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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 (^unknown) was highest around midday, with values of 4.35 ppbh-1 in summer, 3.53 ppbh-1 in spring, 3.13 ppbh-1 in autumn, and 2.05 in winter. Unknown sources made up the largest proportion of all sources in summer (78.55 %), autumn (71.51 %), spring (69.67 %), and winter (55.63 %). 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* ratio, while the J(NO3 R) x pNO3- should be considered for daytime simulations in summer and autumn, or 1/4 x( J(NO3 R) x pNO3-) in spring and winter. Compared with O3 photolysis, HONO photolysis has long been an important source of OH except for summer afternoon. Observation on HONO across four seasons with various auxiliary parameters improves the comprehension of HONO chemistry in southeastern coastal China. [ABSTRACT FROM AUTHOR]

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 Runknown had a positive logarithmic relationship with the photolysis of particulate nitrate in spring, summer, and autumn. However, Runknown 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/NOx 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. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) plays an important role in global climate change and air quality. PM 2.5 (particles with aerodynamic diameters ≤2.5 μm) samples were collected at a mountainous forest site (Mt. Wuyi) in southeastern China between November 2015 and July 2016. Fourteen PM 2.5 -bound SOA tracers, including isoprene, α/β‑pinene, β‑caryophyllene, and toluene, were measured using the gas-chromatography-mass-spectrometry method. The total concentrations of the isoprene, α/β‑pinene, β‑caryophyllene, and toluene SOA tracers were 45.28 ± 65.52, 30.66 ± 24.44, 5.99 ± 7.25, and 0.62 ± 0.72 ng m -3 , respectively. The isoprene SOA tracers exhibited the highest concentration (145.97 ± 53.78 ng m -3 ) and accounted for 76 ± 9% of the total concentration of SOA tracers in summer. In fall-winter, the mass fraction of 2‑methylglyceric acid was significantly enhanced because of the lower temperature and higher NO x level. As later-generation products of α/β‑pinene tracers, high proportions of 3‑hydroxyglutaric acid and 3‑methyl‑1,2,3 butanetricarboxylic acid were observed on Mt. Wuyi, suggesting that the aerosols were highly oxidized. Biomass burning events affected by local and regional sources were identified by analyzing typical SOA tracers. Significant positive correlation (R 2  = 0.74) was found between the β‑caryophyllene tracer and levoglucosan. The average concentration of secondary organic carbon (SOC) as estimated from SOA tracers was 1.46 μgC m -3 . The isoprene SOC accounted for 70% of the total SOC in summer, whereas the β‑caryophyllene SOC was the predominant component in winter. Meanwhile, the estimated toluene SOC accounted for 11.6% of the total SOC during the study period. The study helps understanding the characteristics and the formation of SOA in a mountainous forest area of southeastern China., (Copyright © 2018 Elsevier B.V. All rights reserved.)