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Knowledge of vertical distribution of air pollutants is important for understanding the mechanisms underlying haze pollution. In this paper, we characterize the vertical structure of air pollutants based on ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in the wintertime in Hefei, East China. Both NO 2 and aerosol show exponentially decreasing profile with high concentrations concentrated close to the surface, suggesting the major emissions are at ground level. SO 2 profiles extend to slightly higher altitudes than NO 2 , which might be due to the SO 2 emissions from elevated point sources (e.g., power plants). Elevated HCHO was observed in the middle layer during noon time, probably indicating that the larger effect of the photochemical formation of atmospheric HCHO rather than direct HCHO emissions. Potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) models were used to evaluate the potential sources of Hefei's air pollution in the lower, middle and upper boundary layer. The results revealed that the major regional source of NO 2 and aerosol were located in the northern and eastern part of Anhui and the transport mainly occured in the lower layer. Transported SO 2 from the northwestern regions in the middle layer made great contribution to SO 2 in Hefei. HCHO was mostly produced locally while the transport of HCHO from potential source areas was negligible. To further explore the potential source contributions affecting haze pollution, PSCF and CWT analysis for heavy haze episodes suggested that the potential source areas for aerosol were almost located in the northern region, the northwestern region, and the YRD region. Our findings provide a more comprehensive understanding for regional transport through the study on potential sources at different altitudes, which are useful for designing collaborative air pollution control strategies on a regional scale. • Vertical distribution of air pollutants is investigated to identify potential sources at different altitudes during winter. • Both NO 2 and aerosol show exponentially decreasing profile with high concentrations concentrated close to the surface. • SO 2 and HCHO profile typically extended to higher altitudes than NO 2 and aerosol did during winter. • The main potential sources for aerosol were located in the northern region, the northwestern region, and the YRD region. • This study is useful for designing collaborative air pollution control strategies on a regional scale. [ABSTRACT FROM AUTHOR]

Based on the principle of light propagation, it is necessary to obtain the ratio (R MN) of atmospheric aerosol mass concentration to the imaginary part of the atmospheric equivalent refractive index for measuring aerosol mass vertical flux. In this paper, the factors affecting the R MN values are analyzed by numerical calculation under an ideal state and by using observational data. Numerical calculations show that the R MN is mainly determined by the aerosol particle size distribution (PSD), hygroscopicity parameters and relative humidity. With the same relative humidity, the smaller the geometric mean diameter (Dg) is, the larger the R MN , and when the aerosol particle hygroscopicity parameter is greater than 0.14, the greater the relative humidity is, the smaller the R MN. Comprehensive experiments were conducted in urban Hefei, and the results of observations at Hefei confirm the numerical results. Data from ten cities across China also confirm the numerical results. The order of geometric mean diameter of aerosol particles at the ten sites can be determined by the order of R MN , and the order of hygroscopicity parameters at the ten sites can be determined by the order of the change rate of R MN with relative humidity. The geometric mean diameters of aerosol particles are different among the ten sites, while the hygroscopicity parameters are similar. • The ratio (R MN) of TSP mass concentration to the imaginary part of the atmospheric equivalent refractive index is defined • The R MN is mainly affected by aerosol particle size distribution, relative humidity and hygroscopicity parameter. • The order of geometric mean diameter of aerosol particles at the ten sites can be determined by the order of R MN. • The variation of R MN with relative humidity reflects hygroscopicity parameter (α) of the aerosol particles. [ABSTRACT FROM AUTHOR]