Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

Abstract. This paper describes and evaluates a new framework for modeling kinetic gas-particle partitioning of secondary organic aerosol (SOA) that takes into account diffusion and chemical reaction within the particle phase. The framework uses a combination of (a) an analytical quasi-steady-state treatment for the diffusion–reaction process within the particle phase for fast-reacting organic solutes, and (b) a two-film theory approach for slow- and nonreacting solutes. The framework is amenable for use in regional and global atmospheric models, although it currently awaits specification of the various gas- and particle-phase chemistries and the related physicochemical properties that are important for SOA formation. Here, the new framework is implemented in the computationally efficient Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) to investigate the competitive growth dynamics of the Aitken and accumulation mode particles. Results show that the timescale of SOA partitioning and the associated size distribution dynamics depend on the complex interplay between organic solute volatility, particle-phase bulk diffusivity, and particle-phase reactivity (as exemplified by a pseudo-first-order reaction rate constant), each of which can vary over several orders of magnitude. In general, the timescale of SOA partitioning increases with increase in volatility and decrease in bulk diffusivity and rate constant. At the same time, the shape of the aerosol size distribution displays appreciable narrowing with decrease in volatility and bulk diffusivity and increase in rate constant. A proper representation of these physicochemical processes and parameters is needed in the next generation models to reliably predict not only the total SOA mass, but also its composition- and number-diameter distributions, all of which together determine the overall optical and cloud-nucleating properties.

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Mechanism of the hydroxyl radical oxidation of methacryloyl peroxynitrate (MPAN) and its pathway toward secondary organic aerosol formation in the atmosphere

Physical Chemistry Chemical Physics ◽

10.1039/c5cp02001h ◽

2015 ◽

Vol 17 (27) ◽

pp. 17914-17926 ◽

Cited By ~ 64

Author(s):

Tran B. Nguyen ◽

Kelvin H. Bates ◽

John D. Crounse ◽

Rebecca H. Schwantes ◽

Xuan Zhang ◽

...

Keyword(s):

Hydroxyl Radical ◽

Alkyl Radical ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Particle Phase ◽

Aerosol Formation ◽

Radical Oxidation ◽

Secondary Organic Aerosol Formation ◽

Phase Chemistry

The photooxidation of MPAN forms an energetic alkyl radical that promptly cyclizes, producing an α-lactone that participates in particle-phase chemistry.

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Kinetic modelling of formation and evaporation of SOA from NO<sub>3</sub> oxidation of pure and mixed monoterpenes

10.5194/acp-2020-55 ◽

2020 ◽

Author(s):

Thomas Berkemeier ◽

Masayuki Takeuchi ◽

Gamze Eris ◽

Nga L. Ng

Keyword(s):

Kinetic Model ◽

Gas Phase ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Phase Reaction ◽

Particle Phase ◽

Aerosol Formation ◽

Environmental Chamber ◽

Gas Phase Chemistry ◽

Phase Chemistry

Abstract. Organic aerosol constitutes a major fraction of the global aerosol burden and is predominantly formed as secondary organic aerosol (SOA). Environmental chambers have been used extensively to study aerosol formation and evolution under controlled conditions similar to the atmosphere, but quantitative prediction of the outcome of these experiments is generally not achieved, which signifies our lack in understanding of these results and limits their portability to large scale models. In general, kinetic models employing state-of-the-art explicit chemical mechanisms fail to describe the mass concentration and composition of SOA obtained from chamber experiments. Specifically, chemical reactions involving nitrate radical (NO3) oxidation of volatile organic compounds (VOCs) are a source of major uncertainty for assessing the chemical and physical properties of oxidation products. Here, we introduce a kinetic model that treats gas-phase chemistry, gas-particle partitioning, particle-phase oligomerization, and chamber wall loss and use it to describe the oxidation of the monoterpenes α-pinene and limonene with NO3. The model can reproduce aerosol mass and nitration degrees in experiments using either pure precursors or their mixtures and infers volatility distributions of products, branching ratios of reactive intermediates as well as particle-phase reaction rates. The gas-phase chemistry in the model is based on the Master Chemical Mechanism (MCM), but trades speciation of single compounds for the overall ability of quantitatively describing SOA formation by using a lumped chemical mechanism. The complex branching into a multitude of individual products in MCM is replaced in this model with product volatility distributions, detailed peroxy (RO2) and alkoxy (RO) radical chemistry and amended by a particle-phase oligomerization scheme. The kinetic parameters obtained in this study are constrained by a set of SOA formation and evaporation experiments conducted in the Georgia Tech Environmental Chamber (GTEC) facility. For both precursors, we present volatility distributions of nitrated and non-nitrated reaction products that are obtained by fitting the kinetic model systematically to the experimental data using a global optimization method, the Monte Carlo Genetic Algorithm (MCGA). The results presented here provide new mechanistic insight into the processes leading to formation and evaporation of SOA. Most notably, much of the non-linear behavior of precursor mixtures can be understood by RO2 fate and reversible oligomerization reactions in the particle phase, but some effects could be accredited to kinetic limitations of mass transport in the particle phase. The methodologies described in this work provide a basis for quantitative analysis of multi-source data from environmental chamber experiments with manageable computational effort.

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Modeling kinetic partitioning of secondary organic aerosol and size distribution dynamics: representing effects of volatility, phase state, and particle-phase reaction

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-28631-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 28631-28694 ◽

Cited By ~ 2

Author(s):

R. A. Zaveri ◽

R. C. Easter ◽

J. E. Shilling ◽

J. H. Seinfeld

Keyword(s):

Size Distribution ◽

Rate Constant ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Diffusion Reaction ◽

Aerosol Size Distribution ◽

Particle Phase ◽

Distribution Dynamics ◽

Bulk Diffusivity ◽

New Formulation

Abstract. This paper describes and evaluates a new formulation for modeling kinetic gas-particle partitioning of secondary organic aerosol (SOA) that takes into account diffusion and chemical reaction within the particle phase. The new formulation uses a combination of: (a) an analytical quasi-steady-state treatment for the diffusion-reaction process within the particle phase for fast-reacting organic solutes, and (b) a two-film theory approach for slow- and non-reacting solutes. The formulation is amenable for use in regional and global atmospheric models, although it currently awaits specification of the actual species and particle-phase reactions that are important for SOA formation. Here, the formulation is applied within the framework of the computationally efficient Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) to investigate the competitive growth dynamics of the Aitken and accumulation mode particles. Results show that the timescale of SOA partitioning and the associated size distribution dynamics depend on the complex interplay between organic solute volatility, particle-phase bulk diffusivity, and particle-phase reactivity (as exemplified by a pseudo-first-order reaction rate constant), each of which can vary over several orders of magnitude. In general, the timescale of SOA partitioning increases with increase in volatility and decrease in bulk diffusivity and rate constant. At the same time, the shape of the aerosol size distribution displays appreciable narrowing with decrease in volatility and bulk diffusivity and increase in rate constant. A proper representation of these physicochemical processes and parameters are needed in the next generation models to reliably predict not only the total SOA mass, but also its composition and number size distribution, all of which together determine its overall optical and cloud-nucleating properties.

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Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-2091-2013 ◽

2013 ◽

Vol 13 (4) ◽

pp. 2091-2113 ◽

Cited By ~ 108

Author(s):

J. E. Shilling ◽

R. A. Zaveri ◽

J. D. Fast ◽

L. Kleinman ◽

M. L. Alexander ◽

...

Keyword(s):

Mass Spectrometer ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Chemical Mechanism ◽

Biogenic Emissions ◽

Anthropogenic Emissions ◽

Radiation Balance ◽

Particle Phase ◽

Aerosol Formation ◽

Research Aircraft

Abstract. The CARES campaign was conducted during June, 2010 in the vicinity of Sacramento, California to study aerosol formation and aging in a region where anthropogenic and biogenic emissions regularly mix. Here, we describe measurements from an Aerodyne High Resolution Aerosol Mass Spectrometer (AMS), an Ionicon Proton Transfer Reaction Mass Spectrometer (PTR-MS), and trace gas detectors (CO, NO, NOx) deployed on the G-1 research aircraft to investigate ambient gas- and particle-phase chemical composition. AMS measurements showed that the particle phase is dominated by organic aerosol (OA) (85% on average) with smaller concentrations of sulfate (5%), nitrate (6%) and ammonium (3%) observed. PTR-MS data showed that isoprene dominated the biogenic volatile organic compound concentrations (BVOCs), with monoterpene concentrations generally below the detection limit. Using two different metrics, median OA concentrations and the slope of plots of OA vs. CO concentrations (i.e., ΔOA/ΔCO), we contrast organic aerosol evolution on flight days with different prevailing meteorological conditions to elucidate the role of anthropogenic and biogenic emissions on OA formation. Airmasses influenced predominantly by biogenic emissions had median OA concentrations of 2.2 μg m−3 and near zero ΔOA/ΔCO. Those influenced predominantly by anthropogenic emissions had median OA concentrations of 4.7 μg m−3 and ΔOA/ΔCO ratios of 35–44 μg m−3 ppmv. But, when biogenic and anthropogenic emissions mixed, OA levels were enhanced, with median OA concentrations of 11.4 μg m−3 and ΔOA/ΔCO ratios of 77–157 μg m−3 ppmv. Taken together, our observations show that production of OA was enhanced when anthropogenic emissions from Sacramento mixed with isoprene-rich air from the foothills. After considering several anthropogenic/biogenic interaction mechanisms, we conclude that NOx concentrations play a strong role in enhancing SOA formation from isoprene, though the chemical mechanism for the enhancement remains unclear. If these observations are found to be robust in other seasons and in areas outside of Sacramento, regional and global aerosol modules will need to incorporate more complex representations of NOx-dependent SOA mechanisms and yields into their algorithms. Ultimately, accurately predicting OA mass concentrations and their effect on radiation balance will require a mechanistically-based treatment of the interactions of biogenic and anthropogenic emissions.

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Impact of Aerosol-Cloud Cycling on Aqueous Secondary Organic Aerosol Formation

Atmosphere ◽

10.3390/atmos10110666 ◽

2019 ◽

Vol 10 (11) ◽

pp. 666 ◽

Cited By ~ 3

Author(s):

William G. Tsui ◽

Joseph L. Woo ◽

V. Faye McNeill

Keyword(s):

Atmospheric Aerosols ◽

Organic Material ◽

Organic Aerosol ◽

Time Of Day ◽

Aerosol Formation ◽

Atmospheric Models ◽

Cloud Processing ◽

Aerosol Mass ◽

Phase Chemistry ◽

The Impact

Chemical processing of organic material in aqueous atmospheric aerosols and cloudwater is known to form secondary organic aerosols (SOA), although the extent to which each of these processes contributes to total aerosol mass is unclear. In this study, we use GAMMA 5.0, a photochemical box model with coupled gas and aqueous-phase chemistry, to consider the impact of aqueous organic reactions in both aqueous aerosols and clouds on isoprene epoxydiol (IEPOX) SOA over a range of pH for both aqueous phases, including cycling between cloud and aerosol within a single simulation. Low pH aqueous aerosol, in the absence of organic coatings or other morphology which may limit uptake of IEPOX, is found to be an efficient source of IEPOX SOA, consistent with previous work. Cloudwater at pH 4 or lower is also found to be a potentially significant source of IEPOX SOA. This phenomenon is primarily attributed to the relatively high uptake of IEPOX to clouds as a result of higher water content in clouds as compared with aerosol. For more acidic cloudwater, the aqueous organic material is comprised primarily of IEPOX SOA and lower-volatility organic acids. Both cloudwater pH and the time of day or sequence of aerosol-to-cloud or cloud-to-aerosol transitions impacted final aqueous SOA mass and composition in the simulations. The potential significance of cloud processing as a contributor to IEPOX SOA production could account for discrepancies between predicted IEPOX SOA mass from atmospheric models and measured ambient IEPOX SOA mass, or observations of IEPOX SOA in locations where mass transfer limitations are expected in aerosol particles.

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Mapping gas-phase organic reactivity and concomitant secondary organic aerosol formation: chemometric dimension reduction techniques for the deconvolution of complex atmospheric data sets

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-8077-2015 ◽

2015 ◽

Vol 15 (14) ◽

pp. 8077-8100 ◽

Cited By ~ 7

Author(s):

K. P. Wyche ◽

P. S. Monks ◽

K. L. Smallbone ◽

J. F. Hamilton ◽

M. R. Alfarra ◽

...

Keyword(s):

Phase Composition ◽

Dimension Reduction ◽

Gas Phase ◽

Secondary Organic Aerosol ◽

Ambient Air ◽

Organic Aerosol ◽

Chemical Mechanism ◽

Particle Phase ◽

Aerosol Formation ◽

Holistic View

Abstract. Highly non-linear dynamical systems, such as those found in atmospheric chemistry, necessitate hierarchical approaches to both experiment and modelling in order to ultimately identify and achieve fundamental process-understanding in the full open system. Atmospheric simulation chambers comprise an intermediate in complexity, between a classical laboratory experiment and the full, ambient system. As such, they can generate large volumes of difficult-to-interpret data. Here we describe and implement a chemometric dimension reduction methodology for the deconvolution and interpretation of complex gas- and particle-phase composition spectra. The methodology comprises principal component analysis (PCA), hierarchical cluster analysis (HCA) and positive least-squares discriminant analysis (PLS-DA). These methods are, for the first time, applied to simultaneous gas- and particle-phase composition data obtained from a comprehensive series of environmental simulation chamber experiments focused on biogenic volatile organic compound (BVOC) photooxidation and associated secondary organic aerosol (SOA) formation. We primarily investigated the biogenic SOA precursors isoprene, α-pinene, limonene, myrcene, linalool and β-caryophyllene. The chemometric analysis is used to classify the oxidation systems and resultant SOA according to the controlling chemistry and the products formed. Results show that "model" biogenic oxidative systems can be successfully separated and classified according to their oxidation products. Furthermore, a holistic view of results obtained across both the gas- and particle-phases shows the different SOA formation chemistry, initiating in the gas-phase, proceeding to govern the differences between the various BVOC SOA compositions. The results obtained are used to describe the particle composition in the context of the oxidised gas-phase matrix. An extension of the technique, which incorporates into the statistical models data from anthropogenic (i.e. toluene) oxidation and "more realistic" plant mesocosm systems, demonstrates that such an ensemble of chemometric mapping has the potential to be used for the classification of more complex spectra of unknown origin. More specifically, the addition of mesocosm data from fig and birch tree experiments shows that isoprene and monoterpene emitting sources, respectively, can be mapped onto the statistical model structure and their positional vectors can provide insight into their biological sources and controlling oxidative chemistry. The potential to extend the methodology to the analysis of ambient air is discussed using results obtained from a zero-dimensional box model incorporating mechanistic data obtained from the Master Chemical Mechanism (MCMv3.2). Such an extension to analysing ambient air would prove a powerful asset in assisting with the identification of SOA sources and the elucidation of the underlying chemical mechanisms involved.

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Examining the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee ground site

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-8871-2015 ◽

2015 ◽

Vol 15 (15) ◽

pp. 8871-8888 ◽

Cited By ~ 128

Author(s):

S. H. Budisulistiorini ◽

X. Li ◽

S. T. Bairai ◽

J. Renfro ◽

Y. Liu ◽

...

Keyword(s):

Mass Spectrometry ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Anthropogenic Emissions ◽

Strongly Correlated ◽

Particle Phase ◽

Aerosol Formation ◽

Aerosol Acidity ◽

Southeastern Us

Abstract. A suite of offline and real-time gas- and particle-phase measurements was deployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant and Aerosol Study (SOAS) to examine the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol (SOA) formation. High- and low-time-resolution PM2.5 samples were collected for analysis of known tracer compounds in isoprene-derived SOA by gas chromatography/electron ionization-mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography/diode array detection-electrospray ionization-high-resolution quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). Source apportionment of the organic aerosol (OA) was determined by positive matrix factorization (PMF) analysis of mass spectrometric data acquired on an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average mass concentrations of the sum of quantified isoprene-derived SOA tracers contributed to ~ 9 % (up to 28 %) of the total OA mass, with isoprene-epoxydiol (IEPOX) chemistry accounting for ~ 97 % of the quantified tracers. PMF analysis resolved a factor with a profile similar to the IEPOX-OA factor resolved in an Atlanta study and was therefore designated IEPOX-OA. This factor was strongly correlated (r2 > 0.7) with 2-methyltetrols, C5-alkene triols, IEPOX-derived organosulfates, and dimers of organosulfates, confirming the role of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracer mass was ~ 26 % (up to 49 %) of the IEPOX-OA factor mass, which accounted for 32 % of the total OA. A low-volatility oxygenated organic aerosol (LV-OOA) and an oxidized factor with a profile similar to 91Fac observed in areas where emissions are biogenic-dominated were also resolved by PMF analysis, whereas no primary organic aerosol (POA) sources could be resolved. These findings were consistent with low levels of primary pollutants, such as nitric oxide (NO ~ 0.03 ppb), carbon monoxide (CO ~ 116 ppb), and black carbon (BC ~ 0.2 μg m−3). Particle-phase sulfate is fairly correlated (r2 ~ 0.3) with both methacrylic acid epoxide (MAE)/hydroxymethyl-methyl-α-lactone (HMML)- (henceforth called methacrolein (MACR)-derived SOA tracers) and IEPOX-derived SOA tracers, and more strongly correlated (r2 ~ 0.6) with the IEPOX-OA factor, in sum suggesting an important role of sulfate in isoprene SOA formation. Moderate correlation between the MACR-derived SOA tracer 2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides (NOy; r2 = 0.38) and nitrate (r2 = 0.45) indicates the potential influence of anthropogenic emissions through long-range transport. Despite the lack of a clear association of IEPOX-OA with locally estimated aerosol acidity and liquid water content (LWC), box model calculations of IEPOX uptake using the simpleGAMMA model, accounting for the role of acidity and aerosol water, predicted the abundance of the IEPOX-derived SOA tracers 2-methyltetrols and the corresponding sulfates with good accuracy (r2 ~ 0.5 and ~ 0.7, respectively). The modeling and data combined suggest an anthropogenic influence on isoprene-derived SOA formation through acid-catalyzed heterogeneous chemistry of IEPOX in the southeastern US. However, it appears that this process was not limited by aerosol acidity or LWC at Look Rock during SOAS. Future studies should further explore the extent to which acidity and LWC as well as aerosol viscosity and morphology becomes a limiting factor of IEPOX-derived SOA, and their modulation by anthropogenic emissions.

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Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-26297-2012 ◽

2012 ◽

Vol 12 (10) ◽

pp. 26297-26349 ◽

Cited By ~ 1

Author(s):

J. E. Shilling ◽

R. A. Zaveri ◽

J. D. Fast ◽

L. Kleinman ◽

M. L. Alexander ◽

...

Keyword(s):

Mass Spectrometer ◽

Organic Aerosol ◽

Proton Transfer Reaction ◽

Biogenic Emissions ◽

Anthropogenic Emissions ◽

Radiation Balance ◽

Particle Phase ◽

Aerosol Formation ◽

Aerosol Mass ◽

Research Aircraft

Abstract. The CARES campaign was conducted during June 2010 in the vicinity of Sacramento, California to study aerosol formation and aging in a region where anthropogenic and biogenic emissions regularly mix. Here, we describe measurements from an Aerodyne High Resolution Aerosol Mass Spectrometer (AMS), an Ionicon Proton Transfer Reaction Mass Spectrometer (PTR-MS), and trace gas detectors (CO, NO, NOx) deployed on the G-1 research aircraft to investigate ambient gas- and particle-phase chemical composition. AMS measurements showed that the particle phase is dominated by organic aerosol (OA) (85% on average) with smaller concentrations of sulfate (5%), nitrate (6%) and ammonium (3%) observed. PTR-MS data showed that isoprene dominated the biogenic volatile organic compound concentrations (BVOCs), with monoterpene concentrations generally below the detection limit. Using two different metrics, median OA concentrations and the slope of plots of OA vs. CO concentrations (i.e. ΔOA/ΔCO), we contrast organic aerosol evolution on flight days with different prevailing meteorological conditions to elucidate the role of anthropogenic and biogenic emissions on OA formation. Airmasses influenced predominantly by biogenic emissions had median OA concentrations of 2.2 μg m−3 and near zero ΔOA/ΔCO. Those influenced predominantly by anthropogenic emissions had median OA concentrations of 4.7 μg m−3 and ΔOA/ΔCO ratios of 35–44 μg m−3 ppmv−1. But, when biogenic and anthropogenic emissions mixed, OA levels were dramatically enhanced, with median OA concentrations of 11.4 μg m−3 and ΔOA/ΔCO ratios of 77–157 μg m−3 ppmv−1. Taken together, our observations show that production of OA was enhanced when anthropogenic emissions from Sacramento mixed with isoprene-rich air from the foothills. A strong, non-linear dependence of SOA yield from isoprene is the explanation for this enhancement most consistent with both the gas- and particle-phase data. If these observations are found to be robust in other seasons and in areas outside of Sacramento, regional and global aerosol modules will need to incorporate more complex representations of NOx-dependent SOA yields into their algorithms. Ultimately, accurately predicting OA mass concentrations and their effect on radiation balance will require a mechanistically-based treatment of the interactions of biogenic and anthropogenic emissions.

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The effect of particle acidity on secondary organic aerosol formation from <i>α</i>-pinene photooxidation under atmospherically relevant conditions

Atmospheric Chemistry and Physics ◽

10.5194/acp-16-13929-2016 ◽

2016 ◽

Vol 16 (21) ◽

pp. 13929-13944 ◽

Cited By ~ 14

Author(s):

Yuemei Han ◽

Craig A. Stroud ◽

John Liggio ◽

Shao-Meng Li

Keyword(s):

Gas Phase ◽

Secondary Organic Aerosol ◽

Organic Aerosol ◽

Oxidation Products ◽

Strong Increase ◽

Nitrate Content ◽

Organic Nitrate ◽

Organic Nitrates ◽

Particle Phase ◽

Aerosol Formation

Abstract. Secondary organic aerosol (SOA) formation from photooxidation of α-pinene has been investigated in a photochemical reaction chamber under varied inorganic seed particle acidity levels at moderate relative humidity. The effect of particle acidity on SOA yield and chemical composition was examined under high- and low-NOx conditions. The SOA yield (4.2–7.6 %) increased nearly linearly with the increase in particle acidity under high-NOx conditions. In contrast, the SOA yield (28.6–36.3 %) was substantially higher under low-NOx conditions, but its dependency on particle acidity was insignificant. A relatively strong increase in SOA yield (up to 220 %) was observed in the first hour of α-pinene photooxidation under high-NOx conditions, suggesting that SOA formation was more effective for early α-pinene oxidation products in the presence of fresh acidic particles. The SOA yield decreased gradually with the increase in organic mass in the initial stage (approximately 0–1 h) under high-NOx conditions, which is likely due to the inaccessibility to the acidity over time with the coating of α-pinene SOA, assuming a slow particle-phase diffusion of organic molecules into the inorganic seeds. The formation of later-generation SOA was enhanced by particle acidity even under low-NOx conditions when introducing acidic seed particles after α-pinene photooxidation, suggesting a different acidity effect exists for α-pinene SOA derived from later oxidation stages. This effect could be important in the atmosphere under conditions where α-pinene oxidation products in the gas-phase originating in forested areas (with low NOx and SOx) are transported to regions abundant in acidic aerosols such as power plant plumes or urban regions. The fraction of oxygen-containing organic fragments (CxHyO1+ 33–35 % and CxHyO2+ 16–17 %) in the total organics and the O ∕ C ratio (0.52–0.56) of α-pinene SOA were lower under high-NOx conditions than those under low-NOx conditions (39–40, 17–19, and 0.61–0.64 %), suggesting that α-pinene SOA was less oxygenated in the studied high-NOx conditions. The fraction of nitrogen-containing organic fragments (CxHyNz+ and CxHyOzNp+) in the total organics was enhanced with the increases in particle acidity under high-NOx conditions, indicating that organic nitrates may be formed heterogeneously through a mechanism catalyzed by particle acidity or that acidic conditions facilitate the partitioning of gas-phase organic nitrates into particle phase. The results of this study suggest that inorganic acidity has a significant role to play in determining various organic aerosol chemical properties such as mass yields, oxidation state, and organic nitrate content. The acidity effect being further dependent on the timescale of SOA formation is also an important parameter in the modeling of SOA.

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