Sources and sinks of atmospheric nitrous acid (HONO) in the megacity of Shanghai during COVID-19

The COVID-19 global pandemic has significantly affected air quality due to changes in human behavior. Gaseous nitrous acid (HONO) is a significant precursor of the hydroxyl radicals (OH), powerfully influencing atmospheric oxidization capacity and air quality. However, the impacts on the sources and sinks of HONO during COVID-19 are not well understood. Here, we observed the concentrations of HONO, nitric oxide (NO), and nitrogen dioxide (NO₂) in the suburb of Shanghai during May 2020 (P1), August 2020 (P2), and April 2021 (P3) to analyze seasonal variations of HONO chemistry and also clarify how different pandemic phases influence HONO sources and sinks. The average concentration of HONO during P1, P2 and P3 showed an increasing trend (0.292 ± 0.0078、0.358 ± 0.0115, and 0.415 ± 0.0115 ppb, respectively), with direct emission from vehicles was the most essential source of the nocturnal HONO concentration (38.14%, 47.52%, and 50.95%, respectively), followed by heterogeneous conversion of NO₂. The daytime HONO sources presented noticeable discrepancy among three study periods. In spring, homogeneous reaction was the primary HONO source with a mean production rate of 0.2 ppb h⁻¹, while there was almost no unknown source (P_unknown). In summer, however, the average production rate of homogeneous reaction decreased to 0.15 ppb h⁻¹, while P_unknown was up to 58% of the whole HONO production, demonstrating some strongly enhanced source(s) in the summer season. By comparing HONO budgets between different pandemic phases, the contributions of vertical and horizontal transport doubled from P1 to P3, with the average production rates increasing from 0.03 to 0.06 ppb h⁻¹. Our results also showed that the strict lockdown measures reduced the unknown sources of HONO (P1), and correspondingly, as Shanghai implemented regular epidemic prevention and control measures, the relatively high rate of P_unknown was observed during P3, making up 35% of the whole HONO production. What is more, through the strong correlation with J(HNO₃) (r = 0.9) and J(NO₂) (r = 0.89), it can be argued that the photolysis of nitric acid and photo-enhanced heterogeneous conversion may be a vital production pathway. The HONO photolysis was the major loss pathway, occupying approximately 85% of HONO loss during all campaigns. During noontime, the average photolysis loss rate was 1.07、2.40 and 1.86 ppb h⁻¹, accounting for up to 96% of the HONO sinks. Dry deposition was the second important loss pathway, especially in the morning and before sunset. This study indicates remarkable seasonal variations of HONO and the effects of COVID-19, and has significant implications on controlling measures of air pollutants in megacity.