Abstract. Chemical transport models have historically struggled to accurately simulate the magnitude and variability of observed organic aerosol (OA), with previous studies demonstrating that models significantly underestimate observed concentrations in the troposphere. In this study, we explore two different model OA schemes within the standard GEOS-Chem chemical transport model and evaluate the simulations against a suite of 15 globally distributed airborne campaigns from 2008 to 2017, primarily in the spring and summer seasons. These include the ATom, KORUS-AQ, GoAmazon, FRAPPE, SEAC4RS, SENEX, DC3, CalNex, OP3, EUCAARI, ARCTAS and ARCPAC campaigns and provide broad coverage over a diverse set of atmospheric composition regimes – anthropogenic, biogenic, pyrogenic and remote. The schemes include significant differences in their treatment of the primary and secondary components of OA – a “simple scheme” that models primary OA (POA) as non-volatile and takes a fixed-yield approach to secondary OA (SOA) formation and a “complex scheme” that simulates POA as semi-volatile and uses a more sophisticated volatility basis set approach for non-isoprene SOA, with an explicit aqueous uptake mechanism to model isoprene SOA. Despite these substantial differences, both the simple and complex schemes perform comparably across the aggregate dataset in their ability to capture the observed variability (with an R2 of 0.41 and 0.44, respectively). The simple scheme displays greater skill in minimizing the overall model bias (with a normalized mean bias of 0.04 compared to 0.30 for the complex scheme). Across both schemes, the model skill in reproducing observed OA is superior to previous model evaluations and approaches the fidelity of the sulfate simulation within the GEOS-Chem model. However, there are significant differences in model performance across different chemical source regimes, classified here into seven categories. Higher-resolution nested regional simulations indicate that model resolution is an important factor in capturing variability in highly localized campaigns, while also demonstrating the importance of well-constrained emissions inventories and local meteorology, particularly over Asia. Our analysis suggests that a semi-volatile treatment of POA is superior to a non-volatile treatment. It is also likely that the complex scheme parameterization overestimates biogenic SOA at the global scale. While this study identifies factors within the SOA schemes that likely contribute to OA model bias (such as a strong dependency of the bias in the complex scheme on relative humidity and sulfate concentrations), comparisons with the skill of the sulfate aerosol scheme in GEOS-Chem indicate the importance of other drivers of bias, such as emissions, transport and deposition, that are exogenous to the OA chemical scheme.

<p><strong>Abstract.</strong> Chemical transport models have historically struggled to accurately simulate the magnitude and variability of observed organic aerosol (OA), with previous studies demonstrating that models significantly underestimate observed concentrations in the troposphere. In this study, we explore two different model OA schemes within the standard GEOS-Chem chemical transport model and evaluate the simulations against a suite of 15 globally-distributed airborne campaigns from 2008–2017. These include the ATom, KORUS-AQ, GoAmazon, FRAPPE, SEAC4RS, SENEX, DC3, CalNex, OP3, EUCAARI, ARCTAS and ARCPAC campaigns and provide broad coverage over a diverse set of atmospheric-composition regimes – anthropogenic, biogenic, pyrogenic and remote. The schemes include significant differences in their treatment of the primary and secondary components of OA – a <q>simple scheme</q> that models primary OA (POA) as non-volatile and takes a fixed-yield approach to secondary OA (SOA) formation, and a <q>complex scheme</q> that simulates POA as semi-volatile and uses a more sophisticated volatility basis set approach for non-isoprene SOA, with an explicit aqueous uptake mechanism to model isoprene SOA. Despite these substantial differences, both the simple and complex schemes perform comparably across the aggregate dataset in their ability to capture the observed variability (with an R<sup>2</sup> of 0.41 and 0.44 respectively). The simple scheme displays greater skill in minimizing the overall model-bias (with a NMB of 0.04, compared to 0.29 for the complex scheme). Across both schemes, the model skill in reproducing observed OA is superior to previous model evaluations and approaches the fidelity of the sulfate simulation within GEOS-Chem. However, there are significant differences in model performance across different chemical source regimes, classified here into 7 categories. Higher-resolution nested regional simulations indicate that model resolution is an important factor in capturing variability in highly-localized campaigns, while also demonstrating the importance of well-constrained emissions inventories and local meteorology, particularly over Asia. A comparison of the POA loadings from the complex scheme with SOA loadings from the simple scheme (and vice versa) also suggests that a semi-volatile treatment of POA is superior to a non-volatile treatment. While this study identifies factors within the SOA schemes that likely contribute to OA model bias (such as a strong dependency of the bias in the complex scheme on relative humidity and sulfate concentrations), comparisons with the skill of the sulfate aerosol scheme in GEOS-Chem indicate the importance of other drivers of bias such as emissions, transport, and deposition that are exogenous to the OA chemical scheme.</p>

The composition of ambient particulate matter (PM) and its sources were investigated at the Salton Sea, a shrinking saline lake in California. To investigate the influence of playa exposure on PM composition, PM samples were collected during two seasons and at two sites around the Salton Sea. To characterize source composition, soil samples were collected from local playa and desert surfaces. PM and soil samples were analyzed for 15 elements using mass spectrometry and X-ray diffraction. The contribution of sources to PM mass and composition was investigated using Al-referenced enrichment factors (EFs) and source factors resolved from positive matrix factorization (PMF). Playa soils were found to be significantly enriched in Ca, Na, and Se relative to desert soils. PMF analysis resolved the PM data with four source factors, identified as Playa-like, Desert-like, Ca-rich, and Se. Playa-like and desert-like sources were estimated to contribute to a daily average of 8.9% and 45% of PM mass, respectively. Additionally, playa sources were estimated to contribute to 38-68% of PM Na. PM Se concentrations showed strong seasonal variations, suggesting a seasonal cycle of Se volatilization and recondensation. These results support the importance of playas as a source of PM mass and a controlling factor of PM composition.

Abstract. We present airborne measurements made during the 2014 Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) project to investigate the impacts of the Denver Cyclone on regional air quality in the greater Denver area. Data on trace gases, non-refractory submicron aerosol chemical constituents, and aerosol optical extinction (β ext ) at λ = 632 nm were evaluated in the presence and absence of the surface mesoscale circulation in three distinct study regions of the Front Range: In-Flow, Northern Front Range, and the Denver metropolitan area. Pronounced increases in mass concentrations of organics, nitrate, and sulfate in the Northern Front Range and the Denver metropolitan area were observed during the cyclone episodes (27-28 July) compared to the non-cyclonic days (26 July, 2-3 August). Organic aerosols dominated the mass concentrations on all evaluated days, with a 45 % increase in organics on cyclone days across all three regions, while the increase during the cyclone episode was up to ∼ 80 % over the Denver metropolitan area. In the most aged air masses (NO x / NO y < 0.5), background organic aerosols over the Denver metropolitan area increased by a factor of ∼ 2.5 due to transport from Northern Front Range. Furthermore, enhanced partitioning of nitric acid to the aerosol phase was observed during the cyclone episodes, mainly due to increased abundance of gas phase ammonia. During the non-cyclone events, β ext displayed strong correlations (r = 0.71) with organic and nitrate in the Northern Front Range and only with organics (r = 0.70) in the Denver metropolitan area, while correlation of β ext during the cyclone was strongest (r = 0.86) with nitrate over Denver. Mass extinction efficiency (MEE) values in the Denver metropolitan area were similar on cyclone and non-cyclone days despite the dominant influence of different aerosol species on β ext . Our analysis showed that the meteorological patterns associated with the Denver Cyclone increased aerosol mass loadings in the Denver metropolitan area mainly by transporting aerosols and/or aerosol precursors from the northern regions, leading to impaired visibility and air quality deterioration.

Abstract. Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver metropolitan area as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) campaign during July–August 2014. An Aerodyne cavity attenuated phase shift particle light extinction monitor (CAPS-PMex) was deployed to measure βext (at average relative humidity of 20 ± 7 %) of submicron aerosols at λ = 632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior in various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext ∕ ΔCO) were higher in aged urban air masses compared to fresh air masses by  ∼  50 %. The resulting increase in Δβext ∕ ΔCO for highly aged air masses was accompanied by formation of secondary organic aerosols (SOAs). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&amp;G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.51 to 2.27 m2 g−1, with the minimum and maximum values observed in urban and agriculture-influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During 11–12 August, regional influence of a biomass burning event was observed, increasing the background βext and estimated MEE values in the Front Range.

Refractive index and optical properties of biogenic and anthropogenic secondary organic aerosol (SOA) particles were investigated. Aerosol precursors, namely longifolene, a-pinene, 1-methylnaphthalene, phenol, and toluene were oxidized in a Teflon chamber to produce SOA particles under different initial hydrocarbon concentrations and hydroxyl radical sources, reflecting exposures to different levels of nitrogen oxides (NO x). The real and imaginary components (n and k, respectively) of the refractive index at 375 nm and 632 nm were determined by Mie theory calculations through an iterative process, using the v 2 function to evaluate the fitness of the predicted optical parameters with the measured scattering, absorption, and extinction coefficients from a Photoacoustic Extinctiometer and Cavity Attenuated Phase Shift Spectrometer. Single scattering albedo (SSA) and bulk mass absorption coefficient (MAC) at 375 nm were calculated. SSA values of SOA particles from biogenic precursors (longifolene and a-pinene) were $0.98-0.99 ($6.3% uncertainty), reflecting purely scattering aerosols regardless of the NO x regime. However, SOA particles from aromatic precursors were more absorbing and displayed NO x-dependent SSA values. For 1-methylnaphthalene SOA particles, SSA values of 0.92-0.95 and $0.75-0.90 ($6.1% uncertainty) were observed under intermediate-and high-NO x conditions, respectively, reflecting the absorbing effects of SOA particles and NO x chemistry for this aromatic system. In mixtures of longifolene and phenol or longifolene and toluene SOA under intermediate-and high-NO x conditions, k values of the aromatic-related component of the SOA mixture were higher than that of 1-methylnaphthalene SOA particles. With the increase in OH exposure, k phenol decreased from 0.10 to 0.02 and 0.22 to 0.05 for intermediateand high-NO x conditions, respectively. A simple relative radiative forcing calculation for urban environments at k ¼ 375 nm suggests the influence of absorbing SOA particles on relative radiative forcing at this wavelength is most significant for aerosol sizes greater than 0.4 mm.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; We present airborne measurements made in the Colorado Front Range aboard the NSF C-130 aircraft during the 2014 Front Range Air Pollution and Photochemistry &amp;#201;xperiment (FRAPP&amp;#201;) project. Data on trace gases, non-refractory sub-micron aerosol chemical constituents, and aerosol optical extinction (&amp;#946;&lt;sub&gt;ext&lt;/sub&gt;) at &amp;#955;&amp;#8201;=&amp;#8201;632&amp;#8201;nm in the presence and absence of a surface mesoscale circulation pattern, called the Denver Cyclone, were analyzed in three study regions of the Front Range: In-Flow, Northern Front Range (NFR), and Denver Metropolitan (DM). Pronounced increases in mass concentrations of organics, nitrate, and sulfate in NFR and DM were observed during the cyclone episodes (27&amp;#8211;28 July) compared to the non-cyclonic days (26 July, 02&amp;#8211;03 August). Organics (OA) dominated the mass concentrations on all evaluated days, with a 45&amp;#8201;% increase in OA on cyclone days across all three regions while the increase during the cyclone episode was up to ~&amp;#8201;80&amp;#8201;% for DM, from 3.78&amp;#8201;&amp;#177;&amp;#8201;1.55&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; to 6.78&amp;#8201;&amp;#177;&amp;#8201;1.78&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt;, where sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; is the STP unit of volume of air. Average nitrate mass concentrations were 0.26&amp;#8201;&amp;#177;&amp;#8201;0.27&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; vs. 1.03&amp;#8201;&amp;#177;&amp;#8201;0.74&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; followed by sulfate with an average of 0.58&amp;#8201;&amp;#177;&amp;#8201;0.23&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; vs. 1.08&amp;#8201;&amp;#177;&amp;#8201;0.73&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; on non-cyclone vs. cyclonic days, respectively. In the most aged air masses (NO&lt;sub&gt;x&lt;/sub&gt;/NO&lt;sub&gt;y&lt;/sub&gt;&amp;#8201;&lt;&amp;#8201;0.5), background OA over DM increased by a factor of ~&amp;#8201;4, from 0.93&amp;#8201;&amp;#177;&amp;#8201;0.33&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; to 3.70&amp;#8201;&amp;#177;&amp;#8201;0.28&amp;#8201;&amp;#181;g&amp;#8201;sm&lt;sup&gt;&amp;#8722;3&lt;/sup&gt; due to transport from NFR. Furthermore, enhanced partitioning of nitric acid to the aerosol phase was observed during the cyclone episodes, mainly due to increased abundance of gas phase NH&lt;sub&gt;3&lt;/sub&gt;. During the non-cyclone events, &amp;#946;&lt;sub&gt;ext&lt;/sub&gt; displayed strong correlations (&lt;i&gt;r&lt;/i&gt;&amp;#8201;=&amp;#8201;0.71) with OA and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; in NFR and only with OA (&lt;i&gt;r&lt;/i&gt;&amp;#8201;=&amp;#8201;0.70) in DM while correlation of &amp;#946;&lt;sub&gt;ext&lt;/sub&gt; during the cyclone was strongest (&lt;i&gt;r&lt;/i&gt;&amp;#8201;=&amp;#8201;0.86) with NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt; in DM. Mass extinction efficiency values (MEE) values in DM were similar under cyclone (2.85&amp;#8201;&amp;#177;&amp;#8201;0.63&amp;#8201;m&lt;sup&gt;2&lt;/sup&gt;&amp;#8201;g&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) and non-cyclone (2.72&amp;#8201;&amp;#177;&amp;#8201;0.61&amp;#8201;m&lt;sup&gt;2&lt;/sup&gt;&amp;#8201;g&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) days despite the dominant influence of different aerosol species on &amp;#946;&lt;sub&gt;ext&lt;/sub&gt; (non-cyclone: OA, cyclone; NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8722;&lt;/sup&gt;).&lt;/p&gt;

Improved understanding of the optical properties of secondary organic aerosol (SOA) particles is needed to better predict their climate impacts. Here, SOA was produced by reacting 1-methylnaphthalene or longifolene with hydroxyl radicals (OH) under variable ammonia (NH3), nitrogen oxide (NO x ), and relative humidity (RH) conditions. In the presence of NH3 and NO x , longifolene-derived aerosols had relatively high single scattering albedo (SSA) values and low absorption coefficients at 375 nm independent of RH, suggesting that the longifolene SOA is mostly scattering. In 1-methylnaphthalene experiments, the resulting SSA and SOA mass absorption coefficient (MACorg) values suggest the formation of light-absorbing SOA, and the addition of high NO x and high NH3 enhanced the SOA absorption. Under intermediate-NO x dry conditions, the MACorg values increased from 0.13 m2 g–1 in NH3-free conditions to 0.28 m2 g–1 in high-NH3 conditions. Under high-NH3 conditions, the MACorg value further increased to 0.36 m2 g–1 with an increase in RH. Under dry high-NO x conditions, the MACorg value increased from 0.42 to 0.67 m2 g–1 with the addition of NH3, while with elevated RH, the MACorg value reached 0.70 m2 g–1. The time series of MACorg showed increasing trends only in the presence of NH3. Composition analysis of SOA suggests that organonitrates, nitroorganics, and other nitrogen-containing organic compounds (NOCs) are potential chromophores in the 1-methylnaphthalene SOA. Significant formation of NOCs was observed in the presence of high-NO x and NH3 and was enhanced under elevated RH.