Seasonal variation in particle contribution and aerosol types in Shanghai based on satellite data from MODIS and CALIOP

Although the concentration of atmospheric particulate matter in Shanghai has declined in recent years,aerosols remain one of the major pollutants affecting air quality.Herein,spatio-seasonal variation in aerosol optical properties and aerosol types were studied overa 10-year period(2006-2015)in Shanghai,China,using satellite data from Moderate Resolution Imaging Spectroradiometer(MODIS)and CloudAerosol Lidar with Orthogonal Polarization(CALIOP).The average aerosol optical depth values for central urban and suburban areas of Shanghai were between 0.9 and 1.0.Clear seasonal variation in aerosol concentrations occurred,causing strongest attenuation in summer and weakest attenuation in autumn.Polluted dust,polluted continental(urban/industrial)and smoke aerosols were the main aerosol types.Desert dust aerosols occurred in the Shanghai area at higher altitudes(greater than 3 km)in spring,related to dusty weather in the north;while in winter,smoke aerosols occurred at high altitudes,related to haze pollution in the north.The aerosols detected in autumn were mainly from local sources,comprising polluted dust,polluted continental,and smoke aerosols.Aerosols in Shanghai clearly reflect both local and regional sources at different times.

The Significant Contribution of Small-Sized and Spherical Aerosol Particles to the Decreasing Trend in Total Aerosol Optical Depth over Land from 2003 to 2018

The optical and microphysical properties of aerosols remain one of the greatest uncertainties associated with evaluating the climate forcing attributed to aerosols.Although the trends in aerosol optical depth(AOD)at global and regional scales have been widely examined,little attention has been paid to the trends in type-dependent AODs related to aerosol particle properties.Here,using the aerosol optical component dataset from the Multi-angle Imaging SpectroRadiometer(MISR)instrument,we investigate decadal-scale trends in total aerosol loading as well as AODs for five aerosol components by particle size and morphology during 2003–2018 over land.Relationships between the total AOD(TAOD)trends and type-dependent AOD changes were examined,and the relative contribution of each type-dependent AOD to the overall TAOD trends was quantified.By dividing the TAOD values into four different aerosol pollution levels(APLs)with splits at 0.15,0.40,and 0.80,we further explored the relationships between TAOD changes and interannual variations in the frequency-of-occurrences(FoOs)of these APLs.Long-term trends in FoOs in the different APLs show that there was a significant improvement in air quality between 2003 and 2018 in most land areas,except South Asia,corresponding to a shift from lightly polluted to clean conditions.However,the effects of different APLs on TAOD changes are regionally dependent and their extent of correlation varied spatially.Moreover,we observed that the annual mean TAOD has decreased by 0.47%.a^(-1)over land since 2003(P<0.05).This significant reduction was mainly attributed to the continued reduction in small-sized(<0.7 mm diameter)AOD(SAOD)(-0.74%.a^(-1))and spherical AOD(SPAOD)(-0.46%.a^(-1)).Statistical analysis shows that SAOD and SPAOD respectively accounted for 57.5%and 89.6%of the TAOD,but contributed 82.6%and 90.4%of the trend in TAOD.Our study suggests that small-sized and spherical aerosols composed of sulfate,organic matter,and black carbon play a dominant role in determining interannual variability in land TAOD.

Assessment of column aerosol optical properties using ground-based sun-photometer at urban Harbin, Northeast China

Aerosol plays a key role in determining radiative balance, regional climate and human health.Severe air pollution over Northeast China in recent years urges more comprehensive studies to figure out the adverse effects caused by excessive aerosols. In this study, column aerosol measurements over urban Harbin, a metropolis located at the highest latitude in Northeast China, during May 2016 to March 2017 were conducted using a CIMEL sun-photometer to analyze local aerosol properties and its variation from different aspects. According to the observations, aerosol optical depth at 440 nm（AOD_（440）） ranges from 0.07 up to 1.54, and the large variability in both AOD_（440） and Angstrom Exponent（AE_（440–870）） indicates the frequent change of aerosol types due of different emission sources. Coarse mode particles dominated Harbin during the studying period because of the long-range transported dust and probably the suspended snow crystals in winter. As the wavelength increases, relatively consistent decrease trends of single scattering albedo（SSA） and asymmetry factor（ASY） were observed in spring, autumn, and winter, indicating the presence of absorbing polluted aerosols. Mixed type（MIX） aerosol dominated the study region with a total percentage of 34%, and biomass burning and urban industry（BB/UI）, clean continental（CC）, and desert dust（DD） aerosols were found to be 31%, 27%, and 8%, respectively. The current work fills up the optical characteristics of aerosols in Harbin, and will contribute to the in-depth understanding of local aerosol variation and regional climate change over Northeast China.