Exploring the atmospheric budget of nitrous acid at a rural site in southern China

Nitrous acid (HONO) is an important trace gas in the atmosphere due to its contribution to the cycles of nitrogen oxides (NOx) and hydrogen oxides (HOx). We performed HONO measurements during the PRIDE-PRD2006 campaign in the Pearl River Delta region 60~km north of Guangzhou, China, for 4~weeks in June 2006. HONO was measured by a LOPAP in-situ instrument which was setup in one of the campaign supersites along with a variety of instruments measuring hydroxyl radicals, trace gases, aerosols, and meteorological parameters. The comprehensive datasets allow to investigate the budgets of HONO during day and night time. Average nighttime HONO mixing ratios were above 1~ppb. The nighttime build-up of HONO can be attributed to the heterogeneous NO₂ to HONO conversion on surfaces and the OH+NO reaction. In addition to the high nighttime mixing ratios, measured noontime values were in the range of 50 to 400~ppt indicate the existence of a daytime source higher than the OH+NO->HONO reaction. Using the simultaneously recorded OH, NO, and HONO photolysis frequency, this additional daytime source was calculated to be 0.77~ppb~h^-1 on average. This value compares well to previous measurements in other environments. Our analysis of the source strength shows that the photolysis of adsorbed HNO₃ is a possible explanation for the additional daytime HONO source.