Spatiotemporal Variation Of Air Pollution In Lanzhou And Primary Exploration Of External Sources Of Particulate Matter

In this research,the spatiotemporal variation of air pollutant concentration in Lanzhou was analyzed by using the data of air pollutant concentration from 2013 to 2016.Focusing on the main pollutants of Lanzhou(PM₁₀ and PM_2.5),the paper calculated the 96-hour backward trajectory of the urban representative point and the suburban representative point using HYSPLIT model.Integrating the backward trajectory data with concentration data of particulate matter,this paper analyzed the sensitivity of PSCF method and CWT method to the data resolution and location of monitoring points.The locations of external potential source which were closest to Lanzhou PM₁₀ and PM_2.5 were determined,and the contribution of external source and local source to Lanzhou PM₁₀ and PM_2.5 was calculated.The main conclusions are as follows:（1）Except O3,the concentrations of CO,NO₂,PM₁₀,PM_2.5 and SO₂ concentrations during the heating period were relatively higher than those during the non-heating period.Resulting from sand-dust weather,the concentration of PM₁₀ in April and May were high.The concentration of O3 during the period from March to September was higher.In the aspect of weekly variation,6 kinds of pollutants had different characteristics,but the variation was not obvious.（2）All 6 kinds of pollutants had different characteristics in diurnal change,but they all reached the maximum values during the day.Urban and suburban PM₁₀ concentrations were lowest in the evening.PM_2.5 concentrations in urban and suburban areas were significantly different at night.The concentration of CO in urban area was lowest in evening and the midnight concentration in suburb.The diurnal variation curve of NO₂ had the similar form of "Double Peaks and Double Valleys",but the "valleys" of the suburb midnight were not obvious.And the NO₂ concentration in the suburb was higher in midnight.There was only one "peak" for O3 and SO₂ in both urban and suburban areas,and the urban and suburban changes were quite consistent.（3）CO,NO₂,PM₁₀,PM_2.5 and SO₂ in urban and suburban areas had the highest seasonal concentrations in winter,and the lowest in summer.O3 had the higher seasonal concentration in spring and summer than that in autumn and winter.In addition,the average monthly concentrations of the remaining 5 pollutants in urban areas were higher than those in the suburbs except for the O3 average monthly concentration,indicating that the air pollution in urban areas was severer than that in the suburbs.（4）For the PSCF method,high-resolution concentration data can help avoid some potential source probability amplification.The results of the monitoring points in urban and suburban areas reflected identical potential sources of urban and suburban areas.Wind speed and wind direction of the monitoring point had many effects on the results.The selection of critical percentile concentration had little effect on PSCF probability,but the critical concentration of lower percentile would enlarge the individual source.For the CWT method,the weighted concentrations in urban and suburban areas were consistent in spatial distribution,but the concentration of pollutants in the monitoring points had some influence on the grids with high-value weighted concentration.The CWT weighted concentration chart of different wind speed revealed different high-value areas,but the wind direction had little effect on the CWT result.（5）Gurbantunggut Shamo,parts of Qinghai province and Taklimakan Shamo were the areas with higher weighted concentration,which were the most closely related external sources of particulate matter pollution in Lanzhou.During the period from 2013 to 2016 the contributions of external sources to PM₁₀ and PM_2.5 in Lanzhou were 9% and 6.9%,respectively.And the contributions of local sources were 91% and 93.1%,respectively.