Measurements of PM10 aerosol and gas-phase nitrous acid during fall season in a semi-urban atmosphere

Simultaneous measurements of PM10 (particle mass) aerosol and gas-phase nitrous acid (HNO2) were made over a fall season at a semi-urban site, Kwangju, South Korea, to investigate the effects of agricultural waste burning on their concentrations. The PM10 aerosol and gaseous species samples were collected three times a day for 6- and 12-h durations using a three-stage filter pack during three intensive sampling campaigns, and analyzed for particulate mass, ionic species, and HNO3 concentrations. Hourly HNO2 measurements were also made using a dual channel glass-coil sampling technique and ion chromatography analysis. NO2 (g) was measured using a differential optical absorption spectroscopy (DOAS) to investigate the formation pathway of HNO2. PM10, K+, and Cl− concentrations during the agricultural wastes burning event were substantially higher than during any other sampling periods, whereas the concentrations of secondary-formed aerosols, i.e., NO3−, NH4+, and SO42−, were substantially low during the burning event, compared to those in the non-burning periods. Higher HNO2 concentrations occurred during the night and November sampling periods when PM10 mass concentrations were generally high, and lower mixing layer and more humid condition were found, rather than during the day, September, and October sampling periods. It was also found that rapid increase in HNO2 levels from 0.2 to 2.6 ppb in the afternoon on October 20 was due to direct emissions from the agricultural wastes (straw and stubble) burning practice. Increases in the HNO2/NO2 ratio after sunset, and a close relationship between HNO2/NO2 and the PM10 mass indicate that during the study period NO2 is the precursor of HNO2 production at nighttime.