Simulation and analysis of secondary organic aerosol dynamics in the South Coast Air Basin of California

Abstract

[1] The dynamics of secondary organic aerosol (SOA) formation are analyzed using a species-resolved SOA model for the South Coast Air Basin of California (SoCAB). Updated versions of the Caltech Atmospheric Chemistry Mechanism (CACM) and the Model to Predict the Multiphase Partitioning of Organics (MPMPO) are integrated with the CIT airshed model. The simulations are performed using input data from the 8-9 September 1993 episode. Results show that urban areas with major volatile organic compound (VOC) emission sites experience peaks in SOA levels during morning hours. Downwind locations, such as Azusa and Claremont, experience sustained levels of high SOA concentrations in comparison with coastal areas such as central Los Angeles and Long Beach. Concentrations of condensible organics are higher in inland locations compared to those in coastal locations because of high oxidation capacity and transport of pollutants. Furthermore, SOA constitutes up to 30% of simulated organic particulate matter at inland locations, with maximum contributions occurring during afternoon hours. Anthropogenic sources contribute over 90% of simulated SOA at most locations in the basin. Oxidation products of aromatic compounds from anthropogenic sources constitute over 70% of total simulated SOA. Sensitivity runs indicate strong dependence of SOA on VOC emissions and temperature. Overall, model predictions are in qualitative agreement with recent observations in the SoCAB.

Apportionment of Primary and Secondary Organic Aerosols in Southern California during the 2005 Study of Organic Aerosols in Riverside (SOAR-1)

“…Recently determined SOA yields (e.g., ref 55) close the gap somewhat, but a significant discrepancy still remains. Scaling up the modeling SOA estimates (47)(48)(49)(50)(51) by the measurement/model discrepancies summarized in ref 31 would produce SOA fractions of the order of those in this study. Additionally, estimates based on EC-tracer method calculations could be biased low due to difficulties estimating (OC/EC)p with the ambient regression method.…”

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities

“…First, organic particulate matter over megacities is most likely more complicated than toluene-derived SOM. Toluene and other aromatics can account for a large fraction of nonmethane hydrocarbon emission in urban environments (Singh et al, 1985;Na et al, 2005;Suthawaree et al, 2012), and toluene and aromatics are thought to be one of the main sources of SOM particles in urban environments (Odum et al, 1997;Schauer et al, 2002a, b;Vutukuru et al, 2006;Velasco et al, 2007Velasco et al, , 2009de Gouw et al, 2008;Gentner et al, 2012;Liu et al, 2012;Hayes et al, 2015). Nevertheless, large alkanes and unspeciated nonmethane organic gases also likely play a role in SOM formation in urban environments.…”

Does the POA–SOA split matter for global CCN formation?

“…The relative contributions of POA and SOA to the total OA remains an open issue as different studies report wide ranges of estimates (Kanakidou et al, 2005;Hallquist et al, 2009). Traditionally, models have tended to predict a predominance of POA over SOA (Chung and Seinfeld, 2002;Kanakidou et al, 2005;Pun et al, 2003;Vutukuru et al, 2006), but measurement studies show striking evidence of SOA dominance observed at various locations, even in heavily urbanized locations (Zhang et al, 2005de Gouw et al, 2005;Volkamer et al, 2006). Recent work has suggested that many global models are underestimating OA sources (Heald et al, 2010;Spracklen et al, 2011b) that appear to be SOA, and recent improvements in SOA modeling efforts have addressed this discrepancy (Jathar et al, 2011;Pye and Seinfeld, 2010).…”