Exploration of the atmospheric chemistry of nitrous acid in a coastal city of southeastern China: Results from measurements across four seasons

Abstract. Because nitrous acid (HONO) photolysis is a key source of hydroxyl (OH) radicals, identifying the atmospheric sources of HONO is essential to enhance the understanding of atmospheric chemistry processes and improve the accuracy of simulation models. We performed seasonal field observations of HONO in a coastal city of southeastern China, along with measurements of trace gases, aerosol compositions, photolysis rate constants (J), and meteorological parameters. The results showed that the average observed concentration of HONO was 0.54 ± 0.47 ppb. Vehicle exhaust emissions contributed an average of 1.64 % to HONO, higher than the values found in most other studies, suggesting an influence from diesel vehicle emissions. The mean conversion frequency of NO₂ to HONO in the nighttime was the highest in summer due to water droplets was evaporated under the condition of high temperatures. Based on a budget analysis, the rate of emission from unknown sources (Runknown) was highest at midday, with values of 14.78 ppb h⁻¹ in summer, 6.49 ppb h⁻¹ in autumn, and 2.18 ppb h⁻¹ in spring. Unknown sources made up the largest proportion of all sources in summer (84.92 %), autumn (80.29 %), and spring (49.98 %), whereas the main source in winter was the homogeneous reaction of NO with OH (56.15 %), due to winter having the highest NO concentration of the four seasons. The value of R_unknown had a positive logarithmic relationship with the photolysis of particulate nitrate in spring, summer, and autumn. However, R_unknown was limited by particulate acidity under the condition of photolysis of particulate nitrate (J (NO₃⁻_R) × pNO₃⁻) > 1 µg m⁻³ s⁻¹ in autumn and J(NO₃⁻_R) × pNO₃⁻ > 2 µg m⁻³ s⁻¹ in spring and summer. The variation of HONO at night can be exactly simulated based on the HONO / NO_x ratio, while the main sources should be considered for daytime simulations. Compared with O₃ photolysis, HONO photolysis has long been an important source of OH, particularly in the morning in spring and winter and around noon in summer and autumn. This study draws a full picture of the sources of HONO across all four seasons and improves the comprehension of HONO chemistry in southeastern coastal China.