Anthropogenic enhancements to production of highlyoxygenated molecules from autoxidationHavala O. T. Pyea,b,1, Emma L. D’Ambrob,c, Ben H. Leeb, Siegfried Schobesbergerb,d, Masayuki Takeuchia,2, Yue Zhaob,3,Felipe Lopez-Hilfikerb, Jiumeng Liue,4, John E. Shillinge, Jia Xinga,5, Rohit Mathura, Ann M. Middlebrookf, Jin Liaof,g,6,7,André Weltif,g,8, Martin Grausf,g,9, Carsten Warnekef,g, Joost A. de Gouwf,g,10, John S. Hollowayf,g, Thomas B. Ryersonf,Ilana B. Pollackf,g,11, and Joel A. Thorntonb,c,1

aNational Exposure Research Laboratory, Environmental Protection Agency, Research Triangle Park, NC 27711; bDepartment of Atmospheric Science,University of Washington, Seattle, WA 98195; cDepartment of Chemistry, University of Washington, Seattle, WA 98195; dDepartment of Applied Physics,University of Eastern Finland, 70211 Kuopio, Finland; eAtmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland,WA 99352; fChemical Sciences Division, National Oceanic and Atmospheric Administration Earth System Research Laboratory, Boulder, CO 80305;and gCooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309

Edited by Frank N. Keutsch, Harvard University, and accepted by Editorial Board Member A. R. Ravishankara February 14, 2019 (received for review June22, 2018)

Atmospheric oxidation of natural and anthropogenic volatile organiccompounds (VOCs) leads to secondary organic aerosol (SOA), whichconstitutes a major and often dominant component of atmosphericfine particulate matter (PM2.5). Recent work demonstrates that rapidautoxidation of organic peroxy radicals (RO2) formed during VOCoxidation results in highly oxygenated organic molecules (HOM) thatefficiently form SOA. As NOx emissions decrease, the chemical re-gime of the atmosphere changes to one in which RO2 autoxidationbecomes increasingly important, potentially increasing PM2.5, whileoxidant availability driving RO2 formation rates simultaneously de-clines, possibly slowing regional PM2.5 formation. Using a suite of insitu aircraft observations and laboratory studies of HOM, togetherwith a detailed molecular mechanism, we show that although autox-idation in an archetypal biogenic VOC system becomes more compet-itive as NOx decreases, absolute HOM production rates decrease dueto oxidant reductions, leading to an overall positive coupling be-tween anthropogenic NOx and localized biogenic SOA from autoxi-dation. This effect is observed in the Atlanta, Georgia, urban plumewhere HOM is enhanced in the presence of elevated NO, and predic-tions for Guangzhou, China, where increasing HOM-RO2 productioncoincides with increases in NO from 1990 to 2010. These results sug-gest added benefits to PM2.5 abatement strategies come with NOx

emission reductions and have implications for aerosol–climate inter-actions due to changes in global SOA resulting from NOx interactionssince the preindustrial era.

autoxidation | SOA | monoterpenes | PM2.5 | aerosol

Ambient particulate matter is responsible for a significantfraction of the global burden of disease (1) and affects cli-

mate (2). The organic portion of ambient aerosol forms largelydue to the condensation of low vapor pressure (3) or highlysoluble (4) gases stemming from atmospheric oxidation of vol-atile organic compounds (VOCs). During summer, emissions ofbiogenic VOCs exceed those from anthropogenic activities (5)and monoterpenes are a major class of biogenic VOCs emittedthroughout the year (6). Oxidation of monoterpenes results insignificant formation of particle mass (7) and is a major source ofsecondary organic aerosol (SOA) in the southeastern UnitedStates (8, 9). While monoterpenes are included as an SOAsource in most chemical transport models, many parameteriza-tions (10–12) lack a mechanistic dependence of monoterpeneSOA on NOx and oxidant identity (e.g., OH vs. ozone), and alllack unimolecular RO2 reactions. This limits the ability ofmodels to predict how changes in emissions will affect ambientconcentrations and new particle formation events.Building a mechanistic understanding of SOA formation path-

ways is critical for being able to determine historic and futuredrivers of ambient particulate matter, particularly as the abun-dance of species, such as nitrogen oxides (NOx =NO + NO2), that

Significance

Government organizations set standards for permissible levels ofatmospheric particulate matter (PM) due to its adverse effects onhuman health and the environment. Unimolecular reactions thatefficiently produce organic PM are suppressed by NOx, allowingfor potential increases in PM mass due to controls on anthro-pogenic NOx. Using laboratory experiments and observations ofthe atmosphere, we assemble a conceptual understanding ofhow PM from unimolecular organic reactions is affected by NOx.We demonstrate that the NOx penalty on PM yield can be offsetby reductions in oxidant abundance. As a result, PM fromunimolecular reactions is predicted to decrease as NOx is con-trolled, consistent with declines in ambient organic aerosol ob-served in the United States between 1990 and today.

Author contributions: H.O.T.P. and J.A.T. designed research; H.O.T.P., E.L.D., B.H.L., S.S.,F.L.-H., J. Liu, J.E.S., A.M.M., J. Liao, A.W., M.G., C.W., J.A.d.G., J.S.H., T.B.R., I.B.P., andJ.A.T. performed research; H.O.T.P., J.X., and R.M. contributed new reagents/analytictools; H.O.T.P., E.L.D., B.H.L., M.T., Y.Z., and J.A.T. analyzed data; and H.O.T.P. andJ.A.T. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission. F.N.K. is a guest editor invited by the Editorial Board.

Published under the PNAS license.

Data deposition: The SOAFFEE data, SENEX HOM data, and CMAQ output have beendeposited in the Environmental Protection Agency Science Hub repository (https://catalog.data.gov/harvest/about/epa-sciencehub, DOI: 10.23719/1502525).1To whom correspondence may be addressed. Email: Pye.Havala@epa.gov or thornton@atmos.washington.edu.

2Present address: School of Civil and Environmental Engineering, Georgia Institute ofTechnology, Atlanta, GA 30332.

3Present address: School of Environmental Science and Engineering, Shanghai Jiao TongUniversity, 200240 Shanghai, China.

4Present address: School of Environment, Harbin Institute of Technology, Harbin, 150001Heilongjiang, China.

5Present address: School of Environment, Tsinghua University, 100084 Beijing, China.6Present address: Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard SpaceFlight Center, Greenbelt, MD 20771.

7Present address: Universities Space Research Association, Goddard Earth Sciences Tech-nology and Research, Columbia, MD 21046.

8Present address: Atmospheric Composition Research, Finnish Meteorological Institute,FI-00101 Helsinki, Finland.

9Present address: Department of Atmospheric and Cryospheric Sciences, University ofInnsbruck, A-6020 Innsbruck, Austria.

10Present address: Cooperative Institute for Research in Environmental Sciences and De-partment of Chemistry, University of Colorado, Boulder, CO 80309.

11Present address: Department of Atmospheric Science, Colorado State University, FortCollins, CO 80523.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1810774116/-/DCSupplemental.

Published online March 18, 2019.

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modulate the efficiency of aerosol formation (8, 13) continues tochange. For biogenic VOCs such as isoprene and monoterpenes, amechanistic description of oxidation can inform what is considered anatural source of particle mass and what is anthropogenically con-trolled. Furthermore, the extrapolation of laboratory results to long-term organic aerosol trends may not be reliable without a mecha-nistic understanding of organic aerosol pathways. This issue wasrecently illustrated for the isoprene system in which a robust,mechanistic representation of SOA based on later generation iso-prene epoxydiols resulted in a significantly different magnitudeof (11, 14) and trend in (15) isoprene SOA compared with onepredicted using an older empirical parameterization without mech-anistic underpinnings. Although new information on the monoter-pene aerosol system is emerging, it has not yet been incorporatedinto a model-ready mechanism to allow for similar analyses.RO2 autoxidation, a unimolecular reaction involving one or

more hydrogen shifts and subsequent addition of molecular ox-ygen, occurs for many hydrocarbon–oxidant systems includingOH- and ozone-initiated reactions of VOCs and oxidized VOCsof biogenic and anthropogenic origin (16–22). These intra-molecular reactions can proceed rapidly (in seconds), competingwith or even outpacing bimolecular reactions with NO or HO2(17). Autoxidation can result in highly oxygenated molecules(HOM; O:C ≥ 0.7) after only one reaction with OH or ozone(23). HOM often have low saturation vapor concentrations (C*)and can be classified as extremely low volatility organic com-pounds (ELVOC; C* < 3 × 10−4 μg m−3) or low volatility organiccompounds (LVOC; 3 × 10−4 < C* < 0.3 μg m−3) (24). ELVOCcan contribute significantly to new particle formation, andELVOC, LVOC, and semivolatile organic compounds (SVOC)can contribute to particle mass growth (25, 26).Initial estimates of HOM from α-pinene monoterpene oxida-

tion indicated that ozonolysis produced these species in greateryield (1.7–6.8% by mole) than oxidation by OH (yields of 0.22–0.88% by mole) (27). More recently, reagent ion-dependent de-tection efficiencies of HOM were recognized and the HOM yieldfrom α-pinene + OH oxidation is now estimated to be ≥2.4% bymole (20). The resulting role of autoxidation in the α-pinene +OHsystem in producing SOA mass could be significant as mono-terpene reaction with OH is estimated to account for just overhalf of the daytime monoterpene fate in the southeastern USatmosphere (13), and α-pinene is the most abundantly emittedmonoterpene (6, 10). However, NOx is expected to suppressmonoterpene SOA by inhibiting autoxidation (17).This work reveals that representing autoxidation in models is

key to predicting correct amounts of anthropogenic controls onmonoterpene SOA. To demonstrate this, a molecular mechanism-based analysis of the coupling between autoxidation, OH abundance,and NOx was used to describe how NOx influences prompt HOMformation in the α-pinene + OH system (Materials and Methods).Recent laboratory fast-flow reactor and environmental simulationchamber experiments were used to develop and constrain the de-tailed molecular mechanism of autoxidation pathways. Insights fromthe mechanism-laboratory comparisons were used in conjunctionwith aircraft observations and the Community Multiscale Air Quality(CMAQ) chemical transport model to understand the role of au-toxidation and potential for SOA in different chemical environmentsacross the Northern Hemisphere from 1990 to the present day.

Results and DiscussionMechanistic HOM-RO2 Formation and Yields. A critical parameter inthe description of HOM formation is the autoxidation rateconstant, kautox, which sets the rate at which α-pinene + OH-derived RO2 (C10H17O3) are converted to HOM−RO2 (peroxyradicals with O:C ≥ 0.7). This rate constant also determines howcompetitive autoxidation is with traditional bimolecular RO2fates such as reaction with NO or HO2 that do not produceHOM as efficiently. Autoxidation in the α-pinene + OH system,based largely on the work of Berndt et al. (20), is illustrated inFig. 1. Reaction of α-pinene, at a rate governed by OH oxidantabundance, results in several C10H17O3 peroxy radical isomers.

One of the initial C10H17O3 isomers can undergo autoxidation toC10H17O5 peroxy radicals. These peroxy radicals can undergo anadditional autoxidation step, also dictated by kautox, to produceC10H17O7 HOM-RO2. When HOM-RO2 react with HO2, C10H18O7LVOCs are formed and partition almost entirely toward theparticle resulting in SOA. At each opportunity for autoxidation,reaction with NO or other peroxy radicals can divert mass awayfrom the path to HOM-RO2 formation. Simulations of flow tubeexperiments at short residence times (7.9 s) (20) with the mech-anism developed in this work resulted in predicted HOM-RO2 yieldsconsistent with observations for a kautox of 0.28 s−1 (20). Specifically,box model simulations using this recommended value resulted in apredicted HOM-RO2 yield at 7.9 s of reaction time of 3.3% (Table 1and SI Appendix, Fig. S3), consistent with the observed lower-boundyield of 2.4% (20). As the observed yield is a lower bound, limitedby the ability to detect HOM, a higher, but not lower kautox thanthe default may also be possible (28) (SI Appendix, Table S7).The yield of HOM-RO2 (assuming pseudo steady state for

each peroxy radical) in the absence of bimolecular RO2 reactionswas predicted to be 11%, significantly higher than the few per-cent observed and predicted at short reaction times. The fractionof α-pinene + OH-derived RO2 that undergo autoxidation toproduce HOM-RO2, YHOM-RO2, can be expressed as:

YHOM−RO2 = f�

kautoxkautox + kRO2+NO½NO�+ kRO2+HO2½HO2 +RO2�

�2

[1]

O•O OH

α-pinene

OO

OO

OH

OH

HO

OHO OH

OO•

•OOO

OO

OH

OH

OO

OO

OH

OH

HO

+OH

autoxidation +HO2+RO2+NOor self-terminatingreactions

C10H17O5•

C10H17O3•

autoxidation +HO2+RO2+NOor self-terminatingreactions

+HO2

C10H17O7•

+RO2+NO

C10H18O7gas

particle

otherproducts

C10H18O7

Fig. 1. Schematic of major α-pinene + OH autoxidation pathways leading toHOM SOA. Only one isomer is shown even if many are possible. See SI Ap-pendix, Figs. S1 and S2 and Tables S1–S4 for a complete representation ofthe mechanism developed in this work.

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for steady state conditions and is determined by the assumedfraction of OH-initiated peroxy radicals able to undergo intra-molecular H-abstraction reactions that do not self-terminate ( f =11%) (20, 29) as well as the competition with bimolecular re-action (dictated by the autoxidation rate constant). The maxi-mum molar yield of HOM-RO2 from α-pinene + OH (11%; Fig.2A and SI Appendix, Fig. S1 and Eq. S1) corresponds to a massyield of 20% due to the addition of ∼7 oxygens, and the factor of1.8 increase in molecular weight of C10H18O7 compared withα-pinene (C10H16). As a result, autoxidation induced by OH re-action with α-pinene is expected to be important for particlemass formation.Inclusion of autoxidation in the chemical mechanism used to

describe α-pinene oxidation allowed for vastly improved predic-tions of SOA mass, composition, and volatility in comparison withthat observed during chamber studies of α-pinene + OH at lowNOx from the Secondary Organic Aerosol From Forest EmissionsExperiment (SOAFFEE; Table 1). Without HOM-RO2 fromautoxidation, an explicit mechanism based on the standard masterchemical mechanism (MCM) (30) did not predict meaningfulamounts of SOA (mass yield <1%), consistent with previous at-tempts to use explicit, compound-specific representations of SOAfor this system (31). In addition, the SOA that did form based onthe standard MCM showed a lower degree of functionalization (O:C = 0.59) and significantly higher volatility (C* of 104 μg m−3) thanthat observed during SOAFFEE [O:C of 0.68 and effective C* ofthe peak in thermogram signal corresponding to ∼0.08 μg m−3 withan order of magnitude uncertainty (32)] (Table 1). The em-pirical SOA parameterization used in the current (v5.2) CMAQregional chemical transport model showed similar shortcomings interms of underestimated yields, low degree of functionalization,and high SOA volatility. This high volatility in CMAQ algorithms

implies that monoterpene SOA in models is sensitive to primaryorganic aerosol emissions via absorptive partitioning, and the lowvolatility of the observed aerosol implies monoterpene SOA is notstrongly influenced by semivolatile partitioning.Results from the updated mechanism indicated that prompt

SOA formation from the α-pinene +OH system, at or above 285 K,is essentially entirely dependent upon autoxidation. The au-toxidation mechanism developed here predicted an SOA yield of17% consistent with the observed yield (12 ± 4.6%) and used aninternally consistent representation of molecular compositionwith lower volatility (mass-weighted C* ∼1 μg m−3), which hasnot typically been achieved with explicit chemical mechanisms(30, 33–37). Furthermore, both observations and the updatedmechanism showed significant amounts of species with 10 carbonbackbones and high O:C (0.5–0.7) in the particle (SI Appendix,Fig. S5). Although sufficient time for autoxidation existed in the3.6-h chamber residence time of SOAFFEE, predicted HOM-RO2 yields were 8% (Fig. 2A) instead of the theoretical maxi-mum of 11% due to the presence of elevated levels of HO2radicals (∼100 ppt) that competed with autoxidation throughbimolecular reactions. Even if a lower bound on autoxidationyield was assumed (f = 2.65% and kautox = 2.8 s−1; SI Appendix,Table S7), the autoxidation mechanism still accounted for 29%(range: 21–47%) of the SOA formation during SOAFFEE, fur-ther indicating a substantial role for autoxidation.

Atmospheric Implications. As α-pinene is a major component ofmonoterpene emissions in midlatitude regions (6), and of SOAeven in isoprene dominated regions (8), this system is a proxy forthe behavior of a large fraction of biogenic SOA and potentiallyfor other systems that can undergo autoxidation. The ability ofthe detailed autoxidation mechanism developed here to re-produce several features of α-pinene + OH system observed inthe laboratory broadly supports the combination of mechanisticparameters and allows for an assessment of autoxidation-derivedSOA responses to anthropogenic perturbations on local andregional scales. Here, we demonstrate that autoxidation resultsin aerosol that is enhanced in the presence of NOx, is prevalentin the current ambient atmosphere, and produces aerosol thatis mitigated by controls on anthropogenic NOx.HOM formation in the presence of NOx. Production rates of HOM-RO2 scale with the yield of HOM-RO2 (Eq. 1) and the avail-ability of oxidants, allowing for HOM concentrations to increasewith increasing NOx. Given that HOM are formed promptly atsignificant yields from the reaction of α-pinene + OH, the pro-duction rate of autoxidation-derived HOM, and therefore SOA,can be approximated on short timescales (hours) as:

PHOM−RO2 = kα−pinene+OH ½α− pinene�½OH�YHOM−RO2. [2]

Based on the mechanism and combination of kautox and f evaluatedin this work, we find that although elevated NOx monotonically

Table 1. Predicted and observed properties of the α-pinene OH-initiated system

YHOM-RO2 YSOA SOA O:C SOA nC SOA C*

% by mole % by mass mol mol−1 - μg m−3

MCM v3.3.1 0 <1 0.59 8 104Regional CTM 0 7 0.52 8 55HOM mechanism 3.3 17 0.64 10 1.3Observed ≥2.4 12 ± 4.6 0.68 9 0.08

Gas-phase HOM-RO2 molar yield (YHOM−RO2) at 7.9s for Berndt et al. (20)flow tube conditions as well as SOA mass yield (YSOA), mass-weighted meanmolar O:C, mass-weighted mean number of carbons per molecule (nC), andmass-weighted effective C* of SOA for OH-dominated SOAFFEE laboratorychamber conditions. Regional chemical transport model (CTM) predictionsare based on CMAQv5.2 and OH and O3 oxidation of monoterpenes (11) atthe SOAFFEE-predicted loading of 7 μg m−3.

101 102 103 104

NO [ppt]

0.00

0.02

0.04

0.06

0.08

0.10

0.12

YH

OM

-RO 2 [f

ract

ion]

SOAS CTR day

SOAS CTR AMSOAFFEE

Atlanta plume

5 ppt HO 2

30 ppt HO 2

100 ppt HO 2

101 102 103 104

NO [ppt]

0.00

0.02

0.04

0.06

0.08

0.10

PH

OM

-RO 2 [p

pb h

-1];

YH

OM

-RO 2 [f

ract

ion]

0

2

4

6

8

OH

[mol

ec c

m-3

]

106

PHOM-RO2

YHOM-RO2

OHkautox

10 kautox

A B

Fig. 2. (A) Molar yield of C10H17O7 HOM-RO2 (YHOM−RO2)from α-pinene + OH predicted by the updated explicitmechanism under laboratory (SOAFFEE) and atmo-spherically relevant conditions. Stars (A) representconditions from the 2013 southern oxidant and aero-sol study during the daytime and morning (AM)transition at Centreville, Alabama. The Atlanta plumeNO level is based on SENEX aircraft observations (Fig.3). (B) The YHOM−RO2 (black) and production rate ofHOM-RO2 (PHOM−RO2) (red) for the OH concentrationshown in blue following Thornton et al. (52) methods.Yields and production rates in B are shown for defaultmechanism parameters (kautox = 0.28 s−1, f = 11%,solid) and parameters corresponding to a lower boundyield (kautox = 2.8 s−1, f = 2.7%, dashed).

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suppresses the relative importance of autoxidation as an RO2 fate(Fig. 2A and Eq. 1) as expected (17), the well-known nonlineareffect of NOx on OH radical concentrations (38) offsets this sup-pression and leads to enhancements in the absolute formation rateof HOM (Fig. 2B and Eq. 2) as NOx increases. These enhance-ments in absolute HOM-RO2 production will translate into local-ized enhancements in biogenic SOA due to the low volatility ofresulting products and role of autoxidation in SOA formationdemonstrated in this work. The nonlinear response of OH concen-trations to increasing NOx does lead to a turnover point (∼1 ppb ofNO in Fig. 2B), above which, further increases of NO suppress OHand therefore also local HOM production. However, unless NOx isvery high (>5 ppb in Fig. 2B), the production of HOM-RO2 isfacilitated by NOx in monoterpene-rich regions. The NO concen-tration at which HOM production peaks is different from thatwhere OH concentrations peak and is ultimately a function ofthe autoxidation rate constant with higher kautox values (such askautox = 2.8 s−1) resulting in a yield penalty from NOx emissionreductions (Eq. 1) that is easier to overcome and peak HOM-RO2production at an NO level closer to that of peak OH production.Observations in the Atlanta urban plume (Fig. 3) illustrate

facilitation of HOM in the presence of elevated NO. Gas-phaseconcentrations of C10H18O7 and other HOM compounds(C10H14,16,18O7–8 and C10H15,17NO8–9; see SI Appendix, Fig.S11), likely from monoterpene autoxidation (Fig. 1), were en-hanced over regional background concentrations by a factor of 3to 5 in the urban plume from Atlanta, Georgia, during thesummer of 2013. This enhancement in HOM coincided with adepletion of monoterpenes and occurred in the presence of NO.The NO levels in the Atlanta plume (100–300 ppt) were not sohigh as to shut down autoxidation (Fig. 2A), and enhanced ozonein the plume suggests that the primary OH source would beenhanced as well and lead to increased rates of oxidation. If gas-particle equilibrium is assumed with a typical HOM saturationconcentration of 7.3 × 10−3 μg m−3 (SI Appendix, Table S6), themeasured HOM vapor concentration implies the correspondingC10H18O7 SOA enhancement accounted for 30% (range: 10–40%) of the total OA increase in the Atlanta urban plume. Thesedata provide evidence that anthropogenic enhancements tobiogenic VOC derived HOM exist in the present-day atmosphereand illustrate the importance of understanding multiple roles ofNOx in the production of low volatility compounds that lead toanthropogenically controlled biogenic SOA (39).Significant HOM yields due to autoxidation. The rural southeasternUnited States was predicted to experience near the maximumpossible fraction of RO2 undergoing autoxidation based on ob-served (Fig. 2A) and CMAQ air quality model predicted (Fig. 4)radical abundances. Specifically, Fig. 2A indicates typical day-time conditions in the southeastern US atmosphere as captured by

the 2013 southern oxidant and aerosol study (SOAS) in Centre-ville, Alabama (30 ppt HO2, 40 ppt NO) and conditions during themorning when NO was maximum (300 ppt NO, 5 ppt HO2) (13,40, 41). These observed ambient conditions and those for the ur-ban Atlanta plume resulted in predicted HOM-RO2 yields fromα-pinene + OH just under the maximum with slightly lower valuesduring the morning transition hours due to higher NO. High res-olution simulations for the present day and 20 y of simulationscovering the Northern Hemisphere showed a similar result for therural United States with molar HOM-RO2 yields of 9% for Cen-treville over the past two decades (Fig. 4 A and B).Fig. 4 also indicates variability in the predicted fraction of RO2

converted to HOM-RO2 via autoxidation across the United Statesand Northern Hemisphere as a result of the spatial distribution ofNO emissions, which suppress the relative fraction of RO2 un-dergoing autoxidation (Eq. 1) (17). Model calculations indicatedthat for the past two decades, autoxidation has been an increas-ingly important fraction of the RO2 fate in the Atlanta region (Fig.4B) because of decreasing NO (SI Appendix, Fig. S10), whileHyytiälä, Finland, experienced just under the maximum possibleautoxidation (11% by mole yield of HOM-RO2) for decades. Notethat once NO levels are sufficiently low, the yield of HOM-RO2will not be as sensitive to the NO level (Fig. 2).In China, the contribution of HOM to PM2.5 is unknown, but

α-pinene + OH may serve as an archetype system with lessons foranthropogenic VOCs [e.g., alkylbenzenes (21)] that also undergoautoxidation. Emissions of nitrogen oxides in China have increasedsince 1990 (42) and predictions for both urban (Guangzhou) andsuburban (Sanming) regions showed decreasing fractions of RO2undergoing autoxidation. Even in Guangzhou, with its higher NOlevels and bimolecular RO2 reactions outpacing autoxidation inthe present day, autoxidation was still predicted to lead to signif-icant HOM-RO2 yields on the order of 5% by mole (9% by mass).Monoterpene SOA from OH oxidation predicted by chemicaltransport models would show the opposite trend in yield with timecompared to that shown here for Guangzhou and Atlanta (Fig. 4B)due to current model formulations that assume semivolatile SOAand therefore dependencies of yield on primary organic aerosolabundance (43).NOx emission reduction cobenefits for SOA production from autoxidation.The dual role of NOx in competing with autoxidation andinfluencing oxidant concentrations does not allow for direct ex-trapolation of autoxidation efficiency (e.g., HOM yields fromFig. 4B and Eq. 1) to regional SOA formation rates becauseabsolute HOM-RO2 production rates (PHOM−RO2) also dependon oxidant and precursor hydrocarbon abundance (Eq. 2). Thespatial distribution of predicted HOM-RO2 production (Fig. 4C)reflects the abundance of monoterpenes and OH and illustratesthat increases in OH due to higher NOx can offset decreases inthe autoxidation efficiency such that HOM-RO2 production

Fig. 3. Concentrations of NO, ozone, monoterpenes(MT), organic aerosol (OA), and gas-phase C10H18O7

species measured downwind of the Atlanta, Georgiametropolitan area (A) on June 12, 2013 aboard theNOAA WP-3 aircraft. Pink vertical (B–D) shading in-dicates in-plume conditions characterized by en-hanced NO, CO (generally >190 ppb), ozone, andorganic aerosol (B and C). HOM vapor concentrations(D) are reported as 1-min averages with shading forthe 25th to 75th percentile of the 1-Hz data.

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rates are enhanced in polluted regions otherwise rich in mono-terpene emissions (Fig. 4D). Model results for urban locations(Guangzhou and Atlanta) showed locally increased OH andozone concentrations as well as HOM-RO2 production com-pared with their suburban/rural counterparts (e.g., Sanming andCentreville) due to higher anthropogenic emissions.In addition, the influence of NOx on oxidant abundance was

found to drive the predicted 1990–2010 change in HOM-RO2 forGuangzhou and Atlanta regions (Fig. 4D and SI Appendix, Fig.S10). Changing the mechanism yield of HOM-RO2 able to un-dergo autoxidation to C10H17O7 HOM-RO2 (value of f; Eq. 1)would change the ability of autoxidation to contribute to ambientHOM-RO2 in an absolute sense, but not in terms of trends over20 y. The conclusion that oxidants drive the 20-y trend in HOM-RO2 production is insensitive to the rate of autoxidation forkautox > 0.28 s−1. Taking these NOx–oxidant–autoxidation cou-plings into account, we predicted localized increases in HOM-RO2production rates in China between 1990 and 2010 (Fig. 4D), at thesame time autoxidation decreased relative to bimolecular RO2 fates(Fig. 4B).An oxidant-driven decrease in HOM production of about 20%

was predicted over the 1990–2010 period for the Atlanta region(Fig. 4D). Other work suggests organic aerosol has declinedabout 40% in the United States for this same time period (43). Ifmonoterpenes account for half of ambient organic aerosol (8),then changes in monoterpene SOA due to the NOx–oxidant–autoxidation couplings discussed herein could be responsible for aquarter of the recent downward trend in organic aerosol in thesoutheastern United States. As NO levels continue to decline andreaction with HO2 becomes a major HOM−RO2 fate, the resultingaerosol-phase peroxides could also have implications for publichealth (44) even if particulate matter as a whole declines.

ConclusionsAutoxidation is an effective pathway to substantial amounts oflow volatility organic compounds and recent trends indicateanthropogenic NOx controls will be an effective way to mitigatelocalized enhancements in particulate matter from monoterpeneautoxidation in the present-day atmosphere. Furthermore, dueto the low volatility of aerosol from autoxidation, anthropogeniccontrols on primary organic aerosol emissions are not expected to

significantly influence SOA from autoxidation, in contrast to mostcurrent chemical transport model representations of monoterpene-derived aerosol. Although changes in OH levels between pre-industrial and present day remain uncertain (45), our data suggestthat the autoxidation efficiency of monoterpene peroxy radicalsallows for increasing NOx to enhance biogenic SOA formation onregional scales due to the resulting enhancements in OH andozone. Only in highly polluted urban cores do calculations suggestsuppression of autoxidation by NO is significant enough to out-weigh the locally enhanced monoterpene reactions that can lead toautoxidation.

Materials and MethodsMechanism Development. Additional pathways and products (185 reactions,115 species) were added to MCM v3.3.1 to represent intramolecular H-shift,cyclization, and O2 addition with a focus on α-pinene + OH chemistry leadingto the formation of highly oxygenated peroxy radicals and coproducts.Autoxidation pathways followed those proposed by Berndt et al. (20) andstarted with the double-bond-retaining, OH-initiated C10H17O3−RO2 able toundergo H-shift as indicated by Vereecken et al. (29). The resulting C10H17O5−RO2 could further autoxidize (leading to C10H17O7−RO2). Thus, formation ofHOM-RO2 required two sequential autoxidation steps each with rate constantkautox and resulted in a squared dependence of HOM-RO2 yield on [NO]. kautoxrepresented the effective autoxidation rate constant and was applied to fivedifferent structures with O:C ratios of 0.3–0.5. Although the mechanism de-veloped here performs well for the metrics examined (Table 1), future workshould further constrain the autoxidation rate constant (including its de-pendence on structure and temperature) and overall product distribution forboth OH and ozone reaction under a variety of laboratory and ambient con-ditions. The full mechanism is available in the SI Appendix.

Box Model Framework. The mechanism developed here was implemented inF0AM v3.1 (46) with Washington Aerosol Module (WAM) extension (22).Compounds with saturation concentrations less than 500 μg m−3 were dy-namically partitioned between the gas and aerosol phases using vaporpressures determined by EVAPORATION algorithms (47, 48).

Laboratory Data. The F0AM-WAM model with detailed chemistry was appliedto two laboratory systems: Berndt et al. (20) flow tube experiments (SI Ap-pendix, Fig. S3) and the steady state chamber of the SOAFFEE at PacificNorthwest National Laboratory (parameters in SI Appendix, Table S5). F0AM-WAM simulations specifically focused on conditions where OH oxidation waspredicted to account for 91% of the α-pinene reacted. H2O2 and α-pinenewere continuously injected into the chamber and OH was generated by pho-tolysis. Aerosol mass spectrometer data had a 38% uncertainty (49). A high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS)with Filter Inlet for Gases and Aerosols (FIGAERO) utilizing iodide-adduct ion-ization measured gas and particle-phase composition (32). Loss of vapors andparticles to walls occurred in both the observations and model for SOAFFEE.

Regional Predictions. Specific product yields and rate constants from themechanism were used with two sets of archived hourly oxidant and mono-terpene fields from CMAQ to estimate the yield and production of HOM-RO2.The 12 km by 12 km horizontal resolution contiguous US output for June 1–July 15, 2013, from CMAQ v5.2 (doi:10.5281/zenodo.1167892) were obtainedfrom the work of Xu et al. (9) (Carbon Bond 6 revision 3 chemistry; 2011 EPANational Emission Inventory with year specific emissions when available).CMAQ v5.0 (doi:10.5281/zenodo.1079888) July predictions for the NorthernHemisphere from 1990 to 2010 were obtained from the work of Xing et al.(42) (Carbon Bond 5 chemistry; Emission Database for Global AtmosphericResearch [EDGAR] version 4.2 emissions). All means were calculated whenOH was in the top 50th percentile.

Aircraft Observations. Observations for Atlanta were obtained from theSoutheast Nexus (SENEX) field study on June 12, 2013, (SENEX flight number3) aboard the National Oceanic and Atmospheric Administration (NOAA)WP-3 aircraft. From 11:00 to 13:30 local time on June 12, the wind was from thenorthwest (mean speed of 5.0 m/s). Details on the instrumentation used togenerate observations can be found in Warneke et al. (50) and Lee et al. (51).CIMS signals were converted to abundance using the kinetic limit ionizationsensitivity (lower limit on concentration).

Code and Data Availability. F0AM is available at https://github.com/AirChem/F0AM.

A

D

B

C

Fig. 4. (A) Estimated molar yield of first generation C10H17O7 HOM-RO2 fromα-pinene OH-initiated C10H17O3 peroxy radicals (YHOM−RO2) for present-dayconditions in the contiguous United States; (B) trend in YHOM−RO2 for years1990–2010; (C) production rate of C10H17O7 HOM−RO2 (PHOM−RO2) in theNorthern Hemisphere (year 2010); and (D) trend in PHOM−RO2 for years 1990–2010. Locations marked in A and C, and featured in the time series include:Centerville, Alabama, United States (CTR); Atlanta, Georgia, United States (ATL);Hyytiälä, Finland (FIN); Sanming, Fujian, China (SAN); and Guangzhou,Guangdong, China (GNZ). Predictions are averaged over daytime, summer conditions.

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WAMis available at https://www.atmos.washington.edu/∼thornton/washington-aerosol-module.

CMAQ is available at https://github.com/USEPA/CMAQ.SENEX data are available at https://data.eol.ucar.edu/master_list/?project=

SAS.SOAFFEE data, SENEX HOM data, and CMAQ output have been deposited

in the Environmental Protection Agency Science Hub repository (https://catalog.data.gov/harvest/about/epa-sciencehub) using doi:10.23719/1502525.

ACKNOWLEDGMENTS. The authors thank Thomas Mentel for useful discus-sion, and the US Environmental Protection Agency (EPA) National Com-puter Center for computing resources. H.O.T.P. was supported by an EPAPresidential Early Career Award for Scientists and Engineers. J.A.T. and theUniversity of Washington team were supported by US Department of Energy

Office of Biological and Environmental Research as part of the AtmosphericSystem Research (ASR) program (Grants DE-SC0018221 and DE-SC0006867).E.L.D. was supported by the National Science Foundation Graduate ResearchFellowship under Grant DGE-1256082. S.S. was supported by the Academy ofFinland (Grants 307331 and 310682). M.T. was supported by an interagencyagreement between the EPA and the Department of Energy (DOE) for theOak Ridge Institute for Science and Education (ORISE) Research ParticipantProgram. J.E.S. and J. Liu were supported by the US Department of EnergyOffice of Biological and Environmental Research as part of the ASR program.The Pacific Northwest National Laboratory is operated for DOE by BattelleMemorial Institute under Contract DE-AC05-76RL01830. The EPA through itsOffice of Research and Development collaborated in the research describedhere. The research has been subjected to Agency administrative review andapproved for publication, but may not necessarily reflect official Agencypolicy.

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