Atmospheric oxidation of natural and anthropogenic volatile organic compounds (VOCs) leads to secondary organic aerosol (SOA), which constitutes a major and often dominant component of atmospheric fine particulate matter (PM2.5). Recent work demonstrates that rapid autoxidation of organic peroxy radicals (RO2) formed during VOC oxidation results in highly oxygenated organic molecules (HOM) which efficiently form SOA. As NOx emissions decrease, the chemical regime of the atmosphere changes to one in which RO2 autoxidation becomes increasingly important, potentially increasing PM2.5, while oxidant availability driving RO2 formation rates simultaneously declines, possibly slowing regional PM2.5 formation. Using a unique suite of in situ aircraft observations and laboratory studies of HOM, together with a detailed molecular mechanism, we show that although autoxidation in an archetypal biogenic VOC system becomes more competitive as NOx decreases, absolute HOM production rates decrease due to oxidant reductions, leading to an overall positive coupling between anthropogenic NOx and localized biogenic SOA from autoxidation. This effect is observed in the Atlanta, Georgia urban plume where HOM is enhanced in the presence of elevated NO, and predictions for Guangzhou, China, where increasing HOM-RO2 production coincides with increases in NO from 1990 to 2010. These results suggest added benefits to PM2.5 abatement strategies come with NOx emission reductions and have implications for aerosol-climate interactions due to changes in global SOA resulting from NOx interactions since the pre-industrial era. Datasets include links to CMAQ, F0AM, and WAM model code as well as the SENEX aircraft campaign data archive. Files include data shown in Figure 3 (C10H18O7 HOM from SENEX), data used to construct Figure 4 and S10 (CMAQ model predictions of oxidants and intermediate species), additional supporting data in Figure S11 (SENEX C10 HOM species), and observed particle and gas composition from SOAFFEE laboratory experiments (Figure S6 and elsewhere). This dataset is associated with the following publication: Pye, H., E. D’Ambro, B. Lee, S. Schobesberger, M. Takeuchi, Y. Zhao, F. Lopez-Hilfiker, J. Liu, J. Shilling, J. Xing, R. Mathur, A. Middlebrook, J. Liao, A. Welti, M. Graus, C. Warneke, J.d. Gouw, J. Holloway, T. Ryerson, I. Pollack, and J. Thornton. Anthropogenic enhancements to production of highly oxygenated molecules from autoxidation.. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, USA, 116(14): 6641-6646, (2019).

Recently, we identified seven novel hydroxy-carboxylic acids resulting from gas-phase reactions of isoprene in the presence of nitrogen oxides (NOx), ozone (O3), and/or hydroxyl radicals (OH). In the present study, we provide evidence that hydroxy-carboxylic acids, namely methyltartaric acids (MTA) are: (1) reliable isoprene tracers, (2) likely produced via rapid peroxy radical hydrogen atom(H) shift reactions (autoxidation mechanism) and analogous alkoxy radical H shifts in low and high NOx environments respectively and (3) representative of aged ambient aerosol in the low NOx regime. Firstly, MTA are reliable tracers of isoprene aerosol because they have been identified in numerous chamber experiments involving isoprene conducted under a wide range of conditions and are absent in the oxidation of mono- and sesquiterpenes. They are also present in numerous samples of ambient aerosol collected during the past 20 years at several locations in the U.S. and Europe. Furthermore, MTA concentrations measured during a year-long field study in Research Triangle Park (RTP), NC in 2003 show a seasonal trend consistent with isoprene emissions and photochemical activity. Secondly, an analysis of chemical ionization mass spectrometer (CIMS) data of several chamber experiments in low and high NOx environments show that highly oxidized molecules (HOMs) derived from isoprene that lead to MTAs may be produced rapidly and considered as early generation isoprene oxidation products in the gas phase. Density functional theory calculations show that rapid intramolecular H shifts involving peroxy and alkoxy radicals possess low barriers for methyl-hydroxy-butenals (MHBs) that may represent precursors for MTA. From these results, a viable rapid H shift mechanism is proposed to occur that produces isoprene derived HOMs like MTA. Finally, an analysis of the mechanism shows that autoxidation-like pathways in low and high NOx may produce HOMs in a few OH oxidation steps like commonly detected methyl tetrol (MT) isoprene tracers. The ratio of MTA/MT in isoprene aerosol is also shown to be significantly greater in field versus chamber samples indicating the importance of such pathways in the atmosphere even for smaller hydrocarbons like isoprene. This dataset is associated with the following publication: Jaoui, M., I. Piletic, R. Szmigielski, K.J. Rudzinski, M. Lewandowski, T. Riedel, and T. Kleindienst. Rapid production of highly oxidized molecules in isoprene aerosol via peroxy and alkoxy radical isomerization pathways in low and high NOx environments: Combined laboratory, computational and field studies. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, NETHERLANDS, 775: 145592, (2021).

Recently, we identified seven novel hydroxy-carboxylic acids resulting from gas-phase reactions of isoprene in the presence of nitrogen oxides (NOx), ozone (O3), and/or hydroxyl radicals (OH). In the present study, we provide evidence that hydroxy-carboxylic acids, namely methyltartaric acids (MTA) are: (1) reliable isoprene tracers, (2) likely produced via rapid peroxy radical hydrogen atom(H) shift reactions (autoxidation mechanism) and analogous alkoxy radical H shifts in low and high NOx environments respectively and (3) representative of aged ambient aerosol in the low NOx regime. Firstly, MTA are reliable tracers of isoprene aerosol because they have been identified in numerous chamber experiments involving isoprene conducted under a wide range of conditions and are absent in the oxidation of mono- and sesquiterpenes. They are also present in numerous samples of ambient aerosol collected during the past 20 years at several locations in the U.S. and Europe. Furthermore, MTA concentrations measured during a year-long field study in Research Triangle Park (RTP), NC in 2003 show a seasonal trend consistent with isoprene emissions and photochemical activity. Secondly, an analysis of chemical ionization mass spectrometer (CIMS) data of several chamber experiments in low and high NOx environments show that highly oxidized molecules (HOMs) derived from isoprene that lead to MTAs may be produced rapidly and considered as early generation isoprene oxidation products in the gas phase. Density functional theory calculations show that rapid intramolecular H shifts involving peroxy and alkoxy radicals possess low barriers for methyl-hydroxy-butenals (MHBs) that may represent precursors for MTA. From these results, a viable rapid H shift mechanism is proposed to occur that produces isoprene derived HOMs like MTA. Finally, an analysis of the mechanism shows that autoxidation-like pathways in low and high NOx may produce HOMs in a few OH oxidation steps like commonly detected methyl tetrol (MT) isoprene tracers. The ratio of MTA/MT in isoprene aerosol is also shown to be significantly greater in field versus chamber samples indicating the importance of such pathways in the atmosphere even for smaller hydrocarbons like isoprene. This dataset is associated with the following publication: Jaoui, M., I. Piletic, R. Szmigielski, K.J. Rudzinski, M. Lewandowski, T. Riedel, and T. Kleindienst. Rapid production of highly oxidized molecules in isoprene aerosol via peroxy and alkoxy radical isomerization pathways in low and high NOx environments: Combined laboratory, computational and field studies. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, NETHERLANDS, 775: 145592, (2021).

Dataset contains CMAQv5.3-predicted hourly average concentrations of organic aerosol and other species (HO2, NO, NO3, O3, OH) for Hyytiala, Finland (location of BAECC field campaign) and Centreville, AL, USA (main location of SOAS field campaign) for 2016. How species were created from raw CMAQ output is defined in Table S2 of the paper this data supports. CMAQ v5.3 code is also linked here. The specific configuration of CMAQ is described in the manuscript associated with this data. This dataset is associated with the following publication: Lee, B., E. D'Ambro, F. Lopez-Hilfiker, S. Schobesberger, C. Mohr, M. Zawakowicz, J. Liu, J. Shilling, W. Hu, B. Palm, J. Jimenez, L. Hao, A. Virtanen, H. Zhang, A. Goldstein, H. Pye, and J. Thornton. Resolving Ambient Organic Aerosol Formation and Aging Pathways with Simultaneous Molecular Composition and Volatility Observations. ACS Earth and Space Chemistry. American Chemical Society, Washington, DC, USA, 4(3): 391-402, (2020).

Dataset contains CMAQv5.3-predicted hourly average concentrations of organic aerosol and other species (HO2, NO, NO3, O3, OH) for Hyytiala, Finland (location of BAECC field campaign) and Centreville, AL, USA (main location of SOAS field campaign) for 2016. How species were created from raw CMAQ output is defined in Table S2 of the paper this data supports. CMAQ v5.3 code is also linked here. The specific configuration of CMAQ is described in the manuscript associated with this data. This dataset is associated with the following publication: Lee, B., E. D'Ambro, F. Lopez-Hilfiker, S. Schobesberger, C. Mohr, M. Zawakowicz, J. Liu, J. Shilling, W. Hu, B. Palm, J. Jimenez, L. Hao, A. Virtanen, H. Zhang, A. Goldstein, H. Pye, and J. Thornton. Resolving Ambient Organic Aerosol Formation and Aging Pathways with Simultaneous Molecular Composition and Volatility Observations. ACS Earth and Space Chemistry. American Chemical Society, Washington, DC, USA, 4(3): 391-402, (2020).

EPA did not generate data as part of this work. This dataset is not publicly accessible because: I personally did not generate any new data in this study. It can be accessed through the following means: Via contact with corresponding author. Format: This study was led by the University of North Carolina at Chapel Hill and they plan to distribute through Mendeley Data. Data is not yet posted. This dataset is associated with the following publication: Schmedding, R., M. Ma, Y. Zhang, S. Farrell, H. Pye, Y. Chen, C. Wang, Q. Rasool, S.H. Budisulistiorini, A. Ault, J. Surratt, and W. Vizuete. α-Pinene-Derived organic coatings on acidic sulfate aerosol impacts secondary organic aerosol formation from isoprene in a box model. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, USA, 213: 456-462, (2019).

EPA did not generate data as part of this work. This dataset is not publicly accessible because: I personally did not generate any new data in this study. It can be accessed through the following means: Via contact with corresponding author. Format: This study was led by the University of North Carolina at Chapel Hill and they plan to distribute through Mendeley Data. Data is not yet posted. This dataset is associated with the following publication: Schmedding, R., M. Ma, Y. Zhang, S. Farrell, H. Pye, Y. Chen, C. Wang, Q. Rasool, S.H. Budisulistiorini, A. Ault, J. Surratt, and W. Vizuete. α-Pinene-Derived organic coatings on acidic sulfate aerosol impacts secondary organic aerosol formation from isoprene in a box model. ATMOSPHERIC ENVIRONMENT. Elsevier Science Ltd, New York, NY, USA, 213: 456-462, (2019).

Laboratory data supporting "Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry" by Zhao, Thornton, and Pye. Abstract: Organic peroxy radicals (RO2) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO2 in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO2 and gas-phase dimers from O3-initiated oxidation of α-pinene that ~150 gaseous dimers (C16-20H24-34O4-13) are primarily formed through RO2 cross-reactions, with a typical rate constant of 0.75-2×10-12 cm3 molecule-1 s-1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2-2.5% by mole (0.5-6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤ 50 ppt NO). Given their very low volatility, the gaseous C16-20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5-60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO2, dimers, and highly-oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ~20% of primary α-pinene RO2 from OH reaction and 10% from ozonolysis autoxidize at 3-10 s-1 and ≥ 1 s-1, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO2 autoxdiation are ubiquitous sources of low-volatility organic compounds capable of contributing significantly to atmospheric new particle formation and growth. This dataset is associated with the following publication: Zhao, Y., J. Thornton, and H. Pye. Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry. PNAS (PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES). National Academy of Sciences, WASHINGTON, DC, USA, 115(48): 12142-12147, (2018).

Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are important precursors of secondary organic aerosol (SOA), but the amounts and pathways of SOA generation from these precursors are not well constrained by observations. We propose that the less-oxidized oxygenated organic aerosol (LO-OOA) factor resolved from positive matrix factorization (PMF) analysis on aerosol mass spectrometry (AMS) data can be used as a surrogate for fresh SOA from monoterpenes and sesquiterpenes in the southeastern US. This hypothesis is supported by multiple lines of evidence, including lab-in-the-field perturbation experiments, extensive ambient ground-level measurements, and state-of-the-art modeling. We performed lab-in-the-field experiments in which the ambient air is perturbed by the injection of selected monoterpenes and sesquiterpenes, and the subsequent SOA formation is investigated. PMF analysis on the perturbation experiments provides an objective link between LO-OOA and fresh SOA from monoterpenes and sesquiterpenes as well as insights into the sources of other OA factors. Further, we use an upgraded atmospheric model and show that modeled SOA concentrations from monoterpenes and sesquiterpenes could reproduce both the magnitude and diurnal variation of LO-OOA at multiple sites in the southeastern US, building confidence in our hypothesis. We estimate the annual average concentration of SOA from monoterpenes and sesquiterpenes in the southeastern US to be roughly 2µgm−3. Dataset (csv file) contains CMAQ model predictions for locations in the southeastern US during 2012 and 2013. The species definition file (txt) defines how quantities were obtained from the model. Data in the csv files follows the writesite utility output format (https://github.com/USEPA/CMAQ/tree/5.2.1/POST/writesite). Links to additional datasets are provided. This dataset is associated with the following publication: Xu, L., H. Pye, J. He, Y. Chen, B. Murphy, and N. Ng. Experimental and model estimates of the contributions from biogenic monoterpenes and sesquiterpenes to secondary organic aerosol in the southeastern United States. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, GERMANY, 18(17): 12613-12637, (2018).