Xi'an, Air Pollution Continues To Maintain The Mechanism Of A Preliminary Study

Analyses of the observed data on major pollutants (N0₂, SO₂, PM10) in Xi'an, Shan'xi province in 2002 showed that the accumulated days when PM10 exceeds the National Ambient Air Quality Standard (NAAQS) were up to 192 days and the occurrence probability of exceeding Class III of the NAAQS was 53%. The major air pollutant was PM10 in Xi'an city. The total occurrence frequency of urban persistent air pollution, which is defined as the situation where air quality is equal to or higher than Class III of the NAAQS and the corresponding air pollution lasts more than three days, was 18. The persistent air pollution episode generally occurs in Xi'an with very light wind (0-1 m/s). A typical persistent air pollution episode occurred in Xi'an on Jan.9-12, 2002 was chosen to discuss the role of meteorology conditions. The common features were light wind speed, low air temperature, and large relative humidity without precipitation. The Nested Air Quality Prediction Modeling System (NAQPMS) was used to simulate the episode. Simulations indicate that the NAQPMS successfully reproduced the persistent air pollution episode and persistent light wind without precipitation, stable atmospheric boundary layer, and other related meteorological conditions are key factors to favor formation of persistent air pollution episodes in Xi'an. Comparison of simulations and observations shows that surface pressure, temperature, relative humidity and wind speed are well simulated. Simulated concentrations of PM10, SO₂ and NO2 agree well with observations.