Seasonal Differences in Atmospheric Nitrous Acid near Mediterranean Urban Areas

The major objective of this paper is to provide insights to sources and sinks of nitrous acid in urban areas, and their seasonal dependency on meteorology, photochemistry and long range transport. With this aim, nitrous acid (HONO) mixing ratios and other compounds were measured in Ashdod (south of Tel Aviv, Israel), a typical Mediterranean urban area. Statistical data analysis revealed the expected correlation between HONO and nitrogen oxides during the autumn campaign when HONO sources appeared to be traffic-, harbor-, and industry-related. Conversely, during summer HONO and NO2 were no longer correlated: NO2 at nighttime was probably deposited onto surfaces, soil and plants, whereas HONO at daytime was likely destroyed photolytically contributing to the OH concentration. Photolysis was expected to be the dominant HONO sink at daytime, especially during the summer period. Using modeled photolytical HONO lifetimes we estimate the magnitude of heterogeneous and/or organic electron transfer source reactions of HONO as 6-8 ppbv/h.