Pollution Properties of Atmospheric HNO_2 and Its Effect on OH Radical Formation in the PRD Region in Autumn
Photolysis of HNO_2 in the atmosphere is an important source of OH radical, and plays a crucial role in the formation of photochemical pollution. Based on the online observation of gaseous HNO_2, NOx, O_3, HNO_2 photolysis rate constant, and solar radiation during October of 2013 at the atmospheric supersite in the Pearl River Delta(PRD) region, the variation pattern and pollution properties of HNO_2 were analyzed, and the production rate of OH radical through HNO_2 photolysis was evaluated. During the measurement period, the hourly concentrations of HNO_2 were 0.48 ~9.92 μg/m3, with average concentration of 3.02 μg/m3. The average values of OH radical production rate through HNO_2 photolysis was 1.53 ×107molecule/cm3 s. HNO_2 showed high concentrations at night and low ones during daytime, which was similar to the average diurnal variation of its precursor NOx. The positive relations between HNO_2 and NOxor surface area concentrations of particles were quite good, and the relation between HNO_2 and surface area concentrations of particles became significantly better with relative humidity over 60%. It indicated that heterogeneous reaction of NOxon the particle surface under conditions with high relative humidity contributed a lot in HNO_2 formation. HNO_2 photolysis rate constant was mainly affected by solar radiation,and showed similar average diurnal variation to the latter, with high values from 11:00 to 13:00. However, as the solar radiation became more and more intense, the increase rate of HNO_2 photolysis rate constant turned to be slower. In the average diurnal variation of OH radical production rate, a longer time period of high values were observed, from about 8:00until the high value period of O_3 in the afternoon. The daily maximum hourly average values of HNO_2 concentrations, OH radical production rate, and O_3 concentrations had positive relation, reflecting the important role of HNO_2 in the photochemical pollution.