Abstract: Based on the 6-hourly conventional meteorological observation data and the daily air pollutant concentration data from Ningbo Environment Protection Bureau,the least square method was used to calculate atmospheric mixed layer thicknesses and analyze their temporal variations,along with their relationships with air pollution during 2001 to 2014.The results show that the annual average atmospheric mixed layer thicknesses vary obviously,and the maximum and minimum appear in 2004 and 2007,with the values of 866.1 m and 746.1 m,respectively.Horizontal wind speeds are key factors to atmospheric mixed layer thicknesses over Ningbo.The thicknesses are relatively larger during July and August,whereas smaller during autumn and winter,the minimum appearing in June.As for the diurnal variations,the maximum appears in the noon,whereas the minimum appears at night.The highest frequency of atmospheric mixed layer thicknesses occurs at the levels between 500 m and 1200 m.The pollutants are diluted with increasing the thickness of the mixed layer.In summer,the atmospheric mixed layer thicknesses significantly modulate the concentrations of PM₁₀,SO₂,and NO₂.But in winter,due to the pollutant transport,the maximum concentrations of PM₁₀ and SO₂ are much higher,while the dilution effect of atmospheric mixed layer thicknesses is much weaker than in summer.There is little influence on NO₂ concentrations.In addition,when atmospheric mixed layer thicknesses reach to the levels of 500-1200 m,the range of concentration for the same pollutant is relatively larger for the same mixed layer thickness.

Key words: Atmospheric mixed layer thickness, Least square method, Air pollution, Correlation Analysis