Exploring the HONO source during the COVID-19 pandemic in a megacity in China

HONO is a critical precursor of •OH, but its sources are controversial due to its complex formation mechanism. This study conducted comprehensive observations in Zhengzhou from April 26 to May 11, 2022. Low NO_x concentrations were observed during the Covid epidemic period (EP) (10.4 ± 3.0 ppb), compared to the pre-epidemic period (PEP) (12.5 ± 3.8 ppb). The mean HONO concentration during EP (0.53 ± 0.34 ppb) was 0.09 ppb lower than that during PEP (0.62 ± 0.53 ppb). The decrease in HONO concentration during EP came mainly at night due to the reduction in the direct emission (P_emi) (0.03 ppb/hr), the homogeneous reaction between •OH and NO (P_OH+NO) (0.02 ppb/hr), and the heterogeneous conversion of NO₂ on the ground (0.01 ppb/hr). Notably, there was no significant change in daytime HONO concentration. The daytime HONO budget indicated that the primary HONO sources during PEP were the nitrate photolysis (P_nitrate), followed by the P_OH+NO, P_emi, the photo-enhanced reaction of NO₂ on the ground (P_ground+hv) and aerosol surface (P_aerosol+hv). The primary HONO sources were P_nitrate, P_OH+NO, P_emi, and P_aerosol+hv during EP, respectively. The missing source has a high correlation with solar radiation, there might be other photo-related HONO sources or the contributions of photosensitized reactions were underestimated. In the extremely underestimated cases, HONO production rates from the P_nitrate, P_ground+hv, and P_aerosol+hv increased by 0.17, 0.10, and 0.10 ppb/hr during PEP, 0.23, 0.13, and 0.16 ppb/hr during EP, and P_nitrate was still the primary source during both PEP and EP.