Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron‐sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid‐catalyzed multiphase chemistry of isoprene epoxydiols, particle‐phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process‐related interactions, so that these processes can be accurately represented in atmospheric chemistry‐climate models.

Particles composed of secondary organic material (SOM) are abundant in the lower troposphere. The viscosity of these particles is a fundamental property that is presently poorly quantified yet required for accurate modeling of their formation, growth, evaporation, and environmental impacts. Using two unique techniques, namely a “bead-mobility” technique and a “poke-flow” technique, in conjunction with simulations of fluid flow, the viscosity of the water-soluble component of SOM produced by α -pinene ozonolysis is quantified for 20- to 50-μm particles at 293–295 K. The viscosity is comparable to that of honey at 90% relative humidity (RH), similar to that of peanut butter at 70% RH, and at least as viscous as bitumen at ≤30% RH, implying that the studied SOM ranges from liquid to semisolid or solid across the range of atmospheric RH. These data combined with simple calculations or previous modeling studies are used to show the following: ( i ) the growth of SOM by the exchange of organic molecules between gas and particle may be confined to the surface region of the particles for RH ≤ 30%; ( ii ) at ≤30% RH, the particle-mass concentrations of semivolatile and low-volatility organic compounds may be overpredicted by an order of magnitude if instantaneous equilibrium partitioning is assumed in the bulk of SOM particles; and ( iii ) the diffusivity of semireactive atmospheric oxidants such as ozone may decrease by two to five orders of magnitude for a drop in RH from 90% to 30%. These findings have possible consequences for predictions of air quality, visibility, and climate.

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas–particle equilibrium and (ii) have a short particle-phase lifetime (∼2–4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment. Significance We present online field observations of the speciated molecular composition of organic nitrates in ambient atmospheric particles utilizing recently developed high-resolution MS-based instrumentation. We find that never-before-identified low-volatility organic species, which are highly functionalized, explain a major fraction of the total particle nitrate mass measured by the traditional aerosol mass spectrometer. An observationally constrained box model shows that these organic nitrates are likely derived from oxidation of biogenic hydrocarbons and persist in the particle phase for only a few hours. Given their high rate of loss, their fates have significant implications for the budgets of secondary organic aerosol particles and nitrogen oxides but are currently unknown.

Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM₁) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM₁(with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total nonmethane organic compounds, and PM₁from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM₁emission estimate (1530 ± 570 Gg yr⁻¹) is over 3 times that of the NEI PM_2.5estimate and is also higher than the PM_2.5emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions.

Atmospheric brown carbon (BrC) is a significant contributor to light absorption and climate forcing. However, little is known about a fundamental relationship between the chemical composition of BrC and its optical properties. In this work, light-absorbing secondary organic aerosol (SOA) was generated in the PNNL chamber from toluene photo-oxidation in the presence of NO_x (Tol-SOA). Molecular structures of BrC components were examined using nanospray desorption electrospray ionization (nano-DESI) and liquid chromatography (LC) combined with UV/Vis spectroscopy and electrospray ionization (ESI) high-resolution mass spectrometry (HRMS). The chemical composition of BrC chromophores and the light absorption properties of toluene SOA (Tol-SOA) depend strongly on the initial NO_x concentration. Specifically, Tol-SOA generated under high-NO_x conditions (defined here as initial NO_x/toluene of 5/1) appears yellow and mass absorption coefficient of the bulk sample (MAC_bulk@365 nm = 0.78 m² g⁻¹) is nearly 80 fold higher than that measured for the Tol-SOA sample generated under low-NO_x conditions (NO_x/toluene < 1/300). Fifteen compounds, most of which are nitrophenols, are identified as major BrC chromophores responsible for the enhanced light absorption of Tol-SOA material produced in the presence of NO_x. The integrated absorbance of these fifteen chromophores accounts for 40–60% of the total light absorbance by Tol-SOA at wavelengths between 300 nm and 500 nm. The combination of tandem LC-UV/Vis-ESI/HRMS measurements provides an analytical platform for predictive understanding of light absorption properties by BrC and their relationship to the structure of individual chromophores. General trends in the UV/Vis absorption by plausible isomers of the BrC chromophores were evaluated using theoretical chemistry calculations. The molecular-level understanding of BrC chemistry is helpful for better understanding the evolution and behavior of light absorbing aerosols in the atmosphere.

The chemical composition of secondary organic aerosol (SOA) particles, formed by the dark ozonolysis of α-pinene, was characterized by a high-resolution time-of-flight aerosol mass spectrometer. The experiments were conducted using a continuous-flow chamber, allowing the particle mass loading and chemical composition to be maintained for several days. The organic portion of the particle mass loading was varied from 0.5 to >140 μg/m³ by adjusting the concentration of reacted α-pinene from 0.9 to 91.1 ppbv. The mass spectra of the organic material changed with loading. For loadings below 5 μg/m³ the unit-mass-resolution m/z 44 (CO₂⁺) signal intensity exceeded that of m/z 43 (predominantly C₂H₃O⁺), suggesting more oxygenated organic material at lower loadings. The composition varied more for lower loadings (0.5 to 15 μg/m³) compared to higher loadings (15 to >140 μg/m³). The high-resolution mass spectra showed that from >140 to 0.5 μg/m³ the mass percentage of fragments containing carbon and oxygen (C_xH_yO_z⁺) monotonically increased from 48% to 54%. Correspondingly, the mass percentage of fragments representing C_xH_y⁺ decreased from 52% to 46%, and the atomic oxygen-to-carbon ratio increased from 0.29 to 0.45. The atomic ratios were accurately parameterized by a four-product basis set of decadal volatility (viz. 0.1, 1.0, 10, 100 μg/m³) employing products having empirical formulas of C₁H_1.32O_0.48, C₁H_1.36O_0.39, C₁H_1.57O_0.24, and C₁H_1.76O_0.14. These findings suggest considerable caution is warranted in the extrapolation of laboratory results that were obtained under conditions of relatively high loading (i.e., >15 μg/m³) to modeling applications relevant to the atmosphere, for which loadings of 0.1 to 20 μg/m³ are typical. For the lowest loadings, the particle mass spectra resembled observations reported in the literature for some atmospheric particles.