Heterogeneous conversion of nitric acid to nitrous acid on the surface of primary organic aerosol in an urban atmosphere

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.