<p><strong>Abstract.</strong> The organic aerosol (OA) concentration is simulated in the Guanzhong Basin, China from 23 to 25 April 2013 utilizing the WRF-CHEM model. Two approaches are used to predict OA concentrations: (1) a traditional secondary organic aerosol (SOA) module; (2) a non-traditional SOA module including the volatility basis-set modeling method in which primary organic aerosol (POA) is assumed to be semivolatile and photochemically reactive. Generally, the spatial patterns and

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... mporal variations of the calculated hourly near-surface ozone and fine particle matters agree well with the observations in Xi'an and surrounding areas. The model also yields reasonable distributions of daily PM_2.5 and elemental carbon (EC) compared to the filter measurements at 29 sites in the basin. Filter-measured organic carbon (OC) and EC are used to evaluate OA, POA, and SOA using the OC<span class="thinspace"></span>∕<span class="thinspace"></span>EC ratio approach. Compared with the traditional SOA module, the non-traditional module significantly improves SOA simulations and explains about 88<span class="thinspace"></span>% of the observed SOA concentration. Oxidation and partitioning of POA treated as semivolatile constitute the most important pathway for the SOA formation, contributing more than 75<span class="thinspace"></span>% of the SOA concentrations in the basin. Residential emissions are the dominant anthropogenic OA source, constituting about 50<span class="thinspace"></span>% of OA concentrations in urban and rural areas and 30<span class="thinspace"></span>% in the background area. The OA contribution from transportation emissions decreases from 25<span class="thinspace"></span>% in urban areas to 20<span class="thinspace"></span>% in the background area, and the industry emission OA contribution is less than 6<span class="thinspace"></span>%.</p>