Chemical transport model simulations of organic aerosol in southern California: model evaluation and gasoline and diesel source contributions

Abstract

Gasoline- and diesel-fueled engines are ubiquitous sources of air pollution in urban environments. They emit both primary particulate matter and precursor gases that react to form secondary particulate matter in the atmosphere. In this work, we updated the organic aerosol module and organic emissions inventory of a three-dimensional chemical transport model, the Community Multiscale Air Quality Model (CMAQ), using recent, experimentally derived inputs and parameterizations for mobile sources. The updated model included a revised volatile organic compound (VOC) speciation for mobile sources and secondary organic aerosol (SOA) formation from unspeciated intermediate volatility organic compounds (IVOCs). The updated model was used to simulate air quality in southern California during May and June 2010, when the California Research at the Nexus of Air Quality and Climate Change (CalNex) study was conducted. Compared to the Traditional version of CMAQ, which is commonly used for regulatory applications, the updated model did not significantly alter the predicted organic aerosol (OA) mass concentrations but did substantially improve predictions of OA sources and composition (e.g., POA-SOA split), as well as ambient IVOC concentrations. The updated model, despite substantial differences in emissions and chemistry, performed similar to a recently released research version of CMAQ (Woody et al., 2016) that did not include the updated VOC and IVOC emissions and SOA data. Mobile sources were predicted to contribute 30-40 % of the OA in southern California (half of which was SOA), making mobile sources the single largest source contributor to OA in southern California. The remainder of the OA was attributed to non-mobile anthropogenic sources (e.g., cooking, biomass burning) with biogenic sources contributing to less than 5 % to the total OA. Gasoline sources were predicted to contribute about 13 times more OA than diesel sources; this difference was driven by differences in SOA production. Model predictions highlighted the need to better constrain multi-generational oxidation reactions in chemical transport models.

Figure 1

Total emissions from 4 May to 30 June 2010 for POA, BTEX (aromatics), ALK5 (long alkanes), and IVOCs for gasoline and diesel sources in the Los Angeles and Orange counties for the three OA models: Traditional, VBS, and VBS-IVOC.

Figure 2

Averaged predictions from the VBS-IVOC model for (a) total OA (μg m⁻³), (b) POA fraction, (c) SOA fraction, (d) total gasoline OA (μg m⁻³), (e) total diesel OA (μg m⁻³), (f) biogenic SOA (μg m⁻³), and (g) other OA (μg m⁻³) over southern California.

Figure 3

Scatter plot of VBS-IVOC OA predictions versus 24 h measurements from (a) filters collected at sites in the Chemical Speciation Network (CSN) and (b) HR-AMS measurements at the Pasadena ground site during the CalNex campaign. In panel (a) the model–measurement comparison is for six sites in California (Fresno, Bakersfield, Central Los Angeles, Riverside, El Cajon, and Simi Valley). f.b. is the fractional bias $\left(\frac{1}{N}{\sum }_{i=1}^N\frac{P-M}{\frac{P+M}{2}}\right)$ and f.e. is the fractional error $\left(\frac{1}{N}{\sum }_{i=1}^N\frac{\mid P-M\mid }{\frac{P+M}{2}}\right)$; P is the predicted value, M is the measured value and N is the sample size.

Figure 4

Averaged, normalized composition of OA at the Pasadena ground site as predicted by the Traditional and VBS-IVOC models. Predictions are compared to PMF factors derived from ambient HR-AMS data collected in Pasadena Hayes et al. (2013).

Figure 5

Comparison of predicted and measured campaign-averaged IVOC concentrations at the Pasadena ground site. Measured concentrations are from Zhao et al. (2014). Here, both model predictions and measurements only include primary IVOCs. The predictions of IVOCs include all vapors in equilibrium with POA.

Figure 6

(a) VBS-IVOC predicted campaign-averaged OA concentrations attributable to gasoline and diesel sources at the ground site in Pasadena; the IVOCx1 result for diesel use is from the VBS-IVOC simulation, and the IVOCx3 and IVOCx5 results are from separate sensitivity simulations where IVOC emissions from diesel are scaled by a factor of 3 and 5, respectively, as described in the text. (b) Ratio of gasoline OA to diesel OA over southern California and (c) cumulative distribution functions that show the fractional contribution of gasoline plus diesel OA to total OA in southern California.