mol -1 than that in benzaldehyde. Therefore, the likely origin of the reactivity differences is simply due to the bond enthalpies in the abstraction of aldehydic H atoms. 490 Based on the discussion above, it appears that the preponderance of evidence is consistent with the abstraction of the aldehydic H atom. However, could such an abstraction reaction start via the addition of NO3 to the ring followed by abstraction? To examine this possibility, we carried out quantum mechanical calculations of the reaction pathways in Reaction (1). 495 Stationary points on the potential energy surface (PES) were optimized with the BH&HLYP density functional and 6-311G(d,p) basis set in Gaussian 16 (Becke, 1993;Frisch et al., 2016), with the exception of the NO3 radical, for which the MP2-D3h geometry was used. This level of theory and empirical treatment of NO3 follows Boyd's computational studies with smaller aldehydes, XCHO (X = H, F, Cl, Me) (Mora-Diez and Boyd, 2002). Single point energies were 500 evaluated at the DLPNO-CCSD(T)/cc-pVTZ level of theory with tight SCF convergence and TightPNO cutoffs in Orca (Neese, 2012;. Relative energetics with this basis set are essentially converged, since the effect of using cc-pVQZ on the activation barrier is less than 1 kJ mol -1 . The calculated PES is shown in Figure   6.  The computed reaction pathways show that the NO3 radical is able to form a dearomatized -adduct (Figure 6, LHS). The most stable of these adducts (by more than 10 kJ mol -1 ) occurs 515 at the para-position. While this complexation is exothermic, due to unfavorable entropic effects it is endergonic by 17.0 kJ mol -1 implying this is a readily reversible process. H-atom transfer first forms a pre-reaction complex with the aldehydic group. This complex can undergo H-atom abstraction to yield the stable products observed in our experiments. Application of the Bigeleisen-Meyer equation to this H-atom transfer transition structure (TS) results in a 520 computationally predicted primary KIE of 2.16 (2.17 with Bell's 1D-tunneling correction) (Bigeleisen and Mayer, 1947;Paton, 2016;Rzepa, 2015). This value is almost identical to the measure KIE of 1.92.
Based on this evidence, we suggest that the reactions of NO3 with aromatic aldehydes lead to the abstraction of the aldehydic H-atom. The cleavage of the aldehydic C-H bond in this step, 525 is consistent with the observation that a weaker BDE value is correlated with a larger reaction rate of Reaction (1). CBS-QB3 calculations (SI) imply 2,4-DMBA and 2,5-DMBA, like O-TA, have weaker aldehydic C-H bonds than the other aromatic aldehydes that lack an orthosubstituent. Since the geometry of the formyl radical is more linear than the aldehyde (the C-C-O angle increases by around 4° upon H-atom abstraction), steric strain relief is likely a 530 contributing factor to the C-H bond weakening by an ortho-methyl substituent. Additionally, charge transfer of 0.22e from BA to NO3 occurs in the computed H-atom transfer TS, consistent with the observation that additional electron-donating substituents, such as methyl groups), There are potential future experiments that could shed light on the proposed reaction 535 pathway. They include: (1) measurement of the temperature dependence of the reaction rate coefficient; (2) investigating the influence of various isotopic substitutions (e.g., OD reaction studies); (3) studying further substitution of the aromatic ring, for example with fluorine; and (4) directly detecting the radical formed in the reaction. Further quantum calculations may also be useful. 540

Atmospheric Implications
Once emitted from biomass burning and from incomplete burning of fuel to the atmosphere, the studied aromatic aldehydes could be removed through their reactions with reactive species such as OH, NO3, and chlorine atoms. The atmospheric lifetimes of the 545 aromatic aldehydes studied in this work have been calculated with respect to the NO3 radical reactions using the rate coefficients, karo, obtained from this work at ambient temperature and pressure, in combination with estimated ambient tropospheric NO3 concentrations, [NO3]= 5 × 10 8 molecules cm -3 , (Atkinson, 1991) following the equation: τNO3 = 1/(kVOC[NO3]). We note that the NO3 radical concentration is highly variable as noted by Brown and Stutz (Brown and 550 Stutz, 2012), and we use this value to illustrate the relative loss rates. The calculated lifetimes are shown in Table 2. Based on our measurements, we expect that the aromatic aldehydes' atmospheric lifetimes with respect to NO3 are 35-280 hours (for the assumed NO3 concentrations). Table 2 also presents the lifetime of these seven aromatic aldehydes with respect to OH radicals (Spivakovsky et al., 2000) of 1 × 10 6 molecules cm -3 (again a rough 555 value characteristic of the mid-tropospheric tropical regions) and Cl atoms (Wingenter et al., 1996) of 1 × 10 4 atoms cm -3 , and the rate coefficients taken from Calvert et al. (2011). It is clear that OH radicals contribute more than NO3 and Cl atoms for the oxidation of the aromatic aldehyde in the atmosphere, with NO3 reactions contributing significantly to its removal at night in the polluted area with high NOx.   (Spivakovsky et al., 2000) , [NO3]=5 × 10 8 molecules cm -3 (Atkinson et al., 1991), and [Cl] = 1 × 10 4 (Wingenter et al., 1996). b the rate coefficients were extracted from Calvert et al. (2011). c recommended values in Table S5. 570 This work also found that the aromatic PAN-type compounds, for example, was the main product formed from the reaction of aromatic aldehydes with NO3 radical. Formation of such compounds enables the transport and release of NOx to the remote troposphere, leading to the production of O3. Such a situation may occur in wildland fire plumes. 575

Data availability
The compiled datasets used to produce each figure within this paper are available as Igor Pro files upon request.

Supplement
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Author contributions
YR and ARR wrote the paper with input from all authors. YR and LZ conducted the experiments 585 and analyzed the data, MM helped with the analysis of the data. MI, VD and SSB were responsible for the CRDS instrument. BR and RSP made the theoretical calculations. AM and ARR designed the experiments and led the study. All coauthors commented on the paper.

Competing interests 590
The authors declare that they have no conflict of interest.