Springtime HONO budget and its impact on the O3 production in Zibo, Shandong, China

Nitrous acid (HONO) is a vital source of OH radicals, nevertheless, the potential sources of HONO are still uncertain. To get an insight into the HONO sources and its role in atmospheric photochemistry, a comprehensive field campaign was conducted in Zibo City of the North China Plain in April 2021. A high average HONO concentration of 0.91 ppb was observed during spring. Direct emission was an important HONO source in Zibo City, with an average contribution of 32%. A calculated NO2 heterogeneous conversion efficiency of 0.37% h−1 indicated its limited contribution to the HONO formation. In addition, an unknown HONO source with an average production rate of 1.28 ppb h−1 was found through the daytime budget analysis, and the correlation analysis suggested that the photo-enhanced NO2 heterogeneous reaction on the ground surface and the nitrate (NO3−) photolysis might be the daytime HONO missing source. After incorporating the contributions of the two sources, 65% of the HONO unknown source could be explained, while there was still an unknown source with a production rate of 0.45 ppb h−1 that needed to be further explored. Furthermore, a 0-D box model was used to evaluate the role of HONO in O3 formation chemisry. The results showed that the absence of HONO led to an 83% underestimation of the OH production rate and a 45% underestimation of the net Ox production rate. This study strengthened the understanding of the HONO sources and its impact on atmospheric photochemistry in Zibo City of the North China Plain. •The spring HONO concentration in Zibo reached 4.33 ppb with a mean of 0.91 ± 0.62 ppb.•Correlation analysis indicated PNO3− and PNO2+hυ (g) were likely to be the potential HONO sources.•Parameterizing the two potential sources could explain 65% of the HONO unknown source.•HONO unconstrained in the box modelling led to an underestimation of P(Ox).