Effects Of Land Cover Changes On Summer Meteorology And Air Quality In Lanzhou

In the last decades,urban land expansion has become important themes of China's urbanization development.The urban land expansion can alter regional climate and environments by affecting traffic and emission sources.Different regions and different seasons have different effects of urbanization process on meteorological conditions and air quality.Therefore,it is very important to explore the impact of land cover changes caused by urban construction in specific regions on meteorological conditions and air quality.Lanzhou new area is the first national new area in Northwest China.From 2015to 2020,the urban land area expanded significantly,the resident population increased significantly,and the urbanization process developed rapidly.Therefore,this paper takes the construction of Lanzhou New Area as an example to discuss the impact of land cover changes on meteorological conditions and air quality in Lanzhou.Base on the WRF-CMAQ model,combined with the MEIC anthropogenic source emission inventory data developed by Tsinghua University,and using the emission data output by SMOKE and MEGAN model,this paper simulates the air quality in Lanzhou in July2015 and 2016.Then,sensitivity experiment is conducted to discuss the impact of Lanzhou New Area construction on summer meteorological elements and air quality under the condition that land cover changes and anthropogenic emissions remained unchanged,and the CCM algorithm was used to quantify the impact of meteorological fields on air quality.Through the analysis of the concentration changes of specific PM_2.5,O₃ precursors and other pollutants,this paper discusses how the construction of Lanzhou New Area affects the air quality in Lanzhou through the changes of meteorology.The following conclusions can be obtained:(1)The WRF model overestimates the 2m temperature in Lanzhou.The wind speed of the Lanzhou station is also overestimated to a certain extent,and the surface pressure is underestimated at most stations.However,the WRF model can accurately describe the temporal variation characteristics of 2m air temperature,dew point temperature,10m wind field and surface pressure.In general,the WRF model can better simulate meteorology in Lanzhou.CMAQ model can better represent the changing trend of PM_2.5 and O₃ concentrations in Lanzhou.Although the model has deviation in the estimates of PM_2.5 and O3 concentrations the statistical parameter errors are also within a reasonable range.Therefore,The simulation results of WRF-CAMQ model for the two air pollutants are reliable.(2)In order to explore the impact of Lanzhou New Area construction on summer meteorological elements and air quality in Lanzhou,this paper carried out a sensitivity experiment before and after the construction of Lanzhou New Area.The construction of Lanzhou New Area has a positive impact on local temperature.The impact on local meteorological elements in Lanzhou New Area is higher than the impact on the surrounding areas of Lanzhou New Area,and the impact on temperature gradually weakens with the increase of altitude.The changes of boundary layer height and ventilation coefficient in Lanzhou New Area are similar to the changes of air temperature.The boundary layer height in Lanzhou is affected by temperature.Changes in ventilation coefficient is driven by the combined competition between changes in boundary layer height and horizontal wind speed.The change of relative humidity showed a opposite spatial distribution law to the change of air temperature and boundary layer height.These changes in meteorology in Lanzhou New Area can be attributed to increased surface roughness due to land cover changes and effects on the energy balance between latent and sensible heat fluxes.(3)The changes in the local summer PM_2.5 concentration caused by the construction of Lanzhou are consistent in each year.And the construction of Lanzhou New Area has a significant positive impact on PM_2.5 concentration in Lanzhou,except for Lanzhou urban area during the daytime in 2015.with a high increase in the proportion of urban and building land,the more obvious the increase in PM_2.5concentration in Lanzhou New Area.According to the CCM algorithm,air temperature,ventilation coefficient and relative humidity are the main influencing factors of PM_2.5concentration and specific PM_2.5 such as OC,EC and SO₄^2-in Lanzhou.After the construction of Lanzhou New Area,the change of PM_2.5 concentration in Lanzhou is mainly caused by the change of OC and SO₄^2-.The influence of EC and NO₃^-concentration change on the change of PM_2.5 concentration in Lanzhou urban area cannot be ignored.The main reasons for the changes in the concentrations of PM_2.5 or various specific PM_2.5 in Lanzhou are composed of the following four points:the change of atmospheric diffusion capacity;the change of air temperature leads to the change of atmospheric oxidation capacity and its impact on SO₄^2-;the change of temperature leads to the change of distribution between gas phase and particle phase of semi volatile substances,which has an impact on NO₃^-;effect of relative humidity change on PM_2.5 concentration.(4)The changes in the local summer O₃ concentration caused by the construction of Lanzhou New Area are consistent in each year.The construction of Lanzhou New Area has a positive impact on the O₃ concentration in Lanzhou New Area,and the increase of O₃ concentration during the daytime is more obvious than that at night.The impact of the construction of Lanzhou New Area on Lanzhou urban area is different in two years.In 2015,it is mainly positive,which is also caused by the changes of meteorological factors.The increase of O₃ concentration increases with the proportion of urban and building land,which is similar to the change characteristics of PM_2.5concentration.According to the CCM algorithm,air temperature,ventilation coefficient and relative humidity are also the main meteorological factors that affect the O₃concentration in Lanzhou,and the ventilation coefficient has a significant impact on CO and NO_x.The sensitivity of O₃ generation in Lanzhou new area is more inclined to NO_x.The sensitivity of O₃ generation in Lanzhou urban area during the daytime is more inclined to VOC,and more inclined to NO_x at night.The change of O₃ concentration caused by the construction of Lanzhou New Area can be attributed to the following points:the change of atmospheric diffusion capacity can not only directly affect the O₃concentration,but also indirectly affect the O₃ concentration through the change of NO_xconcentration;the effect of temperature change on atmospheric photochemical reaction;the effect of NO_x concentration change on O₃ concentration.The change of O₃concentration in Lanzhou during the daytime is the result of the joint action of the three,while the influence of temperature is weakened due to the weakening of sunshine at night.In short,the construction of Lanzhou New Area in different regions or years can have different effects on Lanzhou air quality.