Chemical And Physical Characteristics Of Aerosol And Their Effects On Extinction In Beijing-Tianjin-Hebei

In this paper,through long-term observation of atmospheric aerosol in Beijing-Tianjin-Hebei,the chemical and physical characteristics of aerosol and their effects on atmospheric extinction are analyzed.Firstly,to better understand the characteristics of the chemical compositions of aerosols and their impacts on the visibility in Beijing-Tianjin-Hebei,four experimental sampling campaigns were carried out between June 2013 and May 2014,the sampling sites were located in Beijing,Xiangshan,Tianjin,Shijiazhuang,and Qinhuangdao.After analyzing the water soluble ions?WSIs?and carbonaceous aerosol?CA?,the results show that the air pollution episodes mainly occurred during the autumn and winter in Beijing-Tianjin-Hebei.With regard to different particle size distributions,the ratio of anions to cations in the fine size?0.64?was greater than that in the coarse size?0.54?in the NCP.Coefficients of divergence indicate that aerosol pollution had similar characteristics in the five cities of the NCP and that the pollutants were characterized by mutual influences and regional transfer processes.There were different non-linear correlations between the visibility and mass concentrations of PM_2.5.5 at different relative humidity?RH?conditions.When the RH was greater than 70%,the visibility was less than 5 km;this shows that,due to hygroscopic growth in a higher RH,the increase in PM_2.1.1 resulted in a decrease in the visibility.The Interagency Monitoring of Protected Visual Environments?IMPROVE?method was used to estimate the light extinction coefficients?LEC?from the measured concentrations of chemical species.Organic matter?OM?,ammonium nitrate?AN?,and ammonium sulfate?AS?were the three dominant species that contributed to the LEC in Beijing and had the highest proportions of total contributions to the LEC in the summer,accounting for 32.2%,25.9%,and24.4%,respectively.The LEC of sea salts did not change much throughout the year and accounted for 4.1?5.3%in Beijing and 4.8?7.4%in Tianjin.According to the Ambient Air Quality Standard,the days were divided into pollutional days?PD?and attained days?AD?.The increasing concentrations of AN,AS and OM resulted in an increase in the LEC in the NCP,which led to the occurrences of low visibility events during the autumn and winter.NH₄⁺,NO₃^-,and SO₄^2-mainly existed as fine size particles?diameter<2.1?m?in Beijing?73.5%,80.7%,and 78.0%on PD and63.3%,79.4%,and 72.5%on AD,respectively?and Tianjin?81.0%,80.6%,and 82.1%on PD and71.5%,44.3%,and 69.7%on AD,respectively?.However,the CA originating mainly from fuel combustion also played an important part in the visibility impairment in the coarse size?2.1<diameter<9.0?m?.According to a positive matrix factorization?PMF?model,37.1%and26.5%of the LEC came from secondary aerosols on PD in Beijing and in Tianjin,respectively.In addition,biomass burning,fuel combustion and fugitive dust were also important contributing sources of the LEC in Beijing-Tianjin-Hebei.In order to continue to study the effects of aerosol number concentration distribution,chemical composition and environmental RH on the extinction coefficient of aerosol,an autumn case study was conducted in Beijing from 25 August to 05 September,2017.The chemical and physical characteristics of aerosol were measured to explore the effect of those factors on the extinction coefficient in Beijing during four environmental conditions?heavy pollution?HP?;light pollution?LP?;clean?CL?;and rainy?RA??.After the new particle formation events during CL2,the ratio of particle number concentrations in the Accumulation mode?Acc?and Aitken mode?Ait?gradually grew from 0.2?CL2?to 0.7?LP1?,0.9?HP?and 0.8?LP2?.The contributions of particle number concentrations to the extinction coefficient were 0.1%?0.1%?,10.5%?1.4%?and 89.4%?98.5%?in the Nucleation mode?Nuc?,Ait and Acc,respectively,during CL1?HP?,which revealed that particles with an extinction function were mainly in Acc,especially during polluted days?PDs?.The mass percentage of secondary inorganic aerosol?nitrate,sulfate and ammonium?with a greater hydrophilic characteristic was just 12.8%during clean days?CDs?,while this rapidly increased to 56.5%during PDs,which was the main reason for the increase in the extinction coefficient.The sensitivity test of the extinction coefficient calculated using different refractive indices indicated that using the refractive index of the average value of a long observation would cause an underestimation of the extinction coefficient.The hygroscopic factors of the aerosol scattering coefficient?f?RH??were 1.13,1.59,1.49 and 1.62 during CL2,LP1,HP and LP2,when RH=80.0%,respectively.The extinction coefficients calculated by the?-AMS-BC-Mie model and measured by the sum of the scattering and absorption coefficients were 675.7 and 521.2 Mm^-11 under dry conditions during HP,while those values were 855.7 and661.7 Mm^-11 after correcting for RH.When the hygroscopic growth factor?GF?reached 1.1,the range of f?RH?was approximately 1.30 to 1.38 during all campaigns.Therefore,on PDs with high RH,aerosol with a greater number of hygroscopic components,which are mainly distributed in Acc,will produce a much larger extinction coefficient.In order to study the influence of mass concentration,quantity concentration,source of aerosols and RH on extinction coefficient of aerosol,a summer case study of aerosol chemical and physical properties of Xianghe in Hebei Province was carried out from Jun.15 to 26,2018.Visibility is mainly affected by aerosol of scattering coefficient,meteorological conditions and so on.A comprehensive observation of the chemical and physical properties of aerosol was conducted in Xianghe,southeast of Beijing,from Jun.15 to 26,2018.Five specific campaigns are analysed during this observation:?1?rainy?RA?;?2?low visibility during the day?LD?;?3?high visibility during the day?HD?;?4?low visibility at night?LN?and?5?high visibility at night?HN?.The mass percentage of secondary inorganic aerosol?nitrate,sulfate and ammonium?measured by high-resolution time-of-flight aerosol mass spectrometer?HR-ToF-AMS?were 64.5%and 68.3%during LD and LN,which were higher than those?63.6%and 46.1%?during HD and HN,respectively.The mass percentage of secondary organic aerosol?SOA?and primary organic aerosol?POA?to organic aerosol were 76.6%and 23.4%during LD,respectively.The hygroscopicity parameter?were 0.59 and 0.60 during LD and LN,which were a little higher than those?0.57 and 0.58?during HD and HN,respectively,indicating that the variation of aerosol chemical composition had a limited impact on visibility.The average proportion of number measured by single particle aerosol mass spectrometer?SP-AMS?of biomass burning,dust,industry,combustion,sea salt,secondary aerosol and other particles were 12.3%,2.9%,16.2%,36.1%,8.8%,21.0%and 2.7%during this observation.The average number fractions of combustion and secondary aerosol were 42.4%and 19.2%from 0.2 to 1.0?m.Although the variation and height of mixing layer were similar between LD and HD,the clouds reduced the solar radiation during LD,which caused a higher relative humidity?RH??67.5%?.A larger wind speed below 850hpa during HN made the mixing layer height?MLH?reached to 1910 m,which resulted in a lower RH?64.8%?.The light scattering enhancement factor f?RH?during low visibility events were almost greater than those during high visibility events,whether it is during day or night.Aerosol hygroscopic growth factor?GF?maintained a similar trend with RH,indicating that RH was the main reason affecting the hygroscopic growth of aerosol,resulting in the decrease of visibility.