Investigation On Black Carbon Emission And Wet Scavenging Over The North China Plain Region

Black carbon(BC)aerosols can directly absorb solar radiation,and heat the atmosphere,which will alter the local atmospheric dynamic characteristics.In addition,some hydrophilic materials can be deposited on the surface of BC particles through aging process in the atmosphere,which is termed coatings.These coatings,on the one hand,cause "lensing effect" and increase the radiation absorption capacity of BC;on the other hand,the hygroscopicity of BC could be enhanced as well,which will increase its cloud condensation nuclei(CCN)ability,causing indirect effect on the radiation balance of the climate system by aerosol-cloud interactions.The North China Plain(NCP)is one of the regions with most BC emission in China.Although a large number of ground-based experiments have been performed in this region,their results cannot provide the vertical distribution of BC.Additionally,the characteristics of wet scavenging of BC are still subject to large uncertainties.Therefore,three experiments were carried out in this study to deal with the emission and wet scavenging characteristics of BC over the NCP region.In the first experiment,the vertical profiles of BC particles over the NCP were observed through aircraft measurements,which further explored the optical properties and CCN ability of BC particles.The second experiment was simultaneously performed on a surface site and a mountain site,continuously lasting 1 month in both winter and summer respectively.By doing so,we examined the BC seasonal emission structure and wet scavenging characteristics from the surface to the top of boundary layer.The third experiment was conducted on a mountain site in winter,which was intended to probe the wet scavenging of BC particles in the mixed-phase clouds.The results of aircraft measurements showed that the pollution level of BC in Europe was only equivalent to that in NCP region under clean background conditions.The size of BC cores measured in Europe and the NCP region in warm season were significantly smaller than that in cold season,which suggested the contrast in emission that there were more heating activities in NCP regions in cold seasons.The BC coating thickness over NCP was positively correlated with the pollution level and the modelled absorption enhancement by Mie calculation of BC due to coatings could reach as high as 1.9 in heavy pollution conditions.The CCN ability of BC particles calculated by Kohler model show that under heavy pollution conditions,due to the larger particle size and higher hygroscopicity of BC particles,half of the BC number could be activated at a supersaturation of 0.08% only,however in clean conditions,this value needs 0.14%.These results are important parameters to study the CCN ability and wet scavenging efficiency of black carbon in the NCP region.In order to evaluate the evolution and removal process of BC particles during the transport in the NCP region,an experiment was simultaneously performed on a surface site and on a mountain site at the top of boundary layer to explore the transport and wet removal process of BC from surface to the top of boundary layer.By introducing a net ratio of r BC mass divided by carbon monoxide relative to background: BC/CO(?BC/?CO),we found both site peaked at the same ?BC/?CO on the surface for both seasons,however this was not the case for mountain site:?BC/?CO was observed slightly higher on mountain than surface in winter due to being influenced by airmass from wider area;summer was wet with more precipitation and BC was observed to undergo wet scavenging when transported from surface to the top of boundary layer,resulting in a lowered ?BC/?CO on mountain than surface.This wet scavenging process could continuously input 35-62% of r BC mass from the surface emission into the summer moist air or low-level cloud on a daily basis,having important indirect radiative effects.The rest of BC particles after scavenging presented smaller core size but more coatings,resulting in a higher absorption efficiency by 45%.These results are useful for evaluating the representation of boundary layer processing of BC particles and the potential impacts that BC may have on the boundary layer.In the third experiment,a complex BC-mixed phase clouds interaction process during a full precipitation event was captured.It was found that the BC particles with large BC core and thicker coatings are preferentially removed by the droplets,and the BC cores in the interstitial phase of the atmosphere increase steadily during the subsequent snow precipitation.Based on the observation results and the derived conceptual model,a new mechanism of the interaction between BC particles and mixed-phase clouds is proposed whereby the collision behaviors of droplets could merged BC in different droplets into one large droplet and the BC cores in the same droplet could form a larger BC core through coagulations,then in the subsequent snow precipitation,due to the dominant effect of ice phase,BC particles in droplets will be released and thrown back to the interstitial phase by Wegener?Bergeron?Findeisen processes,resulting in the increase of the core size of BC particles in the atmosphere.Based on the observation data,we present the BC core size-resolved scavenging and release rate in the mixed-phase clouds.These results are of great significance for evaluating the wet scavenging of BC particles(and even other aerosols)in the mixed phase cloud.These results above reveal the three-dimensional distribution characteristics of BC emission and microphysical properties in North China by in-situ measurements.The key processes affecting its transport,evolution and wet removal are also analyzed.In particular,this paper point out that there are a large number of BC particles with thick coatings over the polluted North China region,which can be effectively activated as cloud droplets;BC particles from surface emission can be efficiently incorporated into summer moist air or low-level cloud during the transport from surface to the top of boundary layer on a daily basis over this region,exerting important direct and indirect impacts;even though the BC particles in this region can be effectively scavenged by water clouds/warm clouds,the existence of ice phase in mixed-phase clouds will lead to the release of the scavenged BC particles in the droplets to the interstitial air.This result will be useful to constrain the wet scavenging mechanism of BC particles in mixed-phase clouds at high altitude or in cold regions.All these findings provide a theoretical basis for comprehensive assessment of the direct and indirect effects of BC particles from in-situ measurements.