| The hexavalent chromium in the ambient air has a great impact on human health and the ecological environment.However,due to the easily transformation and complicated analysis of Cr（Ⅵ）,few observations were conducted on the concentration of Cr（Ⅵ）in China.Many researches estimate the concentration level of Cr（Ⅵ）in atmospheric particulate matters with the ratio of Cr（Ⅵ）to total Cr in dust（0.13）,which could bring big uncertainties for Cr（Ⅵ）in ambient air.In order to determine the regional concentration level of Cr（Ⅵ）in PM2.5 in China,the sampling and extraction method of the concentration of Cr（Ⅵ）in PM2.5 was optimized,and a laboratory analysis method of Cr（Ⅵ）based on IC-ICP-MS was constructed.Concentration of Cr in PM2.5were simultaneously determined by the ICP-AES method.Two sampling sites in Beijing and Qingdao were selected to make the observation.The concentration of Cr and Cr（Ⅵ）were sampled and analyzed in ambient PM2.5,as well as the ratio of Cr（Ⅵ）and Cr were observed.The main results of this research are as followed:（1）The experimental conditions were optimized for both sampling and extraction processes of Cr（Ⅵ）analysis.The filters were pre-treated in acid before sampling,with rinsing by ultrapure water and soaking by alkaline.The proper temperature for ultrasonic extraction were recommended to be below 25℃.The storage time of samples should be less than 29 days.A IC-ICP-MS method for the determination of Cr（Ⅵ）were established with the detection limit of 0.0015 ng/m3,the spiked recovery of 92.78%,and the accuracy of 7.8%.And compared with the commonly used ion chromatography method,the method established in this study had the shorter the detection time and the lower the detection limit,which can accurately determine the concentration of Cr（Ⅵ）in atmospheric PM2.5.（2）Based on the analysis of samples collected at the sampling sites of the Chinese Academy of Environmental Sciences in Beijing and Qingdao University in Qingdao from September 2020 to December 2020,the concentrations of Cr（Ⅵ）in PM2.5 were（0.140±0.065）ng/m3in Beijing and（0.091±0.073）ng/m3in Qingdao.Also the average concentration of Cr（Ⅵ）in PM2.5in the two cities in December were higher than those of September,October,and November.Based on the analysis of potential sources of Cr（Ⅵ）in autumn and winter,the results showed that in autumn,Cr（Ⅵ）in Beijing mainly come from emissions in the northern part of Beijing and transportation in the northern surrounding areas,while in winter it is affected by the southern and southern surrounding areas of Beijing.In Qingdao,Cr（Ⅵ）in autumn was affected much by marine air mass,and the main potential source in winter was from surrounding areas in Shandong Province.（3）The relative humidity,concentration of ozone and PM2.5 were found to be the influencing factors for Cr（Ⅵ）during the sampling period.When the relative humidity increases,the concentration of Cr（Ⅵ）and the ratio of Cr（Ⅵ）to Cr showed a negative correlation trend.While,the concentration of Cr（Ⅵ）and the ratio of Cr（Ⅵ）to Cr increase slightly with the increase of ozone concentration.The higher concentration of PM2.5 may help the conversion of Cr（Ⅵ）.（4）During the sampling period,the range for the ratio of Cr（Ⅵ）to Cr was 0～0.1,with the mean value of 0.0623±0.0969.The ratio of Cr（Ⅵ）/Cr in Beijing and Qingdao were 0.076±0.104 and 0.041±0.039,respectively.The ratio decreases with the increase of the concentration of Cr.Concentration of Cr（Ⅵ）in 50 cities in China from references were estimated by the value of Cr（Ⅵ）/Cr obtained from this research and those in dust,the average values showed great differences,the concentration of Cr（Ⅵ）calculated based on the ratio of Cr（Ⅵ）to Cr obtained from this research[（0.52±1.04）ng/m3)]is significantly lower than those estimated from dust ratios[（4.04±8.51）ng/m3].The highest concentration of Cr（Ⅵ）in North China was（1.13±1.58）ng/m3,followed by East China at（0.85±1.89）ng/m3,and the lowest was（0.19±0.12）ng/m3in South China.Compared with other countries,the concentration of Cr（Ⅵ）values at the sampling sites in Beijing and Qingdao in this research showed lower values.The average concentration of Cr（Ⅵ）estimated by the ratio of this research in China was lower than that in South Korea and similar with Canada and Australia. |
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