Observation of nighttime nitrous acid (HONO) formation at a non-urban site during PRIDE-PRD2004 in China

Though the importance of HONO as an OH precursor has been recognized for years, its chemical formation pathway is still not well understood. This inhibited the simulations of HONO and observation-based formation rates provided an alternative for the air quality models. However, HONO formation rate derived from certain period may be significantly influenced by uncertainties in transport and emission processes. In this study the use of large sample and scaling methods were recommended in the calculation of HONO formation rate. During the Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD2004), good correlations between HONO and NO2 were found supporting the involvement of NO2 in HONO formation. An average NO2-to-HONO nighttime conversion rate CHONO of was derived at a non-urban site Xinken. This conversion rate was comparable to other field measurements and could not be explained by gas phase reactions only. If assumed that HONO was formed only on the ground surface, the observed conversion rate could be explained by the reactions of NO2 on ground surfaces only if three deposited NO2 lead to one HONO released. The emission factor of HONO and its sampling interferences during the measurements were also evaluated in this article.