PM_2.5 Influence On Urban Heat Island（UHI） Effect In Major Chinese Cities And The Mechanisms

Rapid urbanization has led to increasing problems related to Urban Heat Island（UHI）effect and air pollution in major Chinese cities.The UHI effect can increase the heat stress of urban residents in summer,and the synergy of heat stress and air pollution may pose a greater threat to urban operations and human health.As cities grow,the UHI intensity（UHII）and the atmospheric particle concentrations may simultaneously increase and interact to produce complex thermal-dynamic conditions in the boundary layer over large cities.Although there have been many studies on urban heat island characteristics and their interactions with atmospheric particle,the results are still subject to large uncertainties due to the constraints of observation conditions,especially the mechanism by which atmospheric particle affects UHII remains unclear.In this study,30 provincial capitals in China were taken as the research objects,by combining various environmental and meteorological observation data and reanalysis data,we quantified the variation characteristics and their regional differences of UHII and urban-rural particle pollution at different time scales.The mechanisms of PM_2.5influence on UHII at different time periods and its performance in different seasons and regions were also analyzed by the studies in Beijing and 30 cities（including Beijing）separately.This study has implications for understanding the UHI effect and its interaction mechanism with air pollution in different regions,and can also provide a scientific basis for the synergistic control of UHI effect and urban air pollution.The main findings of the study are as follows:（1）The spatial distribution pattern of UHII in 30 provincial capitals in China from2016 to 2020 shows that UHII is higher in the northern regions than in the southern regions,and from east to west UHII gradually decreases in the northern regions and increases in the southern regions.The UHII drops and rises sharply around sunrise and sunset and remains relatively stable during daytime and nighttime,with nighttime UHII greater than daytime.The UHI in northern cities is winter type,which is strong in winter night and summer day and has obvious seasonal variation;in southern cities,it is mostly autumn type UHI,which is stronger in summer and autumn night and summer day,and weaker in winter and spring night.Seasonal differences of UHII show latitudinal variation,and both seasonal and intra-day variations of UHII are smaller in southern cities than in northern cities.The nighttime UHII in the northern cities is significantly higher than that in the south in winter,but less than that in the south in summer August.Daytime UHII in southern cities is higher than in the north from spring to autumn and lower than that in the north in winter.（2）All cities except Urumqi have higher concentrations of particle in urban areas than their surrounding rural counterparts,and more pollution-exceedance days than in suburban areas.The average differences in PM_2.5 and PM₁₀ concentrations between urban and rural areas in provincial capitals were 6.63 and 13.06μg/m³,respectively,and the concentrations of PM_2.5 and PM₁₀ particle in rural areas account for 83%and84%of those in urban areas,separately.The pollution is more serious in the north than in the south,with PM_2.5 concentrations in northern cities exceeding those in southern cities by 16.69 and 15.23μg/m³ in urban and rural areas,respectively.Urban Pollution Island Intensity（UPII）was constructed based on the PM_2.5 concentration.The spatial pattern of UPII is higher in the north than in the south,with the highest in northern China and the second highest in northeast China.Both pollution levels and UPII are higher in winter than in summer,and the higher the particle concentration,the larger the UPII,and this relationship holds both within and between cities.（3）In Beijing,PM_2.5 weakens daytime UHII in summer,but strengthens it in winter,while PM_2.5 reduces nighttime UHII.The mechanism by which PM_2.5 affects UHII varies seasonally and is regulated by the interactions of aerosols with radiation,evaporation,and the boundary layer.The former two change the surface energy balance via sensible and latent heat fluxes,while the latter affects atmospheric stability and energy exchange.In summer daytime,aerosol-radiation interaction plays an important role,and the energy balance in urban areas is more sensitive to PM_2.5 concentration than in rural areas,thereby weakening UHII.In winter daytime,aerosol-PBL interaction is dominant,because aerosols lower the PBL height and stabilize atmosphere,weaken the heat exchange with the surrounding,with more heat accumulated in the urban areas and the increased UHII.Changes in evaporation and radiation strengthen the relationship.At night,the change of UHII more depends on the energy stored in the urban canopy.Aerosols effectively reduce the incident energy during daytime,and the long-wave radiation from the buildings of urban canopy at night becomes less,leading to a weakened UHII.（4）The analysis of 30 provincial capitals showed regional differences in the effects of PM_2.5 on UHII.Northern PM_2.5 attenuates summer and winter nighttime and summer daytime UHII but enhances winter daytime UHII.Southern PM_2.5 weakened daytime UHII and strengthened nighttime UHII.Aerosol-radiation interaction is the main reason for the weakening of daytime UHII in northern summer and southern summer and winter.The aerosol-boundary layer interaction is less influential,but it plays an important role in daytime UHII in northern winter.Particle reduces incident radiation during daytime and reduces the escape of urban energy at night,the former diminishes nighttime UHII while the latter enhances it.The combined effect of PM_2.5on nighttime UHII is controlled by the PM_2.5 concentration and the intensity of incident radiation.Stronger pollution and weaker incident radiation in northern cities,more reduction of daytime incident energy by particle,less energy in urban canopy,weaker nighttime UHII.The nighttime UHII is enhanced in southern cities where pollution is weak and incident radiation is stronger,and the suppression of urban energy fugacity by particulate matter is more important.The aerosol-radiation interaction makes PM_2.5have a higher impact on nighttime UHII than daytime UHII,while the aerosol-boundary layer interaction,on the contrary,makes PM_2.5 have a greater impact on daytime UHII.