Impact of high PM2.5 nitrate on visibility in a medium size city of Pearl River Delta

Daily PM2.5 were collected at urban Zhongshan, a medium size city in the Pearl River Delta (PRD), and its major chemical components were determined. The contribution of NO3− to PM2.5 concentration (17%) during the campaign in Zhongshan was higher than that of other PRD cities on a seasonal basis. Among the inorganic ions, NO3− elevated dramatically with PM increase. Among the inorganic ions, NO3− elevated dramatically with PM increase. The NO3− was the most abundant species in PM2.5 (19%) during the polluted cases, followed by SO42− (13%) and NH4+ (9%). All haze events were associated with high nitrate and ammonium rich (AR) aerosols; the average [NH4+]/[SO42−] was equal to 3.4 and ranged from 2.4 to 4.8. The nitrogen oxidation rate (NOR) during the haze events was significantly higher than during the non-haze. Potential source contribution function (PSCF) and back trajectory analysis indicated that trans-regional transmission of NO2 and NH3 from the northeast and east of the PRD might contribute to local nitrate formation. The reduced visibility was associated with a higher percentage of NH4NO3. The average contribution of NH4NO3 enhanced markedly from 19% (>20 km) to 54% (<5 km). NH4NO3 became the most important contributor of extinction coefficients (bext) than (NH4)2SO4 when the visibility was less than 10 km, and the estimated b'ext in the dry condition was about 30% lower than that in ambient conditions.