Understanding sources of organic aerosol during CalNex-2010 using the CMAQ-VBS

• Published in: Atmospheric chemistry and physics 2016-03, Vol.16 (6), p.4081-4100
• Main Authors: Woody, Matthew C, Baker, Kirk R, Hayes, Patrick L, Jimenez, Jose L, Koo, Bonyoung, Pye, Havala O. T
• Format: Article
• Language: English
• Subjects: Aerosols; Ageing; Aging; Air pollution control; Air quality; Air quality models; Biomass burning; Burning; Carbon; Chemical speciation; chemical-transport model; Climate change; Combustion; Computer simulation; Control; Cooking; Diurnal; Efficiency; Emission inventories; Emissions; Environmental aspects; ENVIRONMENTAL SCIENCES; Frameworks; Gasoline; Hydrocarbons; los-angeles; m-xylene; Mass spectrometry; mass-spectrometer; Meat; Methods; mexico-city; Monitoring; Nitrogen compounds; Organic carbon; Organic compounds; Oxidation; Oxides; Parameterization; particle composition; Photochemicals; Photochemistry; Pollution sources; Ratios; secondary; Secondary aerosols; Simulation; Speciation; Vehicle emissions; VOCs; Volatile organic compounds; Volatility; volatility basis-set; Wood
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-16-4081-2016

Community Multiscale Air Quality (CMAQ) model simulations utilizing the traditional organic aerosol (OA) treatment (CMAQ-AE6) and a volatility basis set (VBS) treatment for OA (CMAQ-VBS) were evaluated against measurements collected at routine monitoring networks (Chemical Speciation Network (CSN) and Interagency Monitoring of Protected Visual Environments (IMPROVE)) and those collected during the 2010 California at the Nexus of Air Quality and Climate Change (CalNex) field campaign to examine important sources of OA in southern California. Traditionally, CMAQ treats primary organic aerosol (POA) as nonvolatile and uses a two-product framework to represent secondary organic aerosol (SOA) formation. CMAQ-VBS instead treats POA as semivolatile and lumps OA using volatility bins spaced an order of magnitude apart. The CMAQ-VBS approach underpredicted organic carbon (OC) at IMPROVE and CSN sites to a greater degree than CMAQ-AE6 due to the semivolatile POA treatment. However, comparisons to aerosol mass spectrometer (AMS) measurements collected at Pasadena, CA, indicated that CMAQ-VBS better represented the diurnal profile and primary/secondary split of OA. CMAQ-VBS SOA underpredicted the average measured AMS oxygenated organic aerosol (OOA, a surrogate for SOA) concentration by a factor of 5.2, representing a considerable improvement to CMAQ-AE6 SOA predictions (factor of 24 lower than AMS). We use two new methods, one based on species ratios (SOA/ΔCO and SOA/Ox) and another on a simplified SOA parameterization, to apportion the SOA underprediction for CMAQ-VBS to slow photochemical oxidation (estimated as 1.5 × lower than observed at Pasadena using −log(NOx : NOy)), low intrinsic SOA formation efficiency (low by 1.6 to 2 × for Pasadena), and low emissions or excessive dispersion for the Pasadena site (estimated to be 1.6 to 2.3 × too low/excessive). The first and third factors are common to CMAQ-AE6, while the intrinsic SOA formation efficiency for that model is estimated to be too low by about 7 × . From source-apportioned model results, we found most of the CMAQ-VBS modeled POA at the Pasadena CalNex site was attributable to meat cooking emissions (48 %, consistent with a substantial fraction of cooking OA in the observations). This is compared to 18 % from gasoline vehicle emissions, 13 % from biomass burning (in the form of residential wood combustion), and 8 % from diesel vehicle emissions. All "other" inventoried emission sources (e.g., industrial, poi