Investigation Of The Physicochemical Characteristics And Source Attribution For Atmospheric Mercury In Urban Area Of Yangtze River Delta

Mercury is a high toxic and unessential element that can pose serious harm to human health and the ecosystem via bioaccumulation through the food chain.Atmospheric mercury exists in three forms,including gaseous element mercury?GEM?,gaseous oxidized mercury?GOM?and particulate bound mercury?PBM?.Atmospheric mercury pollution is regarded as a global atmospheric environmental problem of increasing concern recently because of its special physical and chemical property and long atmospheric lifetime.Yangtze River Delta?YRD?area is one of most industrialized area in China with highest population density and most serious environmental pollution.However,the characteristics and mechanism of atmospheric mercury was largely ignored in previous studies of pollution in YRD.The comprehensive characteristics of atmospheric mercury in representative urban area of YRD was studied in this dissertation with both of field measurements and numerical simulation.The measurements results in representative urban area in YRD?Nanjing?were analyzed for the variations of total gaseous mercury?TGM=GEM+GOM?,PBM and atmospheric mercury deposition and their impact factors.The mechanism of chemical transformation of atmospheric mercury and its sensitivity was explored with atmospheric chemical box model.The attribution sources and atmospheric processes for atmospheric mercury in East China was qualified by atmospheric environmental model of CMAQ.Following is the main results of this study.?1?The characteristics and impact factors of TGM in urban Nanjing were studied based on measurements.Continuous online measurements of TGM were conducted in urban atmospheric environment observational station of Nanjing University from 18 January to 31 December 2011.The average concentration of TGM was 7.9±7.0 ng m^-3 during the measurement period,which was significantly higher than the continental background values(?1.5 ng m^-3)but comparable to the level in other Chinese cities.The seasonal variation of TGM in urban Nanjing was evident with the high concentration in summer and low concentration in winter,which was quite different from measurements at most other sites around the world.Positive correlations between temperature,solar radiation,and TGM concentration combined with the quite different correlations between CO and TGM in summer and winter respectively provided a strong indication that natural sources from re-volatilization of deposited mercury probably were important for the high concentration of TGM in summer of urban Nanjing.TGM concentration in urban Nanjing also exhibited a noticeable diurnal pattern with the peak of greater than 8 ng m^-3 during 7?10 a.m.with largest variation in spring and insignificant fluctuations in winter.Using HYSPLIT backwards trajectories from six clusters,it was indicated that the highest TGM concentrations(11.9 ng m^-3)was derived from local air masses and the cleanest air masses were advected from the north.?2?The characteristics and impact factors of atmospheric mecury deposition and PBM in urban Nanjing were studied based on measurements.A comprehensive measurement of precipitation and size-fractionated atmospheric particles was conducted from June 2011 to February 2012 to evaluate the characteristics of mercury deposition and PBM in urban Nanjing.The volume-weighted mean?VWM?concentration of mercury in rainwater was 52.9 ng L^-1 with a range of 46.3?63.6 ng L^-1.The wet deposition per unit area was averaged 56.5 ?g m-2 over 9 months,which was lower than that in most Chinese cities,but much higher than annual deposition in urban North America and Japan.Wet deposition in summer was quite high linked with the amount of precipitation,which contributed 82.7%to the total wet deposition over 9 months.A part of contribution to wet deposition of mercury from anthropogenic sources was evidenced by the association between wet deposition of mercury and sulfate as well as nitrates in rainwater.The ions correlated most significantly with mercury were formate,calcium,and potassium,which suggested that natural sources including vegetation and resuspended soil should be considered as an important factor to affect the wet deposition of mercury in Nanjing.The average concentration of PBM during measurement was 1.10±0.57 ng m^-3.A distinct seasonal distribution of PBM concentration was found to be higher in winter as a result of an increase in PM10 concentration.Overall,54.2%of PBM existed in the particle size range less than 2.1?m.The highest concentration of PBM in coarse particles was observed in summer,while PBM in fine particles dominated in fall and winter.The size distribution of averaged mercury content in particulates was bimodal,with two peaks in the bins of<0.7?m and 4.7?5.8 ?m.Dry deposition per unit area of PBM was estimated to be 47.2?g m-2,which was 16.5%less than mercury wet deposition.PBM in coarse particles contributed 52.2%to the total dry deposition due to higher deposition velocities.Negative correlation between precipitation and the PBM concentration reflected the effect of scavenging of PBM by precipitation.?3?The mechanism and sensitivity of atmospheric mercury were studied by box model of CAABA/MECCA.The results revealed that the averaged conversion rate of GEM is 2.7%h^-1 with maximum of 14.0%h^-1 under the condition of the atmospheric composition in Chinese urban area.HgCl and ClHgBr is the dominant gas-phase product of GEM with the concentration level of 10"12 and 10^-14 mol mol^-1.Most of HgO,HgCl2 and HgBr2 can transformed to aqueous phase and react with Br-and Cl-to generate macromolecular compound with low concentration.The oxidations of atmospheric mercury is affected by photochemical reactions deeply with higher oxidation rate of 10 times during the day than night.Numerical sensitivity experiments indicated that the transformation rate of GEM increased by 112%,56%,3%,61%and 12%respectively when extremely high?313K?or low?253K?temperature,high relative humidity?95%?,high solar radiation?SZA=6.5°?,and high liquid water content of aerosol(10-9 g m^-3).On the contrary,the increase of the concentration of 03 and nitrogen oxides can make the transformation of GEM slow down due to the oxidation of more Cl and Br.The gas-phase chemical mechanism of mercury in CMAQ was improved to simulate the spatial distribution of the contribution from the oxidation of O₃?Cl2?Cl?H2O2?2OH?Br and BrO to the transformation of GEM.The results indicated the decrease of oxidation rate of GEM from ocean to inland with the bias of 25%,which was more than 90%dominated by the oxidation by Cl.Beside of Br and BrO,other oxidizers all reacted with GEM most active in summer.The vertical profile of GEM oxidation rate by O₃?Cl?H2O2?OH and BrO was unimodal distribution with the peak between 800?2500 m,while the oxidation of GEM by Br was increasing with height upon 300 m.?4?The characteristics of atmospheric mercury and deposition in YRD and its surrounding regions were simulated by CMAQ with mercury module and studied by processes analysis.The contributions from seven kinds of emission sources including natural emission,cement production,domestic life,industrial boilers,metal production,coal-fired power plants and traffic emission to atmospheric mercury and mercury deposition in East China were quantified in the base year of 2011.Natural emission was responsible for 36.6%for annual averaged GEM concentration which was regard as the most important source for GEM in East China.Among anthropogenic sources,the influence of metal production?8.4%?and coal-fired power plants?7.1%?on GEM were most evident.Anthropogenic sources dominated the variations of GOM and PBM concentration with the contribution of 86.7%and 791.%respectively.The dry and wet deposition of mercury was also controlled by anthropogenic sources with the contribution of 42.7%and 62.4%and industrial boilers was the largest contributor for mercury deposition.The effect of mercury emitted from out of China was indicated by>30%contribution to GEM concentration and wet deposition.Local emission in YRD contributed 8.5%more to GEM and?30%more to GOM and PBM in urban area than those in non-urban area.The influence of nine kinds of atmospheric processes were analyzed.Emission and vertical diffusion affected surface GEM and PBM concentration most and dry deposition was the most important removal process for GOM.Most of anthropogenic sources caused mercury transported and diffused away from urban area by horizontal advection and vertical diffusion,and made gain of mercury in non-urban area by horizontal advection.This study showed a comprehensive analysis on the characteristics,mechanism and impact factor of atmospheric mercury in urban area of YRD.The results revealed that the concentration of TGM and PBM,the wet and dry deposition of mercury in urban Nanjing were close to the levels in other Chinese cities with evident seasonal variation,but higher than those in Europe and North America.The transformation rate of GEM is increased by extremely high or low temperature and the enhancement of relative humidity,solar radiation,liquid water content of aerosol,while the increase of the concentration of O₃ and nitrogen oxides have the opposite effect.Natural emission is the dominant source for GEM in East China,while anthropogenic sources,especially industrial boiler,attribute more to the concentration of GOM and PBM,the wet and dry deposition.Also,transportation of mercury from out of China make more than 30%attribution to GEM concentration and mercury wet deposition.Mercury emitted from urban area lead to the mercury pollution in non-urban area via horizontal advection and vertical diffusion.However,the influence of the emission in YRD area is still more evident in urban area than that in non-urban area.This dissertation provides scientific foundation for controlling atmospheric mercury pollution in urban area.