Spatiotemporal Variation Characteristics Of Black Carbon And Its Influencing Factors Over Shanghai

Black carbon?BC?,which is ranked as the second largest contributor to atmospheric warming after CO₂,has important effects on climate changes and human health.It can easily cause adverse health effects on respiratory system due to its finer particle size.BC is mainly emitted as the by-products of incomplete combustion of fossil fuels and traffic exhaust in urban.This paper was focused on spatiotempora l variation characteristics of atmospheric BC based on stationary and mobile sampling methods in Shanghai which was in the process of rapid urbanization.Meanwhile,associated with other predictor variables,e.g.meteorological conditions,major atmospheric pollutants,land use,traffic network,population and economic factors,etc.,the influence mechanism of spatiotemporal variation characteristics of near-surface atmospheric BC concentrations was analyzed and discussed in Shanghai.Furthermore,based on a land use regression?LUR?model and BC data from mobile sample monitoring,spatial simulations of BC concentrations were conducted to explore the determinants for within-city near-surface atmospheric BC concentrations and their influencing factors over Shanghai.Major results of this paper are as follows:?1?BC daily average concentrations observed at three fixed stations in Shangha i were 1.17±0.61?g·m^-3?Pudong station?,1.11±0.48?g·m^-3?Minhang station?and2.09±0.97?g·m^-3?Qingpu station?,respectively.Integrated observations of three sites,average BC concentration of Shanghai(1.56±0.71?g·m^-3)was 5.23 times of annual average BC concentration observed at Mt.Waliguan(0.30?g·m^-3)during 2000-2011.Affected by human activity,atmospheric BC concentrations of three stations exhibited significant diurnal variations with two peaks and one trough,but there was no significant difference between in weekday and weekend.For monthly and seasonal variations,high atmospheric BC concentrations occurred in winter,and in other seasons,BC concentrations of three stations showed relatively large differences.As regards the spatial distributions for three stations,BC concentration of Qingpu station located in outer suburbs was higher than Pudong and Minhang station,which were situated in the city center and suburban,respectively.This might be related to the spread of exogenous pollution,meteorological conditions and the distance to sea.?2?On the basis of BC observations at three fixed stations,correlation relations hip between concentrations of atmospheric BC and major air pollutants(CO,NO₂,O₃,SO₂and PM_2.5)were analyzed.The results showed CO,NO₂,SO₂ and PM_2.5 were significantly positively correlated with atmospheric BC.Hence,in a sense,BC is homologous with CO,NO₂,SO₂ and PM_2.5.The correlation coefficient between BC and PM_2.5 was the highest?0.74-0.81,p<0.01?.And the correlation coefficients between CO,NO₂ and BC were higher than SO₂?CO:0.43-0.69,NO₂:0.60-0.70,SO₂:0.36-0.44,p<0.01?.It was indicated that the atmospheric BC concentrations in Shanghai was more affected by traffic exhaust than coal-generated emissions.BC concentrations had significantly negative correlations with both wind speed and boundary layer height,-0.42 and-0.37?p<0.01?,respectively.To further analysis BC heavy pollution using backward trajectory model,heavy BC pollutions in summer were mainly affected by air flow from southwest inland,and those in winter and spring were related with local emission and the pollution that air masses transport over long distances.?3?BC median concentrations(BC₅₁)in autumn(11.38±6.32?g·m^-3)and winter(11.95±5.35?g·m^-3)were higher than summer(10.06±7.05?g·m^-3)based on mobile measurements.For the spatial distributions,BC₅₁ concentration in downtown Shangha i was lower than in the suburban,and the high BC₅₁ concentration was concentrated on near highway roads and road junctions.Overall,BC₅₁ concentration showed a rough linear growth trend?R²=0.76,p<0.05?along with the decline of the level of urbanizat io n.The average BC₅₁1 concentration in rural area was the highest(15.69±1.87?g·m^-3),while the average BC₅₁1 concentration in highly urbanized area was the lowest(6.85±1.41?g·m^-3),which might be caused by spatial heterogeneity of industrial emission and highway network.Related to heavy duty diesels on highway,provincial and national roads,the correlations between average BC₅₁ concentration and road density during 100m-3 km buffer performed a decreasing trend,and the largest correlation appeared in the buffer distance of 2.5 km.?4?This paper conducted LUR models for BC₅₁ concentrations observed in the winter of 2016,the summer and autumn of 2017,and observations integrated from three mobile sampling campaigns,based on BC51 mobile measures and various spatial geographic variables.Except for the LUR model for BC₅₁ concentrations in the summer of 2017,other LUR models all performed well with high R²?0.39-0.71?and low RMSE?0.18-0.34?.Models were developed using traffic,economic and population factors,as well as physical geographical variables,which demonstrated that BC concentrations in Shanghai were mainly affected by these factors.Spatial patterns modeled by LUR showed the BC₅₁ concentrations in downtown area was lower because of less local BC emissions.However,high-concentration areas were mainly concentrated in eastern coastal port areas and west and northwest areas,e.g.Jiading,Baoshan and Minhang,etc.For the eastern coastal port areas,local BC emission was relatively large from port transports.And pollution air transport from inland and traffic emissio n played an important role on BC concentrations in west and northwest areas of Shanghai,and so was industrial emission.Additionally,there was a significant seasonal difference in spatial distributions of LUR models.The spatial patterns of BC₅₁ concentrations in autumn and winter was more affected by air mass over a long-distance transport.In summer,BC₅₁ concentrations indicated a homogeneous distribution due to the better atmospheric diffusion conditions.