AN EVALUATION OF THE MECHANISM OF NITROUS-ACID FORMATION IN THE URBANATMOSPHERE

Nitrous acid (HONO) has been observed to build in the atmosphere of ci ties during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urb an atmospheres, such as modeling of urban O3 formation. This paper rev iews the available information on measurements of HONO in the atmosphe re and suggest mechanisms of HONO formation. The most extensive atmosp heric measurement databases are used to investigate the relations betw een HONO and potential precursors. Based on these analyses, the nightt ime HONO concentrations are found to correlate best with the product o f NO, NO2 and H2O concentrations, or possibly the NO, NO2, H2O, and ae rosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N2O3 with moist aerosols (or other surfaces) to form two H ONO molecules. Theoretical considerations of the equilibrium constant for N2O3 formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in u rban atmospheres. For photochemical modeling purposes, a relation is d erived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO2 + H2O --> 2 HONO with a rate constant of 1.6 8 x 10(-17) e6348/T (ppm-2 min-1). This rate constant is based on an a nalysis of ambient measurements of HONO, NO, NO2 and H2O, with a tempe rature dependence from the equilibrium constant for formation of N2O3. Photochemical grid modeling is used to investigate the effects of thi s relation on simulated HONO and O3 concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the propo sed relation results in HONO concentrations consistent with ambient me asurements. Furthermore, the relation represents a conservative modeli ng approach because HONO production is effectively confined to the mod el surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived.