Size-resolved Hygroscopic Behavior Of Submicron Aerosol And The Influence On The Heavy Aerosol Pollution In Wintertime In Beijing

The size-resolved hygroscopic properties of submicron particles?PM₁?were observed by a Hygroscopicity Tandem Differential Mobility Analyzer?H-TDMA?at an urban site in Beijing in December 2016.H-TDMA was operated at the relative humidity?RH?of 30-90%with the particle size range of 30-200 nm.According to the HGFs of the size-selected particles with diameters of 30,50,100,150 and 200 nm at 70-90%RH throughout the measurement period,the probability distribution of the hygroscopic growth factor?HGF-PDF?during the measurement usually showed a bimodal pattern,namely,hydrophobic mode?HGF<1.2?and hydrophilic mode?HGF?1.2?with RH?70%.The hydrophilic mode particles were usually more dominant for the aged particles.The HGF values were size-dependent and varied depending on particle chemical composition,and HGF always increased with the size of aerosols.The volume-weighted mean HGF could better characterize the relationship between hygroscopicity and chemical composition for the bulk aerosols.The particle mixing state could be also known from the width of HGF-PDF and the particles were mainly externally mixed in this study because a major road with heavy traffic was nearby.For the observed new particle formation events,the HGF values of 30 nm diameter particles at 90%RH showed a decreasing trend,indicating the participation of the organics with weak hygroscopic growth behaviors.During the heavy aerosol pollution episode,the particles undergone the the transport stage and cumulative stage.The small particles became more externally mixed and less hygroscopic,which were influenced by the primary emissions.Meanwhile,the aged particles?e.g.,200 nm?changed to a quasi-internal mixing state and were affected by the secondary inorganic formation process,which further enhanced the ability of water uptake.And the formation of nitrate and ammonium played a more important role in the particle hygroscopicity than sulfate in this work.That indicated a positive feedback could exist between secondary inorganic aerosol?SIA?formation process and aerosol hygroscopicity,which facilitated the particle mass concentration reaching to the maximum and fluctuating at high level in cumulative stage.The aerosol liquid water content?ALWC?of submicron particles was calculated by three methods based on the aerosol physical and chemical propertiesmeasurement campaigns in December 2016 and December 2017 in Beijing,including?a?the PM₁ volume difference between the ambient and dry states by applying the particle number size distribution and particle hygroscopicity measurement;?b?the thermodynamic equilibrium model?ISORROPIAII?based on the chemical composition;and ?c?the ?-K???hler theory of chemical composition with a volume mixing scheme.The three methods agreed well with ?20% error range.This resulted in a mean ALWC mass concentration of 44.8?g/m³ during the 2016 campaign,which was almost three times the value in 2017 of 14.4?g/m³.The ALWC showed an exponential trend depending on the RH,and an abundant ALWC was also favored by the high PM₁ mass loading.The contribution of different chemical component to the ALWC was evaluated by the?-K???hler method,which revealed that during the measurement,the inorganics and organics could contribute to ?80% and ?20%,respectively,under ambient RH conditions,with the largest contributor of ammonium nitrate.When the RH was above 85%,the mass concentration of ALWC was comparable to,or even larger than,that of the dry PM₁.During the HPEs the high mass loading of ALWC promoted the secondary formation processes of SIA.The SIA enhanced the ability of aerosol water uptake,which further promoted the heterogeneous reactions.This was a self-amplifying process between the ALWC and SIA that could further enhance the particle mass loading in the ambient and deteriorate the visibility.