Simulations of nitrous acid for the Houston metropolitan area and comparison with data from the Texas Air Quality Study 2006

Nitrous acid (HONO) mixing ratios for the Houston metropolitan area were simulated with the Community Multiscale Air Quality (CMAQ) Model v 4.7 for an episode during the Second Texas Air Quality Study (TexAQS) in August/September 2006 and compared with DOAS measurements at the Moody Tower super site. HONO gas phase formation, direct emissions from mobile sources, as well as heterogeneous formation on urban, leaves, and particle surfaces was taken into account. HONO predictions with older CMAQ versions, which accounted only for gas phase HONO chemistry, resulted in much lower mixing ratios than experimental values. Introduction of emission and heterogeneous sources significantly improved model predictions of HONO, especially during nighttime, when the highest HONO values were observed. The biggest contribution to HONO mixing ratio comes from its formation on urban and leaves surfaces.