Spatio-temporal evolution of urban thermal environment and its driving factors: Case study of Nanjing, China

In recent years, with the rapid urbanization, the urban underlying surface has changed dramatically. Various urban eco-environmental problems have emerged in the world, among which the urban heat island effect has become one of the most obvious urban eco-environmental problems. In this study, Nanjing, China is chosen as the study area. Based on the Landsat8 remote sensing image data in Nanjing from 2014 to 2018, the land surface temperature is retrieved, the spatiotemporal variation track and characteristics of the thermal environment pattern are systematically depicted, and its driving factors are revealed. The results show that: in the past five years, the spatial pattern of heat field in Nanjing has changed from scattered distribution in the periphery of the city to centralized in the center of the city, and the heat island intensity has increased year by year. The changes of administrative divisions, the layout of transportation trunk lines, the transfer of industrial centers, and ecological construction projects are important driving factors for the evolution of land surface thermal environment pattern of these regions. The research results will provide scientific and technological support for similar cities with typical heat island effect in the world to make urban planning and development decisions, and to govern and improve urban ecological environment.

Urban surface thermal environment is not only directly related to the quality of 83 urban living environment and residents' health, but also has a far-reaching impact on urban energy and 84 water consumption, ecosystem process evolution, biological phenology and sustainable economic 85 development [1,[18][19][20][21]. Therefore, it is imperative to study the urban heat island effect and its 86 influencing factors, and at the same time, it becomes a particularly urgent problem to put forward 87 countermeasures to improve a series of risks and negative effects caused by it.

156
Atmospheric profile parameters could be obtained from the website which was provided by NASA 157 (http://atmcorr.gsfc.nasa.gov/).The values of atmospheric upward radiance, downward radiance and 158 transmittance were as follows: 159 Urban heat island intensity 161 The southern regions in China had the common weather of rainy days and cloudy days, the images of the 162 same period in many years were hardly obtained, so theoretically the remote sensing images of the same 163 time phase were also hardly obtained. The research of heat island effect mainly focused on analyzing the 164 spatial distribution characteristics of the relative strength of urban underlying surface temperature, and the 165 difference of seasons only changed the size of land surface temperature instead of its spatial distribution. 166 Therefore, in order to reflect and display the urban land surface thermal environment in a more 167 concentrated way, the concept of urban heat island intensity (UHII) was introduced. The UHII referred to 168 the difference between the average temperature in the center of a city and that in the surrounding suburbs 169 (villages), which was used to indicate the intensity of urban heat island effect. The calculation formula of 170 UHII was as follows: 171 In formula (4), UHII(T) was the urban heat island intensity, was the surface temperature at a certain 173 point in urban area, was the average temperature in the suburbs, and land surface temperature of farmland 174 was used instead of retrieved surface temperature.   215 Meanwhile, the heat island intensity was increased obviously, the heat island effect was significant, and the 216 land surface temperature in central urban areas was obviously higher than that of outer suburbs. Besides the 10 217 main urban areas and Jiangbei New Area, the heat islands of central areas of various towns in southern 218 regions were also strengthened.

227
(1) The heat island intensity of urban areas and riverside regions ascended obviously.

228
The regions of heat island intensity ascending obviously were concentrated around the main urban 229 areas in the southern regions of Yangtze River, Jiangbei New Area and the upstream and downstream 230 southern bank regions of Yangtze River Nanjing Section. Thereinto, in the northern regions of Yangtze 231 River, the regions of heat island intensity ascending obviously showed the distinct strip distribution, and 232 were divided into two-way extension from northeast to southwest direction.

233
(2) The heat island intensity of suburbs and water regions descended obviously.

234
The regions of heat island intensity descending obviously were concentrated around the Donggou 235 Town of Luhe District in suburbs and riverside belts on the north of Tangshan. Meanwhile, the whole 236 Yangtze River Nanjing Section water regions, the southern Shijiu Lake and Gucheng Lake water regions 237 all belonged to the regions of heat island intensity descending obviously.

238
(3) The heat island intensity of outer suburbs kept a stable status.

239
Except for Shijiu Lake and Gucheng Lake water regions in the southern suburbs of Nanjing, the heat