Modeling Secondary Organic Aerosol Tracers and Tracer-to-SOA Ratios for Monoterpenes and Sesquiterpenes Using a Chemical Transport Model.

The community multiscale air quality (CMAQ) model was modified to simulate secondary organic aerosol (SOA) formation from five explicit (α-pinene, β-pinene, d-limonene, Δ 3 -carene, and sabinene) and one lumped monoterpene (MT) species and sesquiterpenes (SQTs). The contribution of each oxidation pathway [including OH, O 3 , NO 3 , and O( 3 P)] was explicitly tracked in the SOA module. Three MT SOA tracers (pinic acid, PA; pinonic acid, PNA; and 3-methyl-1,2,3-butanetricarboxylic acid, MBTCA) and one SQT SOA tracer (β-caryophyllinic acid, BCARYA) were modeled to assess the tracer-to-SOA ratios ( f SOA ) for ambient SOA estimation. Good model performance for BCARYA and MBTCA and reasonable agreement between model predictions and observations of PA and PNA were achieved. The modeled daily f SOA showed significant variations, suggesting that using the chamber-derived constant f SOA could lead to large errors in estimating terpene SOA. Among the four tracers, MBTCA and BCARYA were more appropriate for tracking MT and SQT SOA due to their nonvolatility. Their f SOA values mainly depend on the organic aerosol loading and could be approximated using simple power-law equations. In addition, equations directly linking the tracer concentrations to the corresponding SOA concentrations were proposed and could lead to good SOA estimations. This work provides new insights into the formation of the key MT and SQT SOA tracers and would allow better assessments of the biogenic emissions to regional and global aerosol burden.