| China is one of the land areas with high aerosol optical thickness in the world.High concentrations of particles can enhance the absorption and scattering of the solar radiation,thus decreasing regional visibility.They can also degrade air quality and pose a threat to human health.In recent years,haze events caused by fine particle pollution in megacities have occurred more and more frequently,and also expand to their immediate vicinities.It is found that there are an increasing in frequency,intensity,and duration of pollution events,and there are five heaviest pollutions regions over China,which is Beijing-tianjin-hebei,Yangtze River delta,the Pearl River delta,Sichuan basin and Guanzhong basin.So far,most studies about aerosol and air pollution in China have been made in the North China Plain and southeast of China.And the study of aerosol particles and their interactions with meteorological parameters become more important to further understanding the reason of air pollution in the Guanzhong Basin.In this study,based on the MODIS AOD product,CE318 data,particle number size distribution data,long term meteorological data and Air Quality Index data,we study the spatial and temporal distribution of aerosol optical depth over Guanzhong basin and the characteristics of average particle number concentrations and new particle formation,show the variation of visibility scale,sunshine duration,solar radiation,precipitation and their relationship with aerosol particles.Finally,we discuss the characteristics of air pollution in the past three years and the reason of the occurrence of pollution days.The main conclusions are followed.?1?Spatial and temporal variations of aerosol optical depth?AOD?over Guanzhong basin are investigated using MODIS-derived aerosol data for the period of 2000-2010.The data shows that the distribution of aerosol is largely affected by topography and local economic activities.Heavy aerosol loading and an increasing tendency in AOD is observed in the east part of Guanzhong basin.The variation of aerosol fine mode fraction?FMF?shows that the combined effect of the natural emissions and anthropogenic emissions is the main source of high AOD over Guanzhong Basin.The value of AOD is higher in summer and lower in autumn,which is both observed by CE318 and MODIS aerosol product.The variation of AOD in Jinghe station shows that there are many factors which affect the value of AOD,such as dust storm,emissions of heating in winter time and biomass burning,and high relative humidity etc.The fine mode aerosol emissions increase alongside economic development in recent years over Guanzhong basin.Aerosols are experiencing a transition to anthropogenic aerosols in Guanzhong.?2?The variation of particle number concentration and size distribution in the suburb of Xi'an shows that the annual average particle number concentrations in the nucleation(NNUC),Aitken(NAIT),and Accumulation(NACC)size ranges are 960 cm-3,4457 cm-3,3548 cm-3,respectively.The mean total particle number concentration(NTOT)is 8965 cm-3,which is lower than that in Shanghai and Beijing,similar as that in Shangdianzi and in summer in Jinan.The number concentration is dominated by particles around 67.3 nm in summer,80 nm in spring and fall,while about 90 nm in winter.The diurnal variation of the nucleation mode particles is mainly influenced by New particle formation?NPF?events in summer,while by both traffic densities and NPF events in spring,fall and winter.The diurnal variation of the number concentration of Aitken mode particles correlate with the traffic emission in spring,fall and winter,while in summer it more correlate with contribution of the growth of the nucleation mode particles.The burst of nucleation mode particles typically start in the daytime.The growth rates?GR?of nucleated particles range from 2.8 to 10.7nm h-1 with an average of 5.0±1.9 nm h-1.Among observed 66 NPF events from 347effective measurement days,85 percent of their air masses come from north or northwest China,resulting in a low concentration of pre-existing particles,and only15 percent come southerly from Qingling Mountains.Based on their growth rate,64and 36 percent of their subsequent particles,corresponding to types 1 and 2 NPF events,grow and seldom grow after the burst of nucleation mode particles.For type1 NPF event,the nucleated particles can grow up to 40nm or larger when surface winds shifte from westerly to easterly or southeasterly?from village areas?.For type2 NPF events,the particles keep almost unchanged when the winds stay westerly.?3?The new particle formation?NPF?from two distinct suburban locations in northern China is analyzed during May and June of two consecutive years.The results show that the average of total particle number concentration in the similar size range of 10-487 nm at the suburb of Beijing(9.0×103 cm-3)is about two times higher than those observed at Xi'an(4.7×103 cm-3),and the mean particle mode diameter at Beijing is 1.4 fold higher than that at Xi'an.The estimated total condensation sink?CS?at Beijing(3.11×10-22 s-1)is also higher than at Xi'an(1.13×10-2 s-1).The frequency of NPF events at suburb of Beijing is 24%,lower than that in Xi'an?50%?,and also lower than urban site of Beijing?35%in June?and another suburb of Beijing?over 50%in June?.The NPF events with?Class I?or without?Class II?subsequent growth are both observed at the two suburb sites.The derived GR at the suburb of Beijing(range from 4.6 to 8.6 nmh-1)is a little higher than that at Xi'an(range from 3.3 to 6.7 nmh-1),which are generally comparable to typical values in mid-latitude reported in previous studies.The number size distributions of freshly nucleated particles show clear bimodal distributions at both sites.The case study of NPF events at the two suburb sites shows that the surface area concentration and total scattering coefficient?SC?is significant decreased during the NPF events at both sites.High temperature,low condensations sink and low relative humidity is favor for the occurrence of NPF events,and wind direction shifting is important for the subsequent growth of particles.And low CS may be more important than low relative humidity?RH?for the particle formation under clear day.?4?Based on the meteorological data from the Shanxi meteorological bureau,the variation of visibility scale,sunshine duration,solar radiation and precipitation is analyzed.The result shows that there is a negative correlation between visibility scale and air pollution index,the decrease of visibility scale means the increase of aerosol particles concentration in recent 60 years over Guanzhong.And the fluctuate declining of solar radiation and sunshine duration in recent 60 years have great relation with the increasing of aerosol concentration.Furthermore,the variation of total precipitation over Shanxi province suggest that heavy pollution suppress light rain frequency and amount both in annual and rain season over Shaanxi province,while invigorate moderate and heavy rain events in rain season.Combining the MODIS AOD data and NCEP wind data at 850 hpa,we found that wind from northwest favor the diffusion of aerosol particles,while the warm wind from south east with high relative humidity and high aerosol concentration will aggravate the pollution of Guanzhong Basin.Lower boundary layer will suppress the diffusion of air pollution.?5?Pollution events occur in a total of 175,104 and 138 days in suburb of Xi'an from 2014 to 2016,separately.Heavily and severely polluted days are about 39.1%,35.1%and 53.6%of the pollution events from 2014 to 2016.They mainly occur in deep autumn,winter and early spring.The mean particle number concentration is about 10348 cm-3 in pollution days,more than 1.25 times that in no pollution days.The greatest increase of particle number concentration is in 100-200 nm and 200-500nm size fractions during pollution events,about 1.64 times and 2.91 times than those of no pollution days.The particle volume concentration and surface concentration in pollution and no pollution days are 34.9?m3/cm3,792?m2/cm3,and 15?m3/cm3,430?m2/cm3,As pollution events get more severe,the number concentration of particles smaller than 50 nm decrease,but the particles of 100-200 nm and200-500nm increase.This suggests that the severity of pollution is not simply determined by the increase of particle number concentration,but also closely relate to the size distribution.The diurnal variation of particle number concentration shows a bimodal pattern in pollution days.The case study of different levels of pollution events shows that there is an obviously positive correlation between the number concentration of 100-500nm(N100-500nm)and AQI,and also with PM2.5.These suggest that the increasing of number concentration of accumulation mode particles is the main reason of the occurrence of pollution days.Furthermore,the controlling of the air mass with low temperature,high relative humidity and low wind speed is major meteorological conditions. |
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