The Effects Of Atmospheric Aerosol On Visibility, Clouds And Precipitation

The spatial-temporal variations of aerosol characteristics,as well as its direct and indirect effects have been evaluated using the combination datasets of satellite measurements,aircraft and ground observations in two study areas.One is East Asia-China,which is characterized as the area with rapid increase of aerosol and gaseous pollutants in last decade.And the other is VOCALS-REx(The VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment) study region,which located at the Southeast Pacific.The main content,contribution and innovation of this thesis are described as below.1.The major contributors to the heavy visibility degradation over ChinaA new weather type known as haze has been found as a ubiquitous phenomenon over city clusters in China due to the direct effects of increasing aerosol loading.The spatial variation of visual range over China in 2005 revealed that the annual visibilities of major city clusters were below 7km.The long-term aerosol sampling campaign over Shanghai(a coastal mega-city in eastern China) and Urumqi(an inland city nearby the Asian dust source area in western China) in the four seasons of the past five years was carried out to investigate the relation of visibility with those chemical species in the aerosols and the meteorological factors.The visibility was mainly determined by TWSII(Total Water Soluble Inorganic Ions) and RH(Relative Humidity) in four seasons.While the meteorological factors were excluded,visibility was mostly determined by TWSII.Specially,ammonium was the most significant contributor in most cases over the entire China due to its high hygroscopicity.In the case of Urumqi,it was further revealed that the chemical species,such as NH₄⁺,SO₄^2-, and NO₃^-,rather than PM_2.5,were the key contributors to the heavy visibility degradation.The negative correlation of visibility with oxalate in aerosol indicated that organic aerosols might also play an important role in visibility degradation over China.2.Precipitation changes induced by pollution and associated aerosols—baesd on the Eastern Asia-China study areaThe increasing aerosol loading not only changes the radiative budget and contribute to the local visibility degradation,but also serve as cloud condensation nuclei(CCN) and ice-forming nuclei(IN),which alter the main path of precipitation-forming microphysical processes and the precipitation amount.The Empirical Orthogonal Functions and correlation analysis were applied to analyze the precipitation amount collected from 160 gauge stations during 1951-2007.The temporal patterns of the major four principal components were correlated to Pacific-North American teleconnection pattern(PNA),Pacific Decadal Oscillation (PDO),Arctic Oscillation(AO) and Nino 34 index,respectively.The spatial patterns of the major four EOFs showed much less spatial variation than that of aerosol.Annual trend of rain frequency retrieved from TRMM PR was compared to that of tropospheric NO₂ from GOME and SCIAMACHY and aerosol optical depth from MODIS in the local and continental scale.There was an obvious inverse relationship between the rain frequency and NO₂ and associated aerosols in spring even at continental scale.The spatial-temporal consistency of pollution and rain frequency suggested that precipitation could be changed possibly due to the pollution effects.Comparison between trends in rain frequency and precipitation amount shows that air pollution tends to suppress precipitation occurrence rather than precipitation amount or intensity.We speculate that 1) the increased NO₂ and aerosols(soot particles in particular) could enhance the absorption of solar radiation and stabilize the atmosphere,resulting in reduction of cloud formation and rain frequency;2) the enchancement of CCN from both anthropogenic aerosols and long-range transported mineral dust further suppresses the rain frequency,as favored by topography,wind, and other meteorological conditions.Furthermore,the suppression of precipitation will lead to an increase in moisture and hygroscopic particles in the atmosphere, which would cause regional haze due to insufficient moisture supply or intensive precipitation if water vapor in the atmosphere exceeds a threshold.This hypothesis is supported by the ground observations in China,i.e.,increasing haze days and an increasing trend of intensive precipitation frequency over the Yangtze River Delta region.These findings highlight the threat to hydrological cycle in polluted regions of the world,as in some industrialized areas of China and India,not only locally but also in the downwind regions.3.The aerosol indirect effects on cloud and precipitation using multi-platform measurements—based on the VOCALS-REx study regionCloud-aerosol interaction is the major part of aerosol indirect effects.The aerosol impact on cloud formation,cloud microphysical properties and drizzle formation, precipitation were investigated by utilizing multiple instruments/platforms measurements including aircraft in-situ measurements and satellite measurements. The satellite measurements are validated with the high spatial and temporal resolution data from aircraft in-situ measurements in the VOCALS-REx study region. The increased cloud droplet number concentration,decreased cloud effective radius and raise of top cloud height were observed due to the increase aerosol loading,which verified the first indirect aerosol effect.The similar spatial correlation between wind speed and cloud properties(cloud fraction,cloud optical depth,cloud droplet number, cloud effective radius and cloud top pressure) was found in the case study.It was hypothesized that higher winds would bring more sea salts which contribute to increased CCN and change the cloud microphysical properties.The statistic spatial characteristics based on monthly cloud and aerosol were used to further investigate the aerosol impacts on clouds.The cloud fraction plume borders the coast and extends to the northwest ocean.But the maximum cloud fraction was found at the center of plume rather than the region nearest to the coast,which is similar with the distribution of liquid water path,but different from that of cloud droplet number and cloud top pressure.It implies that the response of "more aerosol-more drops- less coalescence- less rain- higher LWP- higher cloud fraction-longer lifetime" are not always positive.When the aerosol loading is dramatically increased and the moisture is relatively limited,the stratus clouds may experience an evaporation-entrainment feedback due to small radius,resulting in the decrease of cloud liquid water path and cloud fraction.The seasonal averaged rain frequency and rain rate from TRMM PR were studied.The distribution of rain frequency show an inverse relationship with that of cloud fraction plume.The spatial distribution of rain rate was similar to rain frequency but with fewer gradients.The possible reason is that the influences of aerosol on rain rate are less than on rain frequency,which is further corroborated by the smaller decreasing trend of rain rate than rain frequency over land.