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 1.45？% 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 evaporating under high-temperature conditions. Based on a budget analysis, the rate of emission from unknown sources (R_unknown) was highest around midday, with values of 4.51？ppb？h^-1 in summer, 3.51？ppb？h^-1 in spring, 3.28？ppb？h^-1 in autumn, and 2.08？ppb？h^-1 in winter. Unknown sources made up the largest proportion of all sources in summer (81.25？%), autumn (73.99？%), spring (70.87？%), and winter (59.28？%). The photolysis of particulate nitrate was probably a source in spring and summer while the conversion from NO₂ to HONO on BC enhanced by light was perhaps a source in autumn and winter. The variation of HONO at night can be exactly simulated based on the HONO？？NO_x ratio, while the J(NO₃^-_R)？×？pNO₃^- should be considered for daytime simulations in summer and autumn, or (J(NO₃^-_R)？×？pNO₃^-) in spring and winter. Compared with O₃ photolysis, HONO photolysis has long been an important source of OH except for summer afternoons. Observation of HONO across four seasons with various auxiliary parameters improves the comprehension of HONO chemistry in southeastern coastal China.