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 1.45 % 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 (Runknown) 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 / NOx ratio, while the
J(NO3-_R) × pNO3- should be
considered for daytime simulations in summer and autumn, or 1/4× (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.