Abstract
Nitrous acid (HONO), nitric acid (HNO(3)), and organic aerosol were measured simultaneously atop an 18-story tower in Houston, TX during August and September of 2006. HONO and HNO(3) were measured using a mist chamber/ion chromatographic technique, and aerosol size and chemical composition were determined using an Aerodyne quadrupole aerosol mass spectrometer. Observations indicate the potential for a new HONO formation pathway: heterogeneous conversion of HNO(3) on the surface of primary organic aerosol (POA). Significant HONO production was observed, with an average of 0.97 ppbv event(-1) and a maximum increase of 2.2 ppb in 4 h. Nine identified events showed clear HNO(3) depletion and well-correlated increases in both HONO concentration and POA-dominated aerosol surface area (SA). Linear regression analysis results in correlation coefficients (r(2)) of 0.82 for HONO/SA and 0.92 for HONO/HNO(3). After correction for established HONO formation pathways, molar increases in excess HONO (HONO(excess)) and decreases in HNO(3) were nearly balanced, with an average HONO(excess)/HNO(3) value of 0.97. Deviations from this mole balance indicate that the residual HNO(3) formed aerosol-phase nitrate. Aerosol mass spectral analysis suggests that the composition of POA could influence HONO production. Several previously identified aerosol-phase PAH compounds were enriched during events, suggesting their potential importance for heterogeneous HONO formation. (C) 2008 Elsevier Ltd. All rights reserved.