| With the rapid development of China,the urbanization process has been greatly accelerated and the urban agglomeration has changed the nature of the underlying surface.As a result,the urban heat island phenomenon becomes prominent.Due to the large amount of aerosols emitted by human activities,the city center becomes a high-value area of aerosols,especially fine particles,as well as the heat island center,which in turn affects the urban boundary layer.Therefore,it is significant to analyze the relationship between urban heat island and urban fine-grained island for scientifically managing fine particulate matter pollution and improving urban living environment.In this paper,the Yangtze River Delta is selected as the research area.The impact of fine particles on urban temperature structure in the Yangtze River Delta has been investigated through long-term continuous observation data analysis,one-dimensional aerosol radiation convection model and the three-dimensional numerical model.Based on the observation data,the variation characteristics and correlations of fine particle concentration and urban heat island intensity in the Yangtze River Delta region are revealed.The one-dimensional aerosol radiation convection model is used to explore the physical mechanism and sensitivity of urban temperature structure affected by fine particle concentration and composition.The three-dimensional numerical model WRF-Chem was used to quantitatively analyze the total radiation effect,direct effect,indirect effect,and the effects of scattering components and absorbent components on urban temperature structure.WRF-Chem was also conducted to calculate the impact of air pollution control policy on urban heat islands in the Yangtze River Delta region quantitatively.The main results were shown as following:First,based on the observations,the variation characteristics and correlations of fine particulate concentrations and urban heat island intensities in the Yangtze River Delta region were studied.From January 1,2014 to December 31,2017,observations of fine particulate matter concentrations and urban heat island intensities in typical cities in the Yangtze River Delta,Nanjing,Hangzhou,Shanghai,and Hefei show that the annual average fine particle concentration is decreased from 71.21 ?g/m3 in Nanjing,48.74 ?g/m3 in Hangzhou,55.63?g/m3 in Shanghai and 81.72 ?g/m3 in Hefei,which decreased by 48.37%,28.3%,29.41%and 32.5%respectively in 2017.At the same time,the frequency of urban heat islands in the four cities increased year by year from 65.6%to 78.71%in Nanjing,62.1%to 65.74%in Hangzhou,68.56%to 69.9%in Shanghai and 50.78%to 79.34%in Hefei,reflecting the opposite characteristics of fine particle concentration time series.The concentrations of fine particles were higher in winter and lower in summer as the corresponding urban heat island intensities is smaller in summer and larger in winter.The diurnal variations of fine particle concentration were also obvious,which were usually higher at night and lower in the afternoon.The intensities of urban heat islands were characterized as weaker at daytime and stronger at nighttime.The analysis of urban heat island intensities under different fine particle levels shows that it decreased during the day and strengthened at night with the increase of fine particulate concentrations.The correlation analysis between the intensities of urban heat island and the differences of fine particle concentrations between urban and suburban shows that there is a negative correlation between these two quantities during the day,and a positive correlation at night.It reflects the important influence of fine particles on the daytime weakening and nighttime enhancement of urban heat islands.Secondly,a one-dimensional aerosol radiation convection model is developed.The impact of fine particle concentration,species and environmental factors on its optical characteristics and radiation forcings were studied.Using the observation data of fine particles in Nanjing in the four seasons from 2014 to 2015,the optical properties and radiative forcings of Nanjing were simulated and compared with other cases.The results show that the asymmetry factor and the single-scattering albedo are proportional to the ratio of the scattering components in the fine particles and the relative humidity.The optical thickness is proportional to the proportion of the fine particle concentration,relative humidity and absorption components.When the fine particles had 80%scattering ones,the asymmetry factor was about 0.7 in the case of 80%relative humidity and the single-scattering albedo was about 0.88,the asymmetry factor was about 0.6 when the relative humidity was 20%with the scattering albedo was 0.77.When other conditions are the same,the extinction ability of the absorbing fine particles per unit concentration is stronger than that of the scattering fine particles,and the radiative forcings of the fine particles strengthened with the increase of fine particles' concentration and the proportion of the absorbent components.The observation data showed that the the concentration and absorption components of fine particulates in Nanjing were higher in winter and lower in summer,as well as higher in the urban center than the suburban.Based on the results of the one-dimensional model results with the observation data from 2014 to 2015 in Nanjing,it was showed that the radiative forcings of fine particles in Nanjing urban(-7 W/m2)and suburban(-6 W/m2)is different with its uneven distribution.The magnitudes of forcings were higher in the urban center than the suburban.Thirdly,based on the one-dimensional aerosol radiation convection model,the sensitivity factors of urban temperature structure affected by fine particles were studied and the fine particle effect on Nanjing urban temperature structure was calculated.The results of the study showed that the presence of fine particles made the heating rate of the atmosphere generally increase.As aerosols accumulate in the surface and lower atmosphere,the heating rate of the lower atmosphere was greatly affected,and the upper atmosphere is less affected.The low-level atmospheric heating rate is proportional to the fine particle concentration and the proportion of the absorbing component.The fine particles with 80%scattering component of 450?g/m3 increased the heating rate of the lower layer atmosphere by 1?18 K/Day.The fine particles have a cooling effect on the surface air temperature,and the intensity of this action is positively correlated with the concentration and the proportion of absorbent components of fine particles.The urban heat island intensity in the low-level atmosphere was negatively correlated with the concentration and the proportion of absorbent components of fine particles.The fine particles can weaken the urban heat island intensity by about 1?3 K as well as the temperature difference of urban and suburban at the lower layer as 1 K.In the surface and lower atmosphere,the weakening effect of aerosols on urban heat island intensity is positively correlated with fine particle concentrations and absorbing components..Based on the calculation results with the fine particles' concentration and component observation data in Nanjing,the enhanced effect of fine particulates on the heating rate of urban and suburban areas is concentrated near the surface,the urban area was enhanced by 0.4?1.6 K/Day and 0.3?1.5 K/Day in the suburban.The cooling effect of fine particles on the surface air temperature has the seasonal characteristics as stronger in winter than summer as well as stronger in the urban center than suburban.The weakening effect of it on the urban heat island intensity was about 0.1?0.4 K and winter suffered the strongest weakening.Finally,the three-dimensional numerical model WRF-Chem was used to simulate the radiation effects of fine particles on the urban temperature structure in the area of Nanjing,taking the components into account.The influence of air pollution control policy on the intensity of urban heat islands in the Yangtze River Delta region was also quantitatively calculated.The direct,indirect,and total radiation effects of fine particles were studied.The effects of scattering fine particles and absorbing fine particles on temperature profile in Nanjing were also conducted.The simulation results showed that the concentration distribution of fine particles in Nanjing is highest in winter and lowest in summer with the higher values in the urban center than the suburban.The direct radiation effect weakened the urban heat island intensity by 0.04?0.07 K during the daytime and 0.04?0.08 K at night.The indirect radiation effect weakened the urban heat island intensity by 0.04?0.06 K during the daytime and 0.04?0.07 K at night.The effects of direct and indirect effects on urban heat island intensity are comparable,and the impacts had similar seasonal characteristics as higher in summer than in winter,indicating the indirect radiation effect of fine particles like cloud microphysical processes has a certain impact on the intensity of urban heat islands and cannot be ignored.The scattering fine particles with a concentration of about 85%of the total fine particles and the absorbing ones with 9%had the same influence on the intensity of the urban heat island,and both have the seasonal characteristics as higher in summer than in winter.The scattering fine particles weakened the urban heat island intensities by 0.05?0.12 K during the day and enhanced that by 0.03?0.07 K at night.As fine particles accumulate on the surface,their influence was mainly concentrated below 400m.When the height is higher than 800m,the effect of fine particle radiation on urban heat island intensity is almost zero.After the air pollution control policy was conducted in the Yangtze River Delta,fine particulate matter is reduced,leading to weakened surface radiative forcings and increased surface air temperature.During the daytime,the intensity of the urban heat island was enhanced nearly 0.02?0.18 K,and was weakened at night as 0.03?0.15 K.The characteristics of the specific underlying surface of different cities and the fine particles worked together to affect the urban heat islands.In winter,the enhanced intensity of urban heat islands by the reduction of fine particles is highest in Shanghai and lowest in Hefei at daytime.At night,Hangzhou suffered the highest reduction,Hefei and Nanjing suffered the lowest.In summer,the intensity of urban heat islands in Hangzhou increased the most during the daytime,while Nanjing,Shanghai and Hefei increased by the same amount.At night,the intensity of urban heat islands in Hangzhou and Shanghai decreased greatly,while that in Nanjing and Hefei decreased less.In summary,through the data analysis and numerical simulation research in this paper,the fine particles' impact on the urban temperature structure was found.The fine particles weakened the solar radiation reaching the surface,reduced the surface air temperature,and increased the heating rate of the lower atmosphere.Due to the uneven distribution of fine particles in the urban centers and the suburbans,the intensities of urban heat islands were weakened during the day and enhanced at night.As the air pollution control policy reduced the fine particles' concentrations,the urban heat island intensities of the typical cities in the Yangtze River Delta region were increased during the daytime and weakened at night.The effect of fine particles on urban temperature structure was affected by the concentration as well as the composition.The higher concentration and higher absorbing components of the finer particles had greater influence on the temperature structure of urban areas.The research results in this paper can provide a scientific basis for atmospheric environmental planning in urban areas. |
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