Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E(abs)) and mixing state are reported for two California regions. The observed E(abs) is small-6% on average at 532 nm-and increases weakly with photochemical aging. The E(abs) is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E(abs) for BC are possible.

Abstract. An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative Effects Study (CARES) that took place in northern California in June 2010. We present results obtained at Cool (denoted as the T1 site of the project) in the foothills of the Sierra Nevada Mountains, where intense biogenic emissions are periodically mixed with urban outflow transported by daytime southwesterly winds from the Sacramento metropolitan area. During this study, the average mass loading of submicrometer particles (PM 1 ) was 3.0 µg m −3 , dominated by organics (80 %) and sulfate (9.9 %). The organic aerosol (OA) had a nominal formula of C 1 H 1.38 N 0.004 O 0.44 , thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Two distinct oxygenated OA factors were identified via Positive matrix factorization (PMF) of the high-resolution mass spectra of organics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a surrogate for secondary OA (SOA) influenced by biogenic emissions whereas the less oxidized LO-OOA (O/C = 0.42) was found to represent SOA formed in photochemically processed urban emissions. LO-OOA correlated strongly with ozone and MO-OOA correlated well with two 1st generation isoprene oxidation products (methacrolein and methyl vinyl ketone), indicating that both SOAs were relatively fresh. A hydrocarbon like OA (HOA) factor was also identified, representing primary emissions mainly due to local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for 91 % of the total OA mass and 72 % of the PM 1 mass observed at Cool. Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool) were identified using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The average PM 1 mass loading was considerably higher in urban plumes than in air masses dominated by biogenic SOA. The change in OA mass relative to CO ( OA/ CO) varied in the range of 5-196 µg m −3 ppm −1 , reflecting large variability in SOA production. The highest OA/ CO was reached when air masses were dominated by anthropogenic emissions in the presence of a high concentration of biogenic volatile organic compounds (BVOCs). This ratio, which was 97 µg m −3 ppm −1 on average, was much higher than when urban plumes arrived in a low BVOC Published by Copernicus Publications on behalf of the European Geosciences Union. 8132A. Setyan et al.: Characterization of submicron particles environment (∼36 µg m −3 ppm −1 ) or during other periods dominated by biogenic SOA (35 µg m −3 ppm −1 ). These results demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors.

A series of m-xylene/NOx experiments were conducted in the new Bourns College of Engineering-Center for Environmental Research and Technology dual 90 m3 indoor smog chamber to elucidate the role of NOx on the secondary organic aerosol (SOA) formation potential of m-xylene. The results presented herein demonstrate a clear dependence of m-xylene SOA formation potential on NOx, particularly at atmospherically relevant organic aerosol concentration. Experiments with lower NOx levels generated considerably more organic aerosol mass than did experiments with higher NOx levels when reacted m-xylene was held constant. For example, SOA formation from approximately 150 microg m(-3) reacted m-xylene produced 0.6-9.3 microg m(-3) aerosol mass for NOx concentrations ranging from 286 to 10 ppb. The increase in SOA formation was not attributable to changes in ozone and nitrate concentration. A general discussion about possible influences of NOx on SOA formation for this system is included.

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Organonitrate (ON) groups are thought to be important substituents in secondary organic aerosols (SOAs). Model simulations and laboratory studies indicate a large fraction of ON groups in aerosol particles, but much lower quantities are observed in the atmosphere. Hydrolysis of ON groups in aerosol particles has been proposed recently to account for this discrepancy. To test this hypothesis, we simulated formation of ON molecules in a reaction chamber under a wide range of relative humidity (RH) (0 to 90%). The mass fraction of ON groups (5 to 20% for high-NO x experiments) consistently decreased with increasing RH, which was best explained by hydrolysis of ON groups at a rate of 4 day −1 (lifetime of 6 h) for reactions under RH greater than 20%. In addition, we found that secondary nitrogen-containing molecules absorb light, with greater absorption under dry and high-NO x conditions. This work provides the first evidence for particle-phase hydrolysis of ON groups, a process that could substantially reduce ON group concentration in atmospheric SOAs.

The effects of NOx on the volatility of the secondary organic aerosol (SOA) formed from isoprene photooxidation are investigated in environmental chamber experiments. Two types of experiments are performed. In HO2-dominant experiments, organic peroxy radicals (RO2) primarily react with HO2. In mixed experiments, RO2 reacts through multiple pathways, including with NO, NO2, and HO2. The volatility and oxidation state of isoprene SOA are sensitive to and exhibit a nonlinear dependence on NOx levels. Depending on the NOx levels, the SOA formed in mixed experiments can be of similar or lower volatility compared to that formed in HO2-dominant experiments. The dependence of SOA yield, volatility, and oxidation state on the NOx level likely arises from gas-phase RO2 chemistry and succeeding particle-phase oligomerization reactions. The NOx level also plays a strong role in SOA aging. While the volatility of SOA in mixed experiments does not change substantially over time, SOA becomes less volatile and more oxidized as oxidation progresses in HO2-dominant experiments.

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Primary organic aerosol (POA) and associated vapors can play an important role in determining the formation and properties of secondary organic aerosol (SOA). If SOA and POA are miscible, POA will significantly enhance SOA formation and some POA vapor will incorporate into SOA particles. When the two are not miscible, condensation of SOA on POA particles forms particles with complex morphology. In addition, POA vapor can adsorb to the surface of SOA particles increasing their mass and affecting their evaporation rates. To gain insight into SOA/POA interactions we present a detailed experimental investigation of the morphologies of SOA particles formed during ozonolysis of α-pinene in the presence of dioctyl phthalate (DOP) particles, serving as a simplified model of hydrophobic POA, using a single-particle mass spectrometer. Ultraviolet laser depth-profiling experiments were used to characterize two different types of mixed SOA/DOP particles: those formed by condensation of the oxidized α-pinene products on size-selected DOP particles and by condensation of DOP on sizeselected α-pinene SOA particles. The results show that the hydrophilic SOA and hydrophobic DOP do not mix but instead form layered phases. In addition, an examination of homogeneously nucleated SOA particles formed in the presence of DOP vapor shows them to have an adsorbed DOP coating layer that is ∼4 nm thick and carries 12% of the particles mass. These results may have implications for SOA formation and behavior in the atmosphere, where numerous organic compounds with various volatilities and different polarities are present.secondary organic aerosol | single-particle mass spectrometry | morphology A nthropogenic aerosol particles in large urban areas are implicated in air quality and health-related problems and are a source of significant uncertainty in our current understanding of climate change at regional and global scales (1). Recent field studies indicate that organic aerosols (OA) constitute anywhere between 20 and 90% of the total dry fine particulate mass (2). Secondary organic aerosol (SOA), generated from the oxidation reaction products of volatile and semivolatile organic compounds (SVOC), is thought to constitute between 64 and 95% of the total OA mass (3). The development of SOA formation models at present represent a major research activity aimed at reconciling the significant differences between field measured SOA concentrations and those predicted by current SOA models in both polluted urban and regional areas (4). Although a number of attempts have been made to explain this discrepancy, none have yet closed the gap between predicted and observed SOA concentrations (4-6).SOA formation can be enhanced in the presence of primary organic aerosols (POA) that serve as additional organic mass to absorb greater amounts of oxidized organic molecules, thus lowering their vapor pressures and increasing SOA formation yields. For POA to absorb SOA, a single well-mixed SOA þ POA phase must form. However, Song et al. (7) found that SOA format...