Study On Effects Of Nocturnal Atmospheric Boundary Layer On Atmospheric Pollutants' Accumulation And Transport

Planet boundary layer(PBL)is the bottom layer of the earth's atmosphere affecting people's lives,in which atmospheric physical and chemical processes have an important influence on atmospheric environment.The accumulation and transport of atmospheric pollutants largely depend on the PBL structure.Therefore,it is of great significance to study the change of PBL structure on the variation of pollutants.Lots of studies have focused on the effect of daytime PBL structure on spatial-temporal variations of air pollutants,while the nighttime PBL structure variation and its effect on air pollutants attracts less attention.Hence,the Yangtze River Delta and Sichuan Basin as the main areas of air pollution in China,have been selected to explore the nighttime accumulation and transport of PM_2.5 and ozone(O₃)and the effects of nighttime PBL structure changes and cloud longwave radiative based on environmental and meteorological observation data and simulations.The main conclusions are as follows:1)Impact of the cloud radiative effect on nighttime PM_2.5 accumulation in the stable PBLIn comparison with studies on daytime PM_2.5 accumulation,less attention has been paid on nighttime PM_2.5 accumulation varying with surface radiation.Here,we show an observation of a severe PM_2.5 pollution event with the nighttime PM2.5 increases exceeding 100?g m^-3 in several hours after the sunset in a metropolis over the Sichuan Basin,Southwest China.The nocturnal PM_2.5 pollution episodes provide observational evidence that nocturnal surface radiation cooling,modulated by cloud cover change,promotes surface PM_2.5 accumulation,thus worsening the air quality.The decrease(increase)in cloudiness at night would weaken(enhance)downward long-wave radiation,promotes(weaken)radiative cooling at ground level,enhance(reduce)the intensity of near-surface inversion,and increase(slow)the accumulation of PM_2.5.The observational study with 5-year data further confirms the cloud radiative effect on nighttime accumulation of PM2.5 with a significant negative correlation(r=-0.32,p<0.01)between nighttime averages of surface PM_2.5 concentrations and cloud cover fraction.This reveals an important mechanism of cloud radiative effect on accumulation of PM_2.5 at night through changes in downward longwave radiation,surface radiation budget and thermal structure of the boundary layer.This finding extends our understanding of air pollutant accumulation at nighttime with a potential implication on atmospheric environment change.2)Mechanism of nocturnal cloud-top radiative cooling on near-surface O₃increasingNighttime cloud plays a vital in PM_2.5 accumulation in the stable boundary layer(SBL)by modulating surface radiation cooling,while less attention has been paid to the impact of cloud on O₃ in the residual layer at night.In this study,we used measurements and a single-column photochemistry model to investigate O₃ variations within the nighttime cloud-topped boundary layer over the Sichuan Basin,which is the cloudiest region in southwest China.The Santa Barbara Distort Atmospheric Radiative Transfer radiation model was coupled into the single-column photochemistry model to investigate the nighttime radiative effects of clouds.The results showed that the nocturnal cloud-top radiative cooling generated turbulence and enhanced vertical mixing below the cloud-top layer.The cloud-driven turbulent eddies even penetrated through the SBL to the surface.Consequently,it weakened the decoupling of the SBL from the residual layer.O₃-rich air over the SBL was entrained downwards to the surface,decreasing O₃aloft and increasing surface O₃ at nighttime.The cloud-top radiative cooling rates were enhanced with the increase in the cloud liquid water path.Higher cooling rates produced stronger turbulent transport of O₃,leading to higher nocturnal surface O₃ levels.The turbulent transport of O₃ induced by cloud-top radiative cooling is revealed as an important mechanism of vertical transport of O₃ within a cloud-topped boundary layer,contributing to the maximum nocturnal surface O₃,which has great implications for understanding atmospheric O₃ variation.3)Important mechanism of regional O₃ transport in the residual layerNighttime O₃in residual layer contributes significantly to surface O₃pollution by vertical mixing and horizontal transport,while the mechanism of O₃transport in nocturnal residual layer is still unclear.A severe summer smog stretched over the YRD region from 22 to 25 August2016.This event displayed hourly surface O3 concentrations that exceeded 300?g m^-3 on 25August in Nanjing,an urban area in the western YRD.The formation mechanism responsible for this O3 pollution episode over the YRD region,particularly the extreme values over the western YRD,was investigated using observation data and by running simulations with the Weather Research and Forecasting model with Chemistry(WRF-Chem).The results showed that the extremely high surface O3 concentration in the western YRD area on 25 August was largely due to regional O3 transport in the nocturnal residual layer(RL)and the diurnal change in the atmospheric boundary layer.On 24 August,high O3 levels,with peak values of 220?g m^-3,occurred in the daytime mixing layer over the eastern YRD region.During nighttime from24 to 25 August,a shallow stable boundary layer formed near the surface which decoupled the RL above it from the surface.O3 in the decoupled RL remained quite constant,which resulted in an O₃-rich“reservoir”forming in this layer.This reservoir persisted due to the absence of O₃consumption from nitrogen oxide(NO)titration or dry deposition during nighttime.The prevailing easterly winds in the lower troposphere governed the regional transport of this O3-rich air mass in the nocturnal RL from the eastern to the western YRD.As the regional O3transport reached the RL over the western YRD,O3 concentrations in the RL accumulated and rose by 40?g m^-3 over the western Nanjing site during the sunrise hours on 25 August.The development of the daytime convective boundary layer after sunrise resulted in the disappearance of the RL,as the vertical mixing in the convective boundary layer uniformly redistributed O3 from the upper levels via the entrainment of O3-rich RL air down to the O3-poor air at the ground.This net downward transport flux reached up to 35?g m^-3 h^-1,and contributed a considerable surface O3 accumulation,resulting in severe daytime O3 pollution during the summer smog event on 25 August in the western YRD region.The mechanism of regional O3 transport through the nocturnal RL revealed in this study has great implications regarding understanding O3 pollution and air quality change.4)Impact of mountain-plains solenoid on regional transport of nocturnal residual layer O₃The thermally driven regional mountain–plains solenoid(MPS)circulation has an important effect on the accumulation and transport of O₃ in the PBL.Based on the surface observation,reanalysis data of ERA5 and WRF Chem model,we have studied the impacts of MPS on regional O₃ transport in Sichuan Basin(SCB).It is found that the MPS circulation driven by thermal gradients between the Tibetan Plateau(TP)and the SCB is characterized by upslope(easterly)flow at lower levels,rising motion along the east side of the barrier(TP)in the daytime,and downslope(westerly)flow at lower levels,sinking motion along the east side of the barrier in the nighttime.The daytime upslope flow draws air from the SCB toward the eastern slope of TP.This flow transports O₃ from adjacent cities and inner part of SCB toward the foothills where they accumulate,with part of O₃ transported further westward to higher elevations along the eastern slope of TP.After sunset,the flow reverses and nocturnal drainage flows carry polluted air containing high O₃ eastward(downsloping)along the eastern slope of TP into the western SCB,which resulted in a significant increase of O₃ in the residual layer over part of SCB.Under the influence of the prevailing wind in SCB,the high O₃ in the residual layer was transported to the downstream areas,which had a great contribution to the surface O₃concentration by vertical mixing on the following day(?30?g m^–3h^–1).This study reveals the impacts of MPS circulation on the regional transport of O₃ in SCB,which has substantial implications regarding understanding urban O₃ pollution and air quality change.