Long-term observation of atmospheric nitrous acid (HONO) and its implication to local NO2 levels in Shanghai, China

Abstract Long-term observation of atmospheric nitrous acid (HONO) was performed from August 2010 to June 2012 in urban area of Shanghai, China. In total, valid data for nineteen months were obtained. The monthly average HONO mixing ratio varied from 0.60 ppb in February 2011 to 1.20 ppb in December 2010, but no obvious monthly or seasonal variations were observed. The mean mixing ratio of HONO was obtained as 0.92 ± 0.57 ppb, with average HONO mixing ratio of 1.08 ± 0.63 ppb for night-time, and 0.76 ± 0.44 ppb for daytime. It was found that HONO mixing ratio is dependent on NO 2 mixing ratio both in daytime and at night. Besides, high HONO level during daytime implicates that strong daytime sources must be active in Shanghai. The ratio of HONO to NO 2 varied from 0.3% to 42.3% with a typical error of 1% and more than 99% of them below 17%. When NO 2 mixing ratio was above 30 ppb, the HONO/NO 2 ratio (HONO/NO 2 averaged over 5.0 ppb intervals of NO 2 mixing ratio) was nearly constant at 3.3 ± 0.2%. When the NO 2 mixing ratio is below 30 ppb, however, the HONO/NO 2 ratio appeared to increase as the NO 2 mixing ratio decreased. Additionally, HONO/NO 2 ratio depends obviously on relative humidity at night, while there is no obvious correlation between them during daytime. The average conversion frequency of HONO in sixteen selected days was about 0.007 h −1 under different NO 2 pollution scenarios and higher conversion frequency tends to be occurred at lower NO 2 levels.