| Field observations show daytime HONO levels in urban, rural and remote environments are greater than those expected at photostationary state, that is, balance between production by NO+OH reaction and loss by UV-photolysis and OH-oxidation. Studies have interpreted measurements of [HONO]Obs > [HONO]PSS - or equivalently, the rate of HONO loss exceeding that of production - as evidence of a missing, sunlight-driven HONO source. Formation rate inferred from assuming photostationarity indicate a significant source of HONO, which photolyzes to yield OH. Moreover, depending on the mechanism, it may represent a pathway by which deposited nitrogen oxides are repartitioned back into the atmosphere in reactive form.;The accumulation of HONO beneath the nocturnal boundary layer initiates photochemistry in the early morning prior to other HOx precursors. Previous studies have estimated nighttime HONO production rate by attributing the increase in HONO:NOx solely to heterogeneous HONO formation, while treating NOx as an invariant. Moreover, because ambient HONO:NO x exceed what is observed in automobile exhaust, combustion sources are discounted.;In May of 2009, we observed HONO and NO2 mixing ratios in Houston during the SHARP campaign. We demonstrate -- using a chemical box model -- that photostationary state during daytime is not fully established. The reaction/transport time since emission from automobiles is short relative to the lifetime of HONO. The result of assuming PSS is a drastic over-estimation of the so-called missing HONO source. At night, we show that NOx oxidation and emission are significant, thus, NOx cannot be treated as a conservative tracer to infer secondary HONO production.;Nearly-continuous observations of HONO and NO2 at Harvard Forest from December 2010 to December 2011 reveal daytime HONO levels that are comparable to what is expected from known chemistry and much lower than has been reported in similar environments by different measurement techniques. Moreover, HONO fluxes are always below detection limit, indicating daytime HONO production contributes negligibly to the HOx and NOx budgets of the overlying atmosphere at Harvard Forest. Nighttime HONO enhancement is observed, but high night-to-night variability in HONO:NO 2 that is not reasonably explained by the trends in HONO and NO 2 fluxes, suggest a non-NO2, non-ground/canopy-surface related HONO source. |
