Characteristic Analysis And Numerical Simulation Of Regional Source Of Primary Atmospheric Pollutants In Lanzhou

The atmosphere is one of the indispensable and basic environmental factors of human survival.Recently,substantial growth in China's energy consumption and the speeding economy development on which it is based have led to increasingly serious air pollution problems.Lanzhou,once one of the most polluted city in China,has received widespread attention.In 2012,although the local government has taken a series of strict measures to improve the atmospheric environment,which achieved remarkable results,with the increasing number of motor vehicles,the inherent industrial structure of the city and the special valley terrain,the problem of air pollution in lanzhou is still serious.To evaluate the air quality and analyze the temporal and spatial characteristics of primary air pollutants in Lanzhou based on the national air quality standard,the hour concentrations of six air pollutants(PM₁₀,PM_2.5,SO₂,NO₂,CO and O₃)were collected from five national air-quality monitoring sites in Lanzhou from 2014 to 2016.Meanwhile,according to functional area,the monitoring sites are divided into three categories:industrial area,city commercial area and rural background area to discuss the concentration distribution and change characteristics of atmospheric pollutants in different functional areas.Finally,the Hybrid Single Particle Lagrangian Integrated Trajectory Model?HYSPLIT?and the PM_2.5-to-PM₁₀ ratio model were used to determine the potential source areas of sandstorm imported from outside Lanzhou and its contribution to particulate matter(PM₁₀ and PM_2.5).Through the above analysis,the following main conclusions are drawn:?1?The air quality index?AQI?of Lanzhou ranged from 29 to 500 in 2014-2016,the number of days in which the air quality has reached the national secondary standard accounts for 72%of the total days,and the number of days of moderate and above pollution is only 5.3%,suggesting that the air quality in Lanzhou is relatively good during the study period.AQI has obviously annual,seasonal,monthly and daily change trends,and the trend of the three changes is consistent with the bimodal pattern.The air quality standard rate in Lanzhou in 2014-2016 was 71%,77%and 68%,respectively.Analysis of monthly change showed that air quality is the worst in November,December and January?standard rate was below 50%?,and the air quality is better in6-10 months?the rate is around 90%?.It is worth noting that the air quality standard rate was not the lowest during the March-May period,but the frequency of severe and above pollution was the highest due to sandstorms.In addition,the highest frequency of air pollutants in Lanzhou in2014-2016 was PM₁₀,followed by PM_2.5,O₃ and NO₂,and CO and SO₂ were lowest.?2?The analysis in three functional areas of the city indicated that the monthly variation in PM_2.5.5 displayed a unimodal U pattern?the highest value was during the heating period?,whereas that of PM₁₀ displayed a bimodal pattern?the primary peak appeared in the spring,and the secondary peak appeared in the winter?.The source difference is the main reason that causes their monthly changes to show different trends.Daily changes are significantly different from monthly changes,the daily PM₁₀ and PM_2.5.5 concentration variations displayed a bimodal pattern,the peak values appeared at 11:00-13:00 and 22:00-1:00,respectively,and the lowest values appeared at4:00-6:00 and 16:00-18:00,respectively.On the monthly,seasonal and daily scales,the PM concentrations exhibited similar patterns in the industrial,urban and rural areas,indicating that they were partly controlled by the regional natural environment.?3?Analysis of the overall change of atmospheric pollutants in Lanzhou and its three functional areas during 2014-2016 revealed that SO₂ and CO showed a downward trend,and NO₂and O₃ showed an upward trend.The monthly variation characterizes of SO₂,CO,and NO₂ in the three functional areas varied unimodally,while O₃ presented on the contrary.The daily concentration variations of four air pollutants displayed two patterns,in which NO₂ and CO had obvious peaks in morning and evening related to traffic sources,while SO₂ and O₃ had a unimodal U pattern,and the peaks are around 12:00 and 15:00,respectively.Further analysis showed the spatial distribution of atmospheric pollutants and particulate matter in three functional areas were inconsistent,there were spatial distribution differences on different time scales,which are mainly influenced by a series of human activities,such as the urban industrial layout,the number of motor vehicles,and the distribution of population.At the annual,seasonal and daily scales,the minimum of SO₂ and NO₂ occurred in rural areas but their maximum was affected by the emission intensity of pollutants.the concentration of SO₂ was the highest in industrial area followed by in urban areas?except for January?,NO₂ is highest in cities in spring and summer,and highest in industrial areas in autumn and winter.The spatial distribution of CO and O₃ concentration was different from the above two atmospheric pollutants,the CO concentration in the three functional areas was the highest in urban areas?significantly higher than that in industrial and rural area?and the lowest in industrial and rural area alternately.On the summer at annual and seasonal scales and at 11:00to 18:00 at daily scale,concentration of O₃ in industrial and urban area was greater than that in rural area.At other times,O₃ concentration followed the order:rural area>industrial area>urban area.?4?The cluster analysis of the HYSPLIT model showed that there are mainly three sources of dust storms which have a significant impact on Lanzhou City:the Gobi desert area in the Sino-Mongolian border and the Hexi Corridor region is the most important source area;followed by the Qaidam Basin,and the Taklamakan Desert has the least impact The dust material from the first source area was mainly transported along the Hexi Corridor to Lanzhou,the paths of the latter two source areas were consistent,which were transmitted from the west of Lanzhou City.Meanwhile,the PM_2.5-to-PM₁₀ ratio model results suggested that the contributions of dust storms to PM₁₀ and PM_2.5 in the spring of 2014-2016 were 51-53%and 31-34%,respectively.