Study On The Pollutant Transport Over North China Plain And Yangtze River Delta Based On MAX-DOAS Observation Network

With the economy developing,urban scale expanding,and emission sources concentrating,the air pollution situation in China has taken on regional and complex pollution characteristics,which poses a huge threat to social development and human life.The establishment of a high-precision atmospheric monitoring network is an important prerequisite for quantifying pollution concentrations,formulating targeted environmental policies,and improving air quality.China National Environmental Monitoring Centers(CNEMCs)can monitor the near-surface distribution of various atmospheric components,but cannot reveal pollution variation at high altitudes.Satellite remote sensing can measure the horizontal distribution of pollutants in a large scale,but it is difficult to characterize their vertical distribution based on satellite observations.While model simulation provides important auxiliary data for monitoring,analyzing and predicting atmospheric phenomena,there are considerable uncertainties in estimating pollutant vertical distribution.Many significant atmospheric physicochemical reactions and pollution phenomena[e.g.,pollution transport and formaldehyde(HCHO)secondary generation]occur at high altitudes,and thus characterizing the vertical distribution of atmospheric components is crucial for monitoring atmospheric environment.In addition,the detection space range of in situ measuring instruments is limited,and they can only measure the pollution concentration at the sampling point.Satellite remote sensing,however,is limited by spatiotemporal resolution and is unable to conduct continuous observation of smallscale areas.To address the aforementioned problems,we must investigate a new monitoring method to expand the measurement spatial range,extend the observation period,and further enhance the spatiotemporal coverage of monitoring data.Differential optical absorption spectroscopy(DOAS)technology can simultaneously achieve qualitative and quantitative analysis of multiple atmospheric components by utilizing the absorption characteristics of gases in the ultraviolet and visible light bands.By obtaining pollutant vertical profiles,Multi-axis DOAS(MAXDOAS)can effectively compensate for the shortcomings of the current monitoring techniques.Compared with other monitoring technique,MAX-DOAS has the advantages of simple design,low cost,low power consumption,high automation,and minimal maintenance requirements,making it ideal for long-term uninterrupted observation.Pollutant transport has a substantial impact on the atmospheric environment in megacity clusters.However,owing to the lack of knowledge of vertical pollutant structure,quantification of transport processes and understanding of their impacts on the environment remain inadequate.In this study,we retrieved the vertical profiles of aerosols,nitrogen dioxide(NO2),and HCHO using MAX-DOAS and analyzed three typical transport phenomena over the North China Plain(NCP)and Yangtze River Delta(YRD).We found the following:(1)the main transport layers(MTL)of aerosols,NO2,and HCHO along the southwest-northeast transport pathway in the Jing-Jin-Ji region were approximately 400-800,0-400,and 400-1200 m,respectively.The maximum transport flux of HCHO appeared in Wangdu(WD),and aerosol and NO2 transport fluxes were assumed to be high in Shijiazhuang(SJZ),both urban areas being significant sources feeding regional pollutant transport pathways.(2)The NCP was affected by severe dust transport on 15 March 2021.The airborne dust suppressed dissipation and boosted pollutant accumulation,decreasing the height of high-altitude pollutant peaks.Furthermore,the dust enhanced aerosol production and accumulation,weakening light intensity.For the NO2 levels,dust and aerosols had different effects.At the SJZ and Dongying(DY)stations,the decreased light intensity prevented NO2 photolysis and favored NO2 concentration increase.In contrast,dust and aerosols provided surfaces for heterogeneous reactions,resulting in reduced NO2 levels at the Nancheng(NC)and Xianghe(XH)stations.The reduced solar radiation favored local HCHO accumulation in SJZ owing to the dominant contribution of the primary HCHO.(3)Back-and-forth transboundary transport between the NCP and YRD was found.The YRD-to-NCP and NCP-to-YRD transport processes mainly occurred in the 500-1500 and 0-1000 m layers,respectively.This transport,accompanied by the dome effect of aerosols,produced a large-scale increase in PM2.5,further validating the haze-amplifying mechanism.This study indicates that obtaining the vertical distribution characteristics of atmospheric pollution is of great significance for analyzing pollution transport,highlighting the unique advantages of MAX-DOAS in monitoring high-altitude pollution phenomena.In summary,MAX-DOAS technology can fill in the data gaps in both the temporal and spatial dimensions of the current monitoring networks,providing a reference for establishment and improvement of high-precision environmental monitoring networks.