Long-term Variations Of Aerosol Over Wuhan And Their Impacts On Cloud Phase

Aerosols are the suspended tiny particles or droplets in the atmosphere.They can cool the earth by scattering or absorbing solar radiation or impact the earth radiation budget and precipitation by modulating the formation,changing the phase and microphysics of clouds.In addition,particles with diameter less than 2.5?m,can be inhaled;thus,they can be harmful for human health.Therefore,it would be of great importance to investigate the concentration,types,vertical distribution,and temporal evolutions of aerosols.Chinese government had imposed strict regulations on air quality improvement.To comply the national actions,Wuhan local government also took actions of“embrace the blue sky”to improve air quality.These measures would not only alter aerosol distribution near the surface,but also change aerosol vertical distribution.It's well-known that such changes would alter the atmospheric environment;however,it's still not clear for their impacts on meteorology and climate,which is of vital importance for Wuhan suffering from severe rain for a long time.In order to investigate aerosol vertical variations under the background of clean air actions and their impacts on cloud phase,Mid-Upper Atmosphere research team of Wuhan University has conducted long-term continuous observation since 2010 by a self-developed polarization lidar at Mountain Luojia.An automatic lidar processing algorithm was developed to analyze the huge amount of lidar data.Furthermore,to mitigate the interference of multiple scattering(MS)effects on cloud phase identification,a MS model was developed to investigate the features of MS effects in clouds on lidar signal.Finally,the ice nucleating effectiveness of dust and non-dust,pure dust and polluted dust were analyzed by using POLIPHON algorithm.The main research contents are summarized as below:(1)An automatic lidar processing algorithm was developed by combining a calibration-free cloud detection algorithm and an improved Rayleigh fit algorithm.This algorithm has been successfully applied in the long-term lidar data analysis,which is huge time-consuming for manual analysis.The overall mean difference and standard deviation between lidar-derived aerosol optical depth(AOD)and Terra/Aqua AOD were less than 0.05 and 0.2,respectively,which states the automatic lidar processing algorithm results are robust and reliable.(2)A multiple-scattering(MS)model was developed to investigate the MS induced effects on lidar signal by clouds.The modeling results show that volume linear depolarization ratio is increased along the increase of penetration depth into the clouds.Meanwhile,the increased value tends to be saturated finally and the saturated value is associated with cloud optical properties.(3)The annual mean PM_2.5of Wuhan from 2014-2020 has declined 46.5%,from 70.0?g?m^-3to 37.4?g?m^-3 by investigating the ground station data.In addition,the reductions of PM₁₀ and SO₂ are also substantial,which corroborate the effectiveness of clean air actions.On the other hand,O₃ increased dramatically,which states the goal for air quality improved should be changed respectively to deal with the new situation.Furthermore,inter-annual decrease rate of AOD is 0.049 for Wuhan from 2011-2020,which is the largest decrease rate among fifteen megacities worldwide and is 10 times of that for Europe and US,due to the local-wide and national-wide measures for air pollution control in China.By separating the total AOD into boundary layer(BL)and free troposphere(FT)parts,it can be concluded that?80%of the total AOD decline was contributed by BL AOD,which proves the effectiveness of emission reduc-tion.It should be noted that BL AOD was always larger than FT AOD;however,since the implementation of COVID-19 city lockdown at February 2020,this relationship was reversed and the BL AOD researched its minimum level of 0.125.This states COVID-19 city lockdown imposed large impacts on human activity and emission.(4)The ability of aerosols to serve as ice nucleating particles(INP)is negatively correlated with cloud top temperature by inspecting the relationship between ice-containing cloud frac-tions and cloud top temperature.At temperature ranging from-5?-30°C,the effectiveness to trigger heterogenous ice formation of dust particles is stronger than that of non-dust particles.It can lead to 20%more ice-containing cloud fractions in total.Meanwhile,90%of dust-asso-ciated clouds are contributed by polluted dust instead of pure dust.Effective INPs could be the lowest in summer by inspecting the seasonal distribution of ice-containing cloud fractions.In-terestingly,there could be some unknown effective INPs in autumn,which can trigger hetero-genous ice formation at temperature above-5°C.Through comparisons of ice-containing cloud fractions at Wuhan and Leipzig(Germany),which feature different air pollution level,it can be concluded that non-dust particles might not be effective INPs and polluted Asian dust could be more effective than polluted Saharan dust in triggering heterogenous ice formation.