Modeling organic aerosols in a megacity: comparison of simple and complex representations of the volatility basis set approach

Abstract

The Weather Research and Forecasting model coupled with chemistry (WRF-Chem) is modified to include a volatility basis set (VBS) treatment of secondary organic aerosol formation. The VBS approach, coupled with SAPRC-99 gas-phase chemistry mechanism, is used to model gas-particle partitioning and multiple generations of gas-phase oxidation of organic vapors. In addition to the detailed 9-species VBS, a simplified mechanism using 2 volatility species (2-species VBS) is developed and tested for similarity to the 9-species VBS in terms of both mass and oxygen-to-carbon ratios of organic aerosols in the atmosphere. WRF-Chem results are evaluated against field measurements of organic aerosols collected during the MILAGRO 2006 campaign in the vicinity of Mexico City. The simplified 2-species mechanism reduces the computational cost by a factor of 2 as compared to 9-species VBS. Both ground site and aircraft measurements suggest that the 9-species and 2-species VBS predictions of total organic aerosol mass as well as individual organic aerosol components including primary, secondary, and biomass burning are comparable in magnitude. In addition, oxygen-to-carbon ratio predictions from both approaches agree within 25 %, providing evidence that the 2-species VBS is well suited to represent the complex evolution of organic aerosols. Model sensitivity to amount of anthropogenic semi-volatile and intermediate volatility (S/IVOC) precursor emissions is also examined by doubling the default emissions. Both the emission cases significantly under-predict primary organic aerosols in the city center and along aircraft flight transects. Secondary organic aerosols are predicted reasonably well along flight tracks surrounding the city, but are consistently over-predicted downwind of the city. Also, oxygen-to-carbon ratio predictions are significantly improved compared to prior studies by adding 15 % oxygen mass per generation of oxidation; however, all modeling cases still under-predict these ratios downwind as compared to measurements, suggesting a need to further improve chemistry parameterizations of secondary organic aerosol formation

Modeling the meteorological and chemical effects of secondary organic aerosols during an EUCAARI campaign

“…In the version of COSMO-ART applied for the current study, there is the option of choosing between two different SOA modules: The SORGAM which is the default SOA module of COSMO-ART, analytically described in Schell et al (2001) and used in several applications (Grell et al, 2005;Stern et al, 2008;Elleman and Covert, 2009;Zhao et al, 2012;Chuang et al, 2011;Herwehe et al, 2011) and the VBS, which is the newly added SOA module in COSMO-ART, based on the framework proposed by Donahue et al (2006) and employed in other models (Robinson et al, 2007;Lane et al, 2008;Murphy and Pandis, 2009;Tsimpidi et al, 2010Tsimpidi et al, , 2011Shrivastava et al, 2011).…”

“…An efficient way to describe the chemical evolution of the numerous semi-volatile organic compounds in models is to treat them as lumped species that span a range of effective saturation concentrations (Donahue et al, 2006). Recent model applications using different volatility basis set (VBS) approaches confirm their ability to provide improved predictions of OA concentration (Murphy and Pandis., 2009;Tsimpidi et al, 2010Tsimpidi et al, , 2011Shrivastava et al, 2011;Farina et al, 2010;Murphy et al, 2011;Bergström et al, 2012).…”

Verification of Chemical Transport Models for PM2.5 Chemical Composition Using Simultaneous Measurement Data over Japan

“…First, the VBS model considers direct emissions of SVOCs and IVOCs from combustion sources and calculates the partitioning of primary OA (POA), SVOCs, and IVOCs by classifying these species on the basis of effective saturation concentrations ranging from 10 -2 to 10 6 µg m -3 . Emission profiles of POA, SVOCs, and IVOCs in the VBS model were taken from Shrivastava et al (2011). Second, the VBS model treats chemical aging of directly emitted SVOCs and IVOCs, as well as chemical aging of SVOCs and IVOCs generated by oxidation of volatile organic compounds (VOCs).…”

Sources and Characteristics of Summertime Organic Aerosol in the Colorado Front Range: Perspective from Measurements and WRF-Chem Modeling

“…Similar to other aerosol particles, SOA particles deteriorate air quality and visibility and impact the climate directly through absorption and scattering of radiation and indirectly through interactions with clouds (Monks et al, 2009). Despite recent advances in the measurement and modeling aspects of SOA and their precursors (e.g., Donahue et al, 2006;Ervens and Volkamer 15 2010;Hodzic et al, 2010a;de Gouw et al, 2011;Hodzic and Jimenez 2011;Shrivastava et al, 2011;Ahmadov et al, 2012;Isaacman et al, 2012;Yatavelli et al, 2012;Ehn et al, 2014;Ensberg et al, 2014;Fast et al, 2014;Lopez-Hilfiker et al, 2014;Williams et al, 2014), the full extent of SOA sources, formation processes, and therefore their impact on air quality, human health, and climate are not fully understood.…”