Vertical Transport Of Gaseous Pollutants In Deep Convective Clouds And Its Impacts On The Acidity Of Cloud And Rain Drops

Deep convective clouds are the main carrier for vertical transport of atmospheric mass,can transport the various pollutants-containing air from their sources in the lower troposphere to the upper troposphere or even the lower stratosphere in a relatively short time,and thus influence the regional to global atmospheric environment and climate.However,the most studies of convective transport focus on the analysis of vertical distribution characteristics of trace gases,and the domestic research on the vertical transport is relatively small.Thus,a cloud model with detailed bin microphysics as well as gases transmission process was used to simulate a deep convective cloud,which occurred on 30 July 2014 over northweat Nanjing.We investigated the impact of convective system on the transport and redistribution of chemical compositions,and learnt what processes in the convection influence the distribution of different trace gases.Besides,we also discussed how the acidity of cloud droplets and rain drops of different sizes are changed during transport of acidic gases such as CO₂ and SO₂.The main conclusions are as following:The vertical distribution of insoluble gases was related to cloud draft structure,and the contribution of upward transport of boundary layer tracers?0–2.1 km?to the upper troposphere was comparable to that of mid-tropospheric tracers?2.1–4.5 km,4.5–7.5 km,7.5–10.8 km?.The vertical distribution of soluble species also depends on the chemical and physical processes within the convective system.The results showed that cloud draft structure is the dominant factor controlling the distribution of trace gas in its initial layer,and the maximum mean change rate reached-4.6 mol s^-1.The chemical and physical processes played an important role in the distribution of gases in other altitudes,and the contributions of them increased with the solubility of gases,and the key microphysical processes that dicated these results were the evaporation of cloud water and sublimation of ice.The retention coefficient?RC in this study?of gases also influence the vertical distribution of gases,and the sensitivity of gas transport to RC increased as solubility increases.For highly soluble gases(H^*>10⁶ mol dm^-3 atm^-1),a change in RC from 0 to 1 led to a 60%decrease in gas transport but a 40%decrease when the aerosol concentration increased by 50%,which related to the stronger evaporation of ice crystals with higher mass concentrations under high aerosol loading.The sensitivity of vertical transport of tracer gases to retention coefficient and solubility were mainly influenced by aerosol originating from the boundary layer.The concentration of H⁺was higher in the relatively larger droplets at the initial stage of the cloud,whereas it had an opposite dependence on the drop size at the later stage of the cloud,and the ratios of H⁺concentrations between raindrops and cloud droplets were almost unchanged with values around 0.11 after 70 min.The spectrum distribution of drop acidity also highly varied with space,reflecting the different effects of macro-and micro-physical processes on drop acidity.For the entire simulation period,evaporation process contributed the most to the decreasing in pH values of drops?especially for the drops smaller than 100?m?,which related to the decreasing in cloud water.Condensation and collision-coalescence played an important role in modifying the chemical rates of hydrogen ion production,leading to changes in the pH values,and the contributions of them to the changes in pH values are+0.0034 s^-1 and+0.0025 s^-1 at cumulus stage,respectively.The sensitivity tests revealed that background aerosol loading has a significant impact on the pH values and sulfuric acid production in drops.