Transition from high- to low-no x control of night-time oxidation in the southeastern US

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1 In the format provided by the authors and unedited. SUPPLEMENTARY INFORMATION DOI: /NGEO2976 Transition from high- to low-no x control of night-time oxidation in the southeastern US P. M. Edwards, K. C. Aikin, W. P. Dube, J. L. Fry, J. B. Gilman, J. A. de Gouw, M. G. Graus, T. F. Hanisco, J. Holloway, G. Hübler, J. Kaiser, F. N. Keutsch, B. M. Lerner, J. A. Neuman, D. D. Parrish, J. Peischl, I. B. Pollack, A. R. Ravishankara, J. M. Roberts, T. B. Ryerson, M. Trainer, P. R. Veres, G. M. Wolfe, C. Warneke and S. S. Brown The main text describes an analysis of the nocturnal oxidative environment observed in the Southeast U.S. residual layer, and its implications for regional biogenic volatile organic compound (BVOC) processing. The following sections provide more detail on these insights and the methods used in this analysis. Table S1 shows the reaction rate constants for each of the three oxidants investigated in this study (O 3, NO 3 and OH) with isoprene, α-pinene and β-pinene. NO 3 (cm 3 molecule -1 s -1 ) O 3 (cm 3 molecule -1 s -1 ) OH (cm 3 molecule -1 s -1 ) Isoprene 6.5 x x x α-pinene 6.2 x x x β-pinene 2.5 x x x Table S1 Reaction rate constants (at 298 K) for the three dominant primary BVOCs in this study (isoprene, α-pinene and β-pinene) with NO 3, O 3 and OH. S1. Observations The chemistry of the residual layer is determined by the composition of the boundary layer at sunset, with only power plant emissions capable of providing any significant emission after stratification. Three night flights were conducted as part of the SENEX project. One of these flights (on July 3 rd 2013), however, was heavily influenced by biomass burning so has been omitted from this analysis of the background state of the Southeast U.S. nocturnal atmosphere. The data presented in the main text is thus from the remaining 2 night flights, approximately 6 hours duration each, on 19 th June and 2 nd July Figure S1 shows the distribution of boundary layer (< 1 km altitude) observations made within 2 hours of sunset on these two flights. NATURE GEOSCIENCE Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 a) b) c) d) e) Figure S1 Distributions of isoprene (a), monoterpene (b), NO 2 (c), O 3 (d) mixing ratios and calculated PNO 3 (e) for observations below 1 km altitude and within 2 hours of sunset during the 19 th June and 2 nd July flights. Note x-axis log scales on NO 2 and PNO 3. The nocturnal residual layer contains the majority of the atmospheric mass that constitutes the daytime mixed layer, and thus the nocturnal chemistry in this layer, isolated from the surface, can have significant impacts on following day mixed layer composition. Wagner et al used aerosol extinction measurements from SENEX and another aircraft project in the southeast US during summer 2013 to estimate daytime mixed and transition layer height ranges of between m above ground level (AGL) and m AGL respectively. A typical residual layer depth during SENEX can be estimated using 15 vertical profiles from the two night flights studied (Fig. S2). These vertical profiles show nocturnal residual layer depths extending from m AGL to m AGL, implying that typically >85 % of the daytime mixed layer mass was located in the residual layer at night for the flights studied here. As shown in the main manuscript, this residual layer chemistry is dependant on the quantity of NO x and BVOC in the daytime mixed layer at the onset of nocturnal stratification. Estimating the fraction of total emitted BVOC available for oxidation in the residual layer is difficult, due largely to uncertainties and variability in local and regional BVOC emissions. However, previous work in the 2

3 northeast U.S. has estimated that approximately 20% of total emitted BVOC remains in the residual layer at sunset and is available for oxidation 2. The regional observational estimate is similar to, but slightly higher than, estimates of the fraction of isoprene nitrates attributable to NO 3 chemistry in continental 3 or global 4 scale models. Figure S2 Potential temperature as a function of altitude (left), color indicates local time since sunset, on July 2 nd flight. Average isoprene (middle) and monoterpene (right) mixing ratios as a function of altitude for July 2 nd flight show these species to be mixed through the residual layer to a height of approximately 1500 m. In the nocturnal boundary layer, below ~200 m, monoterpene mixing ratios show a stronger increase relative to the residual layer than isoprene, as would be expected from their nocturnal emission. Altitude is quoted in km above ground level (AGL), this was estimated by subtracting the altitude above sea level of Smyrna, TN (166 m) from the measured altitude above sea level. A previous analysis of polluted residual layer chemistry in high BVOC conditions in the northeast U.S. 2 observed a strong anti-correlation between observed isoprene and NO 3, which was shown to be due to rapid isoprene oxidation in the presence of NO x. The SENEX nocturnal data show a similar anti-correlation (example shown in Fig. S3a), although the low observed NO 3 concentrations result in the majority of points being at or below the instrument limit of detection. Sufficient monoterpene concentrations during SENEX enable a similar anti-correlation with NO 3 to be observed (example shown in Fig. S3b). The anti-correlation plot for isoprene shows a small number of points with both non-zero isoprene and non-zero NO 3, indicating NO x -containing plumes in which isoprene oxidation via NO 3 was still occurring at the time of sampling. The monoterpene correlation plot shows fewer of these points, likely due to the more rapid oxidation of monoterpenes, which have NO 3 reaction rate coefficients 3-13 times greater than isoprene (table S1). 3

4 a) b) Figure S3 Plots of (a) isoprene and (b) monoterpenes vs. NO 3 in the residual layer from the July 2nd SENEX flight. Similar anti-correlation was seen on other night flights. In addition to the anti-correlation shown in the plots in Fig. S3, NO 3 and O 3 loss mechanisms have been investigated for the SENEX night flights on June 19 th (Fig. S4a and S4b) and July 2 nd (Fig. S4c and S4d) by calculating the NO 3 first-order loss rate, kno 3 (eq. S1), to its observed reaction partners, including both NO 3 -VOC reactions and heterogeneous uptake of N 2 O 5 to aerosol 5.!NO! =!!"!!!"#$!"#! +!!" [!"! ]!!!!!!!! (S1) The k NO3+VOCi are the bimolecular rate constants for reaction of NO 3 with different VOCs 6, K eq is the temperature dependent equilibrium for NO 2, NO 3 and N 2 O 5, and k N2O5 is the first order loss rate constant for N 2 O 5 loss to aerosol calculated from its uptake coefficient 7,8. The latter has been calculated using measured aerosol surface area and an uptake coefficient of γ N2O5 = This value is likely an overestimate due to the high organic content of the aerosol during SENEX 1,9. For comparison the same calculations have also been performed for the other dominant nocturnal oxidant O 3. This analysis shows that reaction with primary biogenics (Σ(isoprene+αpinene+β-pinene)) dominates residual layer NO 3 loss, resulting in the low observed NO 3 and N 2 O 5 concentrations. Previous studies have shown variable mechanisms for NO 3 loss, N 2 O 5 heterogeneous uptake dominant in some regions (e.g. 5 ) and NO 3 loss to BVOCs dominant in others (e.g. 10 ). However, none of the previously published NO 3 budgets from ground, ship or aircraft campaigns show a contribution from BVOC reaction as large as that in Fig. S4. In comparison, nocturnal O 3 loss mechanisms are dominated by reaction with NO 2, producing NO 3, with only 20-25% of the calculated loss being to BVOCs. 4

5 a) Mean Median b) Mean Median 5

6 c) Mean Median d) Mean Median Figure S4 Calculated NO 3 and O 3 loss rates for the 19 th June (a and b respectively) and 2 nd July (c and d respectively) night flights. Pie charts show mean and median loss fractionation for samples after sunset and below 1 km altitude. Photolytic loss of O 3 is estimated to be 10% of O 1 D photolysis channel, as other photolysis channels are not a true loss. Reaction of O 3 with NO prior to sunset is also not included based on the assumption that this reaction is in photo-stationary steady state with O 3 production from NO 2 photolysis. The oxygenated VOC component consists largely of BVOC oxidation products. 6

7 As described above, the use of γ N2O5 = 0.02 means that the heterogeneous loss of N 2 O 5 term is likely overestimated, making the fractional loss of NO 3 to BVOC slightly greater than shown above. The role of unmeasured VOC oxidation products and peroxy radicals 11 as an NO 3 sink could reduce the calculated BVOC loss to NO 3 shown in Fig. S4, however, the modelling work performed in this analysis supports the assumption that NO 3 sinks during SENEX are dominated by the observed primary BVOCs. The rapid NO 3 loss rate, with typical NO 3 lifetimes of a few seconds, means the NO 3 concentration can be considered to be in steady state between production and loss. The high NO 3 loss rate combined with warm temperatures, which favour NO 3 in its equilibrium with N 2 O 5, makes steady state a valid approximation 12. As stated in the main text, the low NO 3 concentrations observed during SENEX do not prevent an observational constraint on the NO 3 steady state concentration, which can be calculated via Eqn. S2. [NO 3 ] SS = PNO 3 /kno 3 (S2) Where [NO 3 ] ss is the steady state NO 3 concentration, kno 3 is the observed total NO 3 loss rate (Eqn. S1 & Fig. S4), and PNO 3 is the NO 3 production rate calculated using observed O 3 and NO 2 concentrations via Eqn. S3. PNO 3 = k NO2+O3 [O 3 ][NO 2 ] (S3) Here, k NO2+O3 is the bimolecular rate constant for this reaction, and the quantities in square brackets are concentrations. The steady state assumption enabled the instantaneous rate of BVOC loss to NO 3 to be calculated (Fig. 2(c) and (d)) as simply the fraction of NO 3 loss toward BVOC multiplied by PNO 3. This assumes all NO 3 losses are known, but the rapid NO 3 loss to reaction with primary BVOCs (Fig. S4) means that the effect of small changes to the other loss routes will be small. S2. Box model calculations of nocturnal BVOC oxidation For residual layer WAS samples on the 19 th June and 2 nd July flights (45 and 54 samples per flight respectively), sunset concentrations of NO 2, O 3, isoprene and monoterpenes were estimated via a set of 1 st -order loss calculations. These estimated values were then used as initial conditions for a box model simulation, initialized 2 hours prior to sunset, run forward in time until the corresponding WAS sample time. The model then compared calculated concentrations of NO 2, O 3, isoprene, α-pinene, β-pinene, methacrolein (MACR) and methyl-vinyl ketone (MVK) with the observations, and iteratively adjusted the initial conditions until calculated and observed concentrations agreed to within 1%. This condition was relaxed for monoterpenes when their observed and calculated concentrations were below the instrument s limit of detection. The observed concentrations of all other constraining species were always above the instrumental limit of detection within the residual layer. The model was also constrained to the observed concentrations of the anthropogenic VOCs and physical parameters listed in table S2. As the reaction of O 3 and NO 3 with the vast majority of the observed non-biogenic VOCs is significantly slower than with the BVOCs 6 the concentrations of these species were maintained at the observation throughout each simulation for simplicity. Once a set of initial 7

8 conditions was found, the model was run forward for 11.5 hours, in order to simulate the residual layer chemistry from 2 hours before sunset through a full 9.5-hour night. Constrained species Fixed Mean value Isoprene (ppt) Sample time 566 Alpha-pinene (ppt) Sample time 6 Beta-pinene (ppt) Sample time 15 NO 2 (ppt) Sample time 923 O 3 (ppb) Sample time 47.5 MVK (ppt) Sample time 748 MACR (ppt) Sample time 403 i-butane (ppt) Throughout run 46 n-butane (ppt) Throughout run 110 Ethane (ppt) Throughout run 1569 Ethene (ppt) Throughout run 112 Ethyne (ppt) Throughout run 130 n-hexane (ppt) Throughout run 13 i-pentane (ppt) Throughout run 78 n-pentane (ppt) Throughout run 48 Propane (ppt) Throughout run 44 Propene (ppt) Throughout run 31 Acetone (ppt) Throughout run 2633 Benzene (ppt) Throughout run 32 Ethanol (ppt) Throughout run 1654 MEK (ppt) Throughout run 357 Methanol (ppt) Throughout run 3637 Pentane_224tm (ppt) Throughout run 10 Toluene (ppt) Throughout run 16 Press (hpa) Throughout run 917 Table S2 Observed species used to constrain model simulations for analysis described in main text (Nocturnal competition between NO 3 and O 3 oxidation). Initial isoprene, α-pinene, β-pinene, methylvinylketon (MVK), methacrolien (MACR), NO 2 and O 3 concentrations were iteratively adjusted to match observed values at WAS sample time at time t after sunset to within 1%. All other VOCs were fixed at the observed value throughout the entire simulation. Average observed mixing ratios across all samples used in this analysis from night flights on 19 th June and 2 nd July are shown in right column. The model contains the full Master Chemical Mechanism v3.3.1 chemistry scheme 13, with photolysis rates calculated using the Tropospheric Ultraviolet and Visible radiation model 14. For the purpose of photolysis rate calculations during the 2-hours prior to sunset used for model spin-up, the location for all simulations was fixed at N, W. Pressure was set for each simulation to the average observed value over the WAS sample time. Available observations of other reactive nitrogen species (ClNO 2 and HNO 3 ) were compared with model predictions but not constrained due to lack of explicit aerosol representation within the model. 8

9 Figure S5 shows example calculated isoprene and monoterpene (α-pinene+β-pinene) concentrations during the 9.5-hour simulated night, together with the cumulative oxidation mechanisms responsible for their removal. These three example WAS samples illustrate low, medium and high NO x regimes. Mixing ratios of NO x, MVK, MACR, O 3, NO 3, OH, and calculated PNO 3 for each of these simulations are show in Fig. S6. The top plots in Fig. S5 show a simulation under lower NO x concentrations (NO 2 = 0.35 ppbv, ~3.3 hours after sunset), typical of the background or slightly polluted air, with PNO 3 of 0.08 ppb hr -1 (sunset value). The subsequent calculated oxidant balance results in integrated 9.5-hour nocturnal isoprene removals of 43, 30 and 27 % via O 3, NO 3 and nocturnal OH respectively. Nocturnal monoterpene oxidation is evenly split between O 3 and NO 3 (47% each), with nocturnal OH accounting for 6 %. Even under these lower NO x conditions, NO 3 reaction is still a significant contributor to BVOC removal. The low NO x, and thus slower oxidation, results in incomplete BVOC oxidation, with 36 and 6% of sunset isoprene and monoterpenes present at dawn, respectively. The nocturnal OH comes predominantly from BVOC ozonolysis reactions, accounting for over 57 % of the total nocturnal OH source, with a further 20 % coming from reactions of HO 2 with other radicals. The middle plots in Fig. S5 show a simulation with NO 2 of approximately 1 ppbv at 1.25 hours after sunset, representative of residual layer air with urban influence. Sunset PNO 3 = 0.15 ppb hr -1 and NO 3 was responsible for 78 % of the integrated nocturnal isoprene loss over the 9.5-hour simulated night, with O 3 and nocturnal OH responsible for 13 and 9%, respectively. Nocturnal OH is produced predominantly through reaction of HO 2 with O 3 and NO 3 in addition to BVOC ozonolysis (30, 16 and 48 %, respectively). Nocturnal HO 2 is produced during the oxidation of BVOCs by all three oxidants, with the largest source (31%) from decomposition of the alkyl radical produced by NO 3 oxidation of isoprene, followed by OH + CO (14 %). Integrated nocturnal monoterpene loss is 15 % via O 3 and 83 % via NO 3, with nocturnal OH accounting for 2 %. Primary BVOC present at sunset was completely removed during the 9.5-hour night. The lower plots in Fig. S5 show a simulation of a high NO x power plant plume (NO 2 = 5.9 ppb, ~3.6 hours after sunset), with sunset PNO 3 of 1.2 ppb hr -1. Calculated plume age (as per 15 ) indicates the sampled plume in this example was emitted into the residual layer near sunset. Nocturnal isoprene oxidation is dominated by NO 3 (89 %), with O 3 and nocturnal OH accounting for 6 and 5 % respectively. Monoterpene oxidation occurs 95 % by NO 3, with 4 % via O 3 and 1% via nocturnal OH. Nocturnal OH comes predominantly from HO 2 + NO 3 (50%) and HO 2 + O 3 (12%), with the remainder largely from BVOC ozonolysis. The HO 2 is predominantly produced from radical decomposition reactions during the NO 3 and O 3 initiated BVOC oxidation, with an additional 10 % coming from OH + CO. The highly oxidative environment modeled in this simulation resulted in BVOC concentrations of 48 pptv for isoprene and below the instrument limit of detection for monoterpenes at the point of sampling, meaning initial BVOC mixing ratios used are more uncertain than in the previous examples. This does not significantly change the calculated oxidative pathways of the BVOCs. 9

10 OH O 3 NO 3 Background Background Isoprene (ppbv) Isoprene (ppbv) Time since sunset (hours) Urban Monoterpenes (pptv) Monoterpenes (pptv) Time since sunset (hours) Urban Time since sunset (hours) Time since sunset (hours) Isoprene (ppbv) Power plant Monoterpenes (pptv) Power plant Time since sunset (hours) Time since sunset (hours) Figure S5 Example model BVOC oxidation for background air NO 2obs = 0.35 ppbv (WAS sample 31, ) (Top); Urban air NO 2obs = 1.0 ppbv (WAS sample 12, , downwind of Birmingham, AL) (Middle); Power plant plume NO 2obs = 5.9 ppbv (WAS sample 38, ) (Bottom). Green line shows model calculated BVOC (isoprene left plots, sum-monoterpenes right plots), with black dot indicating WAS BVOC and time since sunset. The shaded regions show the integrated loss of BVOC to NO 3 (blue), O 3 (red) and nocturnal OH (grey) from sunset through a 9.5-hour night. 10

11 The stratified nature of the nocturnal atmosphere means that deposition and dilution effects in the residual layer are assumed to be minimal 16. For this reason, and the lack of additional constraints on mixing rates, the simulations described in this work do not include a dilution or depositional term. The effect of this approximation has been investigated with a set of simulations performed that include a 1 st -order loss rate for all calculated species equivalent to a 24-hour lifetime with respect to physical removal. As expected, the inclusion of a physical loss mechanism results in a reduction in the total amount of BVOC lost via reaction with each of the nocturnal oxidants, due to competition from the physical loss process, and O 3 concentrations also show a noticeable decrease over the simulated night (examples shown in Fig. S6). Calculated NO 3 and OH concentrations generally show a decrease with the inclusion of physical loss, except in the high NO x case where NO 3 mixing ratios show a slight increase due to reductions in the size of the NO 3 sink to BVOC despite a reduction in PNO 3. Despite these differences, the inclusion of this physical loss term does not significantly alter the overall relative importance of the 3 nocturnal oxidants with respect to each other, and thus we feel that the approximation used is valid in the absence of any further constraint. 11

12 12 Figure S6 Model output NO3, N2O5 and OH concentrations (left), PNO3, O3 and NO2 concentrations (centre) and Isoprene, (methylvinylketone+methacrolien) and monoterpene concentrations (right) for the same example observational time points shown in Fig. S5. Background air (WAS sample 31, ) (top); Urban air (WAS sample 12, , downwind of Birmingham, AL) (middle); Power plant plume (WAS sample 38, ) (bottom). Solid lines are from simulations with no physical removal of species (as per simulations discussed in main text), and dashed lines show model simulations with the addition of a 24-hour lifetime for all species with respect to physical removal. Plots show 2 hours prior to sunset used for model spin-up and 9.5-hour simulated night

13 The integrated model analysis was applied to all WAS samples and supports the conclusion that oxidation of primary BVOCs in the SENEX residual layer occurred predominantly through NO 3 (58 %) and O 3 (28 %), with a lesser contribution from nocturnal OH (14%). Nocturnal oxidation was responsible for removal of >65 % of primary BVOCs present at sunset in all simulations over the 9.5 hour night, with 32 of the 99 simulations showing nocturnal oxidation of >95 % of the sunset primary BVOC. The top plots in Fig. S7 show the mass of BVOC oxidized during the model simulated 9.5-hour nights compared with the mass of BVOC present in the simulated air mass at sunset (for each WAS sample from each night flight). The pie charts show the fractions of the oxidized mass attributable to isoprene and monoterpene removal by either NO 3, O 3 or nocturnal OH. The lower plots in Fig. S7 show the same but with the inclusion of the observed oxidation products MVK and MACR. The MVK and MACR in the model are constrained in the same way as the primary BVOCs, using the model to iteratively determine the initial concentration that reproduces the observed concentrations to within 1% at the sample time (examples shown in Fig. S6). This increases the amount of BVOC present in the simulation at sunset, representing the major products of photochemical isoprene oxidation. The relative reaction rates of NO 3 with MVK and MACR vs. primary BVOCs, compared to those for O 3 and OH result in a reduction in the contribution of NO 3 oxidation to total biogenic mass oxidation by approximately 10% when MVK and MACR are included, and an increase in the amount of BVOC present after the simulated night. The MVK and MACR that is nocturnally oxidized in the model comes from a combination of concentrations present in the initial air mass that forms the residual layer and secondary production from the OH oxidation of isoprene in the modeled 2 hours prior to sunset. Only the fraction of MVK and MACR present at sunset is included in the mass calculation in order to avoid counting the nocturnally oxidized isoprene mass twice. 13

14 a) b) c) d) Figure S7 a) (061913) and b) (070213) are histograms of model calculated mass of primary BVOC (isoprene + monoterpenes) oxidized over the 9.5 hours from sunset for each WAS sample simulated (grey), and mass of primary BVOC present in simulations at sunset (green). c) (061913) and d) (070213) are histograms of model calculated mass of primary BVOC + methylvinylketone (MVK) + methacrolien (MACR) oxidized over the 9.5 hours from sunset for each WAS sample simulated (grey), and mass present in simulations at sunset (green). The pie charts show the mean mass fractions attributable to each of the oxidants (see legends). S3. Investigating the high-to-low NOx transition in overnight BVOC oxidation As described in the main text, the oxidative fate of BVOCs in the residual layer is determined by the NO 2 /BVOC ratio. Figure 3(a) in the main text illustrates the agreement between model calculated loss fractions and those calculated using the observed BVOC, O 3 and NO 2 mixing ratios. In these model simulations BVOC and O 3 concentrations were fixed at values representative of the SENEX residual layer (isoprene 0.5 ppbv, monoterpene 0.03 ppbv, O 3 40 ppbv). Each model was initialized with a different, fixed NO 2 across the range ppbv. The models were then run for an arbitrary time period to simulate instantaneous oxidation rates. The lack of observational constraints on the magnitude of nocturnal OH means its impact cannot be included in these calculations. Thus, the NO 3 and O 3 loss fractions calculated using the observations show a high bias. For the analysis shown in Fig. 3(a) the role of nocturnal OH was estimated by applying the model calculated BVOC loss to OH fraction at a given NO 2 /BVOC ratio (grey line in Fig. 3(a)) to the BVOC losses calculated from the observations. Figure S8 shows the modelled and uncorrected BVOC loss fractions to NO 3 and O 3, with the apparent high bias of the observations caused by the lack of inclusion of nocturnal OH. 14

15 Figure S8 Model calculated fraction of BVOC loss to NO 3 (blue), O 3 (red) and OH (grey) against model [NO x ]/[BVOC] ratio (lines). Fraction of BVOC loss to NO 3 (blue) and O 3 (red) calculated from observations of O 3, BVOC and NO 2 from both night flights against observed [NO x ]/[BVOC] ratio. The role of nocturnal OH on the observed BVOC loss fractions not been included in this plot (in contrast to Fig. 3(a)), resulting in the high bias of the observed fractions compared to the model. The relative importance of NO 3, O 3 and OH in the nocturnal removal of BVOCs depends on both the abundance of these radicals and the BVOC speciation. This dependence on the BVOC is due to both the relative reaction rates of different BVOCs with the oxidants and, more importantly, the relative reaction rates of O 3 with the BVOCs mixture and O 3 with NO 2 to produce NO 3. Isoprene is the dominant BVOC globally, and typically accounted for >90 % of the sunset BVOC mixture in the southeast US during SENEX (Fig. S1). This isoprene dominance is not the case in all locations, however, with the relative emission of isoprene to monoterpenes depending on tree speciation 17. Figure S9 shows the results from 3 sets of model simulations, similar to those shown in Fig. 3a and Fig. S8, but with single component BVOCs in order to quantify the potential impact of BVOC speciation on the high-tolow NO x transition in NO x /BVOC space. This shows that the largest change is seen when the BVOC is emitted as α-pinene, shifting the point at which NO 3 out competes O 3 as the dominant oxidant to a NO 2 /BVOC ratio of ~ 3, from the ~0.5 found for an isoprene dominated system. 15

16 Figure S9 Model calculated fraction of BVOC loss to NO 3 (blue), O 3 (red) and OH (grey) against model [NO x ]/[BVOC] ratio for isoprene, α-pinene, β-pinene and the average BVOC mixture observed in the residual layer during SENEX. The dependence of nocturnal oxidation to the BVOC speciation shown in Fig.S9 has implications for regions outside the Southeast U.S. Isoprene dominated regions of the world with currently increasing NO x emissions 18, such as Southeast Asia and certain regions of India 19, will likely follow a similar response curve to that presented in this work for the Southeast U.S., albeit in the opposite direction. Some regions, such as the Pearl River delta in China, that show increasing NO x and large emissions of anthropogenic VOCs, also have significant daytime regional isoprene concentrations 20. As it is the sunset mixed layer that determines the composition of the residual layer, and reactions of NO 3 and O 3 with isoprene are generally significantly faster than with most anthropogenic VOCs, these regions may also follow similar nocturnal NO x response to that shown here. Regions where monoterpenes are the dominant VOC, such as Northern Europe 21, however, will likely follow curves similar to those for the monoterpenes in Fig. S9. The plots in Fig. 3(b) and (c) show results from a separate set of 101 box model simulations used to illustrate the high and low NO x nocturnal BVOC oxidation regimes. These simulations are used to simulate BVOC oxidation over an entire 9.5 hour night and initialize with isoprene, monoterpene and O 3 mixing ratios of 710 ppt, 37 ppt and 40 ppb respectively, equal to the average observed SENEX sunset mixing ratios below 2 km. Initial NO 2 mixing ratios were varied between 0 10 ppb in 0.1 ppb increments to provide a range of NO x /BVOC between 0 and 11. The MCM chemistry scheme used provides a near-explicit description of the gas-phase BVOC oxidation, but does not contain any heterogeneous processes. Although the 16

17 partitioning of the organic nitrates produced by the chemistry scheme to the aerosol phase is uncertain, the heterogeneous uptake of N 2 O 5 is well established. In order to represent the heterogeneous uptake of N 2 O 5 in the model, the simulations use a firstorder loss rate coefficient of k N2O5 = 10-4 s -1 for the reaction N 2 O 5 2HNO 3 and does not include production of ClNO 2. The k N2O5 is arbitrary but approximately equivalent to an uptake coefficient, γ(n 2 O 5 ) = 0.01 and an aerosol surface area of 200 µm 2, consistent with measured aerosol surface area during the July 2 SENEX flight and previous determinations of N 2 O 5 uptake coefficients 22. The N 2 O 5 loss rate coefficient was varied to determine the model sensitivity, and it was found that the transition between organic and inorganic dominated regimes is insensitive to a factor of 3 increase or decrease in this rate constant. The products of the inorganic chemistry (N 2 O 5, ClNO 2 and HNO 3 ) are, however, very sensitive to the uptake coefficient and the ClNO 2 yield. Therefore, the inorganic nitrate (sum of HNO N 2 O 5 ) shown represents the potential for inorganic nighttime chemistry relative to BVOC oxidation. S4. Comparison between daytime research flights over Atlanta As described in the main text, research flights of the NOAA P-3 during the 1999 Southern Oxidant Studies (SOS) provide a comparison to the 2013 study. Figure S10 shows daytime research flights over Atlanta, GA in July 1999 and June Daytime flights are compared due to the absence of nighttime flights over urban areas in the 1999 data. The comparison is instructive for nighttime chemistry because the composition of the nighttime residual layer at sunset is similar to that of the late afternoon convective boundary layer. The time series plots on the left show NO x and isoprene mixing ratios over Atlanta in 1999 and 2013 on the same scale, while the maps in the center show P-3 flight tracks color and size coded by NO x mixing ratio. Average mixing ratios of isoprene were approximately a factor of 2 lower on the July 1999 flight, although temperature and sunlight were similar on the two days. Isoprene emissions depend on temperature and sunlight. The average temperature was 0.9 C larger below 1 km altitude in the 2013 flight, and the measured NO 2 photolysis rate constant, j(no 2 ), a proxy for sunlight intensity, was 35% greater in the 2013 flight. The difference in the j(no 2 ) is within combined measurement uncertainty between the two flights. Mixing ratios of NO x were significantly different between the two flights, accounting for the largest difference in the ratio of NO x /isoprene. The right plot shows the distribution of the NO x to isoprene ratio, which determines the transition from high to low NO x nighttime chemistry. The 2013 data have median NO x /isoprene near 0.6, slightly above the value of 0.5 that represents the transition, in accord with the analysis of data from the night flights. The 1999 data shows median NO x /isoprene of approximately 10. At sunset on this day in 1999, the nighttime chemistry would have been strongly NO x dominated, and the isoprene lifetime in the residual layer much shorter than the duration of a single night. Nighttime oxidation would have been NO x dominated even at the 10 th percentile of the NO x /Isoprene ration in the 1999 example. The difference in NO x /isoprene ratio is considerably larger than that inferred from changes in NO x emissions alone during the 14 years between these flights. Part of the difference may arise from differences in meteorology between these two particular days. Part of the difference may also be indicative of more rapid photochemical isoprene oxidation rates due to OH recycling reactions at higher NO x, which would serve to suppress isoprene and enhance the NO x /isoprene ratio. 17

18 Figure S10 Time series of NO x and isoprene for flights in July, 1999 (a) and June 2013 (b) over Atlanta, GA isoprene measurements were from analysis of canister samples alone, while 2013 data for canister samples and higher resolution PTRMS measurements are both shown. Note that the left scale is the same for both flights, and the NO x and isoprene are shown on the same scale. Maps showing the P-3 flight tracks on these days (c and d), color and size coded by NO x on the same scale. Distribution of the NO x to isoprene ratio from the two flights using isoprene data for the canister samples only (e). Bars, boxes and whiskers represent median, 25 th and 75 th percentiles, and 10 th and 90 th percentiles, respectively. 18

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