Scavenging Of Submicron And Black Carbon Particles By Cloud/fog At A Background Site In Southern China

Aerosols show effects on atmospheric environment,climate change and human health.It’s significant to study the removal of aerosols from the atmosphere for it allows us to know about the distribution and lifetime of aerosols.The pathways of aerosol removal include dry deposition and wet scavenging（cloud/fog and precipitation processes）,in which wet scavenging dominated,scilicet aerosols are removed（total wet scavenging）by serving as cloud condensation nuclei（CCN）and ice nuclei（IN）in cloud/fog formation（in-cloud scavenging）or washed out by rainfall（below-cloud scavenging）.Researches have pointed out that in-cloud scavenging is more effective than below-cloud scavenging.Black carbon aerosol（BC）,which originate from incomplete combustion of fuels,owns the greenhouse effect only second to carbon dioxide（CO₂）,high radiative forcing,other environmental climatic effects,and thus receive extensive concern.Currently,many scientific problems remain ambiguous.For instance,how scavenging efficiency（SE）evolve in cloud events and what factors may influence it,and so forth.Cloud residue particles（RES）are sampled by a ground-based counterflow virtual impactor（GCVI）on cloudy days while interstitial particles（INT）,cloud free particles are sampled via a PM_2.5 cut off on cloudy days and non-cloudy days in Jan,Oct-Nov,2016,May-Jun,2017 and May-Jun,2018,respectively.An aethalometer（AE-33）,a scanning mobility particle sizer（SMPS）and a single particle aerosol mass spectrometer（SPAMS）were used to analyze the mass concentration of BC,size distribution and single particle chemical compositions of particles,respectively.The databases gained are used to analyze the scavenging mechanisms of submicron particles,interactions between cloud and BC while influences of long-range transport on BC.Besides,off-line collected cloud/fog water and rainwater were analysed by SPAMS.Detailed research results are as follows:（1）In-cloud scavenging and influencing factors of submicron particles:SMPS data showed that the concentrations and size distributions varied from different cloud events:smaller particles were more observed in RES and less obsereved in INT particles in long cloud events compare to short ones.In the complete cloud event,number scavenging efficiency（NSE）increased gradually with the development of the cloud（with an average value of 9.4%）,NSE increased to the maximum when the cloud was stable（with an average value of 21.1%）and decreased gradually when it tended to dissipate（with an average value of 15.1%）.NSE was influenced significantly by particle size and LWC.NSE showed a parabolic-like relationship with size,which decreased in 20-50 nm,dominated by impaction scavenging and increased in larger than 50 nm,highlighted the increasing role of nucleation scavenging.NSE of particles in size range of 20-30 nm and larger than 50 nm can be well fitted with LWC by a power function.（2）Influence of long-range transport on concentration and aging of BC:Firstly,seasonal variation was analysed via mass concentration of equivalent BC（EBC）during Oct-Nov,2016 and May-Jun,2018.After that,cluster analysis for trajectory,potential source contribution function（PSCF）and concentration weighted trajectory model（CWT）were used to analyze the source of air masses and their relative contributions in different seasons.Finally,data from urban Guangzhou was also analysed to reveal the aging of BC during the transport.Results showed that mass concentration of EBC in RES in Oct-Nov,2016 was higher than that in May-Jun,2018,while those in INT and cloud free particles are lower in Oct-Nov,2016.The source of air masses and relative contributions varied with seasons.In cloud free conditions,air mass from northwest contributed smaller in May-Jun,2018（12.5%）than that in Oct-Nov,2016（24.9%）whereas RES and INT mainly originated from ocean and no significant difference was observed.With the increasing influence of the ocean,the mass concentration of EBC in RES and INT increased gradually.Mass concentration of EBC in cloud free particles was remarkedly impacted by incomplete combustion（fire dots）,high concentration usually appeared in air masses with dense fire dots.The aboundance of sulfate and nitrate were significantly higher in background site than those in urban site,indicating the sufficient aging of BC.（3）In-cloud scavenging and influencing factors of BC particles:The evolution of scavenging efficiency（SE）of BC in a complete cloud event,influence of concentration of INT,size,chemical composition（mixing state）and liquid water content（LWC）on the SE of BC and the impact of cloud processing on BC size and chemical composition were investigated.Results showed that SE in long cloud events was higher than that in short ones.In the complete cloud event,SE firstly increased and then decreased gradually as the cloud ongoing.In the range of larger than 0.4μm,SE increased with the increase of size,revealing that nucleation was the dominating mechanism to scavenge BC.Generally,SE increased with LWC at all specific size bins,they correlated significantly with each other by a power fitting（R²>0.8）,which pointed out that LWC is a momentous factor influencing SE.SE decreased with the increasing INT BC concentration,which can be attributed to the competition for the limited water vapor,hindering particles to grow up to incorporate into cloud.BC coated with more organics was less scavenged than those coated with sulfate and nitrate,the phenomenon revealed that organics are to the disadvantage of particle scavenging.In the formation stage of the cloud event,size showed overwhelming influence on the SE of BC whereas chemical composition displayed increasing impact.After cloud events,BC tended to grow larger and the abundance of sulfate increased slightly.（4）Research on chemical composition of particles in cloud/fog water and rainwater based on SPAMS:Cloud/fog water and rainwater samples collected were atomised by an aerosol atomiser,dried with a silica gel dryer and analysed by SPAMS.The particle types and the discrepancies between cloud/fog water and rainwater were investigated.Results showed that 8 types of particles（biological aerosols,dust,sea salt（SS）,mixture of iron and organics（OC）,amines,mixture of BC and OC,OC and others）were detected in both cloud/fog water and rainwater.The mass spectra characteristics were used to judge the sources.CCN and IN mainly originate from SS,dust and biological aerosols（～70%）.SS（35.7%）,OC（20%）and iron（6.4%）contributed more in cloud water while dust（35.6%）and the mixture of BC and OC（18.9%）contributed more in rainwater.