Inversion Of Aerosol Optical-radiometric Properties In Beijing Based On Sky Radiometer Observations

This study assesses the performance of SKYNET(Skyrad Network)skyradiometer in comparison to AERONET(Aerosol Robotic Network)sunphotometer for retrieving aerosol optical properties in Beijing,China.The results obtained from simultaneous measurements compare well(RMSE of 0.010?0.020)and show high correlation coefficients(>0.996)for aerosol optical depth(AOD)at each wavelength;The highest correlation coefficient for(?)ngstr(?)m exponent is 0.992,at 440?870 nm,with the smallest RMSE of 0.042.The RMSE of single scattering albedo(SSA)is as low as0.018 at 440 nm,with high correlation coefficient(0.851);The real and imaginary parts of the refractive index show deviations of 0.031?0.055 and 0.003?0.005 respectively for all the wavelengths.The fine mode and coarse mode dominated volume size distribution patterns derived from the two instruments are both bimodal.The aerosol optical properties under three weather conditions in Beijing are discussed.The AOD shows high consistency on the clean day,light-pollution condition and heavy-pollution condition,and the RMSE are about 0.005,0.006 and 0.018 respectively.The RMSE of SSA are 0.022,0.046,0.020 and for(?)ngstr(?)m exponent are 0.229,0.289,0.060,respectively.Based on the continuous observation of the skyradiometer from October2018 to September 2019 in Beijing,the aerosol optical properties and frequency distribution characteristics were analyzed throughout this study period.AOD at 500 nm is high from February to July,the highest value is 0.71±0.46 in June;the highest SSA is 0.96±0.04 in August and lowest value is 0.89±0.05 in May;(?)ngstr(?)m exponent in summer(1.11±0.26)is higher than that in spring(0.89±0.32).According to the frequency distribution,the frequency of AOD(>1.0)accounts for 14.69%and 15.77%in spring and summer,respectively.The low value(<0.6)of the(?)ngstr(?)m exponent in spring and winter account for 20.28%and 12.05%,respectively,while the high value(>1.2)in summer and autumn accounts for 37.88%and 36.47%,respectively.The frequency of SSA(>0.96)is less than 11.00%in spring but it is more than 33.83%in summer.The volume size distribution patterns in four seasons are typical bimodal,and the coarse-mode volume concentration is highest in spring(0.16±0.19?m³·?m^-2).Statistics found that the pollution days only account for 17%,of which 62%are light pollution days;The values of PM_2.5 concentrations under pollution weather condition and clean weather condition are 107.22±40.89?g·m^-3 and 47.16±18.45?g·m^-3;The values of AOD at 500 nm are 0.85±0.37 and 0.49±0.39 under pollution and clean weather conditions,respectively;The SSA at 500 nm is 0.96±0.04 and 0.92±0.06,respectively;The value of(?)ngstr(?)m exponent under pollution weather condition(1.02±0.22)is larger than that under clean weather condition(0.91±0.32)in winter while the value of(?)ngstr(?)m exponent under pollution weather condition(0.87±0.24)is smaller than that under clean weather condition(0.90±0.33)in spring.Skyradiometer retrieved data,combined with Sunphotometer data,PM concentrations data,ERA-Interim reanalysis data,meteorological field data,CALIPSO satellite data,and lidar data are used to analyze serious pollution events over Beijing.The following conclusions are obtained:the poor meteorological conditions(low wind speed and high relative humidity),the hygroscopic growth of aerosol,aerosol secondary transformation,local emissions and regional transportation lead to the haze pollution event in winter;The dust pollution event occurs in spring,dust is the main pollutant,with strong absorption and large size.Low wind speed in the surface is conducive to the continuous accumulation of particulate matter which leads to high PM₁₀ concentrations(751?g·m^-3),decreased visibility and worsen pollution.