Impact Of Urban Heat Island Research Articles

Understanding the linkages between spatio-temporal urban land system changes and land surface temperature in Srinagar City, India, using image archives from Google Earth Engine.

Land-use and land-cover (LULC) is an important component for sustainable natural resource management, and there are considerable impacts of the rapid anthropogenic LULC changes on environment, ecosystem services, and land surface processes. One of the significant adverse implications of the rapidly changing urban LULC is the increase in the Land Surface Temperature (LST) resulting in the urban heat island effect. In this study, we used a time series of Landsat satellite images from 1992 to 2020 in the Srinagar city of the Kashmir valley, North-western Himalaya, India to understand the linkages between LULC dynamics and LST, derived from the archived images using the Google Earth Engine (GEE). Furthermore, the relationship between LST, urban heat island (UHI), and biophysical indices, i.e., Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI), was also analysed. LULC change detection analysis from 1992 to 2020 revealed that the built-up area has increased significantly from 12% in 1992 to 40% in 2020, while the extent of water bodies has decreased from 6% in 1992 to 4% in 2020. The area under plantations has decreased from 26% in 1992 to 17% in 2020, and forests have decreased from 4 to 2% during the same period. Urban sprawl of Srinagar city has resulted in the depletion of natural land covers, modification of natural drainage, and loss of green and blue spaces over the past four decades. The study revealed that the maximum LST in the city has increased by 11°C between 1992 and 2020. During the same period of time, the minimum LST in the city has increased by 5°C, indicating the impact of urbanization on the city environment, which is reflected by the observed changes in various environmental indices. UHI impact in the city is quite evident with the maximum LST at the city centre having increased from 13.03°C in 1992 to 22.01°C in 2020. The findings shall serve as a vital source of knowledge for urban planners and decision-makers in developing sustainable urban environmental management strategies for Srinagar city.

A toolchain to evaluate the impact of urban heat island and climate change on summer overheating at district level

This paper aims to quantify and analyse the impact of urban heat island and climate change on summer indoor thermal comfort in buildings at district level. By leveraging the models of outdoor climate and dynamic building simulation, a toolchain is developed to quantify the overheating hours under different outdoor climate scenarios in rural and urban context. Future climate data are generated for the city of Roeselare (Belgium) under RCP4.5 and RCP8.5 scenarios in 2030 and 2050 respectively, and a residential district with 265 buildings is analysed by incorporating characteristics data of individual buildings, instead of using a traditional archetype approach. The results clearly demonstrate that overheating risks exist in most of the buildings of the analysed district under all climate scenarios. Mutual shading plays a non-negligible role in the overheating analysis. Scheduled opening of operable windows and controlled solar shading devices can effectively reduce the overheating hours at the district level under all climate scenarios. The impact of urban heat island is less important for the situation with adaptation measures, and the number of overheating hours could be even larger in rural areas in certain circumstances. The results of overheating hours could be different by using different quantification methods.

The Impact of Urban Heat Island on Cooling Needs in Urban and Suburban Areas of Hyderabad Sindh Pakistan

The urban heat island phenomenon is causing numerous problems, and studying its effects in rapidly developing cities is crucial to meet the Sustainable Development Goal 14 for sustainable building and cities. This research focuses on investigating the characteristics of the Urban Heat Island in Hyderabad Sindh, specifically in a densely populated commercial area and a rural suburb known as TandoJam. The study utilized weather station data from the Pakistan Meteorological Department and temperature sensors from Qasimabad to compare air temperature differences between the urban and reference rural areas. The UHI intensity was determined by calculating the air temperature difference between urban and rural areas. Results showed that the UHI effect was more prominent during winter and nighttime than during summer and daytime, with only a few months, like June, showing some UHI effects during daytime. The study also used cooling degree days and cooling degree hours as a measure of energy required for cooling, and all areas observed higher numbers of CDDs and CDHs. However, urban areas had more CDDs than rural areas, indicating that the maximum cooling degree days in urban areas are a direct consequence of the UHI. The 7-year average data revealed that TandoJam experienced the least number of CDDs compared to its urban area, suggesting better thermal comfort in the rural area. The study’s findings were further validated through a day UHI analysis in the town of Qasimabad Hyderabad, which showed high CDHs and higher UHI intensity in Qasimabad than in other rural towns in Hyderabad.

An Integrated Decision-Making Framework for Mitigating the Impact of Urban Heat Islands on Energy Consumption and Thermal Comfort of Residential Buildings

Urban heat island (UHI) is a zone that is significantly warmer than its surrounding rural zones as a result of human activities and rapid and dense urbanization. Excessive air temperature due to the UHI phenomenon affects the energy performance of buildings and human health and contributes to global warming. Knowing that most of the building energy is consumed by residential buildings, therefore, developing a framework to mitigate the impact of the UHI on residential building energy performance is vital. This study develops an integrated framework that combines hybrid micro-climate and building energy performance simulations and multi-criteria decision-making techniques. As a case study, an urban area is analyzed under the Urban GreenUP project funded by the European Union’s Horizon 2020 Programme. Four different strategies to mitigate the UHI effect, including the current situation, changing the low-albedo materials with high-albedo ones, nature-based solutions, and changing building façade materials, are investigated with a micro-climatic simulation tool. Then, the output of the strategies, which is potential air temperature, is used in a dynamic building energy simulation software to obtain energy consumption and thermal comfort data of the residential buildings in the case area. Finally, a multi-criteria decision-making model, using real-life criteria, such as total energy consumption, thermal comfort, capital cost, lifetime and installation flexibility, is used to make a decision for decreasing the UHI effect on residential energy performance of buildings. The results showed that applying NBSs, such as green roofs and changing existing trees with high leaf area density ones, have the highest ranking among all mitigation strategies. The output of this study may help urban planners, architects, and engineers in the decision-making processes during the design phase of urban planning.

An Analysis of the Spatial and Temporal Evolution of the Urban Heat Island in the City of Zhengzhou Using MODIS Data

A rapid increase in urbanization has caused severe urban heat island (UHI) effects in China over the past few years. Zhengzhou is one of the emerging cities of China where residents are facing strong impact of UHI. By utilizing MODIS data on land surface temperature (LST) and employing 3S technology, this study investigates the UHI phenomenon in Zhengzhou over a 10-year period (2012–2021), aiming to analyze the spatio-temporal evolution characteristics of the UHI effect and the associated land cover changes. To the best of our knowledge, this study represents the first attempt to investigate annual and seasonal changes in different areas of Zhengzhou. It is noted that in the night-time, the intensity of the heat island is stronger than in daytime, which has moderate and weak heat island areas. Seasonal variation showed that in autumn, Zhengzhou has the strong heat island intensity, followed by summer, and the lowest is in winter and spring. The analysis reveals that built-up (construction) areas exhibit the highest LST, whereas forested land and water bodies have the lowest temperature levels. The findings of this study can serve as reference for reducing UHI and increasing thermal comfort in cities.

Analysis of a City's Heat Island Effect on the Micro-Climate Parameters within Cities

The Urban Heat Island (UHI) is a micro-climatic phenomenon that influences the urban areas by elevating its temperature. UHI not only causes the thermal discomfort but also exert serious health issues along with aggravation of urban microclimate. Although a lot of research has been done on this phenomenon but UHI effect on micro scale is still less explored. This paper attempts to make a contribution in UHI studies of micro-climate. It consists of examination of UHI impact on microclimate of Aligarh city areas using mobile traverse method. This study determined the presence and extent of UHI’s microclimate variation within urban communities of different environmental layout and functional uses. The UHI effect started to appear from early afternoon and continue to rise with maximum UHI intensity recorded at early night. The highest recorded UHI intensity was 3.1 °C (at 21:00 hrs.), and the lowest was 0.6 °C (at 09:00 hrs.). A comparison of two districts of the same city located at a distance of 3 km and differing in population density, the number of buildings and landscaping showed that in the L1 area with more dense population and low landscaping, the temperature was consistently higher during the daily period; also in the L1 region had less humidity, which, combined with the already high temperature, makes it difficult to breathe and control the microclimate. These findings can be used for consideration for the future sustainable development of the affected area in regard of thermal comfort, environmental health and urban planning.

Machine Learning Techniques to Map the Impact of Urban Heat Island: Investigating the City of Jeddah

Over the last decades, most agricultural land has been converted into residential colonies to accommodate the rapid population expansion. Population growth and urbanization result in negative consequences on the environment. Such land has experienced various environmental issues due to rapid urbanization and population increases. Such expansion in urbanization has a big impact on worsening the residences soon and in the long term, as the population is projected to increase more and more. One such issue is the urban heat island (UHI), which is computed based on land surface temperature (LST). The UHI effect has fundamental anthropogenic impacts on local areas, particularly in rapidly growing cities. This is due to the unplanned shifts in land use and land cover (LUALC) at the local level, which results in climate condition variations. Therefore, proper planning based on concrete information is the best policy in the long run to remedy these issues. In this study, we attempt to map out UHI phenomena using machine learning (ML) algorithms, including bagging and random subspace. The proposed research also fulfills the sustainable development goals (SDGs) requirement. We exploit the correlation and regression methods to understand the relationship between biophysical composition and the UHI effect. Our findings indicate that in the megacity of Jeddah, Saudi Arabia, from 2000 to 2021, the urban area enlarged by about 80%, while the UHI increased overall. Impervious surfaces significantly impact the UHI effect, while vegetation and water bodies have negative implications for the UHI effect. More than 80% of the total parts in Jeddah have been classified by extremely high UHI conditions, as determined by the bagging and random subspace models. In particular, the megacity’s south, north, and central-east parts were categorized by very high UHI conditions. This research is not only expected to assist in understanding the spatial patterns of the UHI in Jeddah, but to assist planners and policymakers in spatial planning. It will help to ensure sustainable urban management and improve life quality.

Impact of urban heat islands on morbidity and mortality in heat waves: Observational time series analysis of Spain's five cities

Urban heat islands (UHIs) have become an especially relevant phenomenon as a consequence of global warming and the growing proportion of people living in cities. The health impacts that are sometimes attributed to the rise in temperature generated in an UHI are not always adequately justified.The objective is to analyse what effect UHIs have on maximum (Tmax) and minimum daily temperatures (Tmin) recorded in urban and non-urban observatories, and quantify the impact on morbidity and mortality during heat waves in Spain's five cities.Data were collected on natural-cause daily mortality and unscheduled emergency hospital admissions (ICD-10: A00-R99) registered in these 5 cities across the period 2014–2018. We analysed daily Tmax and Tmin values at urban and non-urban observatories in these cities, and quantified the impact of Tmax and Tmin values during heat waves in each of these cities, using GLM models that included Tmax only, Tmin only, and both. We controlled for air pollution and other meteorological variables, as well as for seasonalities, trend and the autoregressive nature of the series.The UHI effect was observed in Tmin but not in Tmax, and proved to be greater in coastal cities than in inland and more densely populated cities. The UHI value in relation to the mean Tmin in the summer months ranged from 1.2 °C in Murcia to 4.1 °C in Valencia (difference between urban/non-urban observatories). The modelling process showed that, while a statistically significant association (p < 0.05) was observed in inland cities with Tmax for mortality and hospital admissions in heat waves, in coastal cities the association was obtained with Tmin, and the only impact in this case was the UHI effect on morbidity and mortality.No generalisations can be made about the impact of UHI on morbidity and mortality among the exposed population in cities. Studies on a local scale are called for, since it is local factors that determine whether the UHI effect will have a greater or lesser impact on health during heat-wave events.

Spatio-temporal analysis of urban heat island (UHI) and its effect on urban ecology: The case of Mekelle city, Northern Ethiopia

The bio-geophysical effects of land cover classes are considered to be an important factor in land surface temperature variations between urban and suburban areas. This means that major cities are significantly warmer than surrounding suburban or rural areas, which is known as the Urban Heat Island (UHI) effect. The aim of this study was to assess and analyze the spatiotemporal variation, correlation and impact of UHIs on Mekelle city (1990–2020) using remote sensing techniques. The study's primary objective was accomplished using the a number of techniques, including the extraction of LULC classes, estimation of the seasonal LSTs, assessment of UHI and UTFVI, and showing the relationship between LULC and LST as well as the interactions between UHI, UTFVI, and urban LULC classes. By analyzing TIRs/OLI thermal band data after calibrating uncertainty in the images and validating it using the concept of theoretical relationships and least squares fitting method, estimates of local LST, UHI (both in Mekelle and periphery rural areas), and UTFVI were obtained. The result of standard multiple regression models showed that impervious urban land surface, built-up areas, and dry bare soil highly contribute and influence variation at LST intensity caused for the formation of UHI in the study area. The result showed that the maximum UHI value in Mekelle city was 2.73 °C during the dry season in 1990. It decreased slightly to 2.53 °C in 2000 and then increased regularly to 2.83 °C and 2.98 °C in 2010 and 2020, respectively. To determine the city's UHI status in comparison to eight selected peripheral suburban areas, a trend analysis has been done. The UHI intensity of Mekelle city was higher relatively to that of most of the periphery suburban districts, particularly in 2020 (both dry and rainy seasons); this could be due to the city's explosive growth. It's worth noting that the research area affected by the urban heat island effect has grown over time, and as a result, the study area also has severe microclimate conditions that primarily damage the quality of urban life and create the worst conditions for thermal discomfort. The results of this study provide major conceptual understandings of how improper distribution and use of urban land affects the urban environment and fuels the creation of the UHI and thermal discomfort phenomena. Hence policy makers and urban planners should consider the effects of LST and UHI and integrate UHI comprehensive mitigation strategies with urban development patterns, and current and projected local climate changes in order to create sustainable urban environments, cities, and communities. In conclusion, compared to the conventional method, satellite remote sensing provides a faster and more efficient method for researching LST and UHI.

Geopolymer as an Innovative Material for Green Roofs - A State-of-the-Art Review

To help minimize water temperature variations and stop excessive evaporation, effective insulation materials are frequently needed during the design and construction of ponds. In contrast to conventional materials, geopolymer mortars have great insulating qualities, making them an attractive option. An alkaline activator and aluminosilicate materials react chemically to create geopolymers, which have a three-dimensional network structure and high thermal insulation properties. Geopolymer mortars are used in ponds to promote energy efficiency and sustainability while also improving temperature stability and reducing water loss from evaporation. An alkaline activator and aluminosilicate materials react chemically to create geopolymers, which have a three-dimensional network structure and high thermal insulation properties. The use of geopolymer mortars as an insulating material in pond construction is discussed in this abstract, along with its composition, characteristics, and advantages over more conventional choices. Geopolymer mortars are used in ponds to promote energy efficiency and sustainability while also improving temperature stability and reducing water loss from evaporation. The utilization of geopolymers as cutting-edge insulating materials for structures with green roofs is thoroughly examined in this publication. As sustainable construction techniques become more prevalent, selecting the right insulating materials is essential for enhancing energy efficiency and overall building performance. Geopolymers, which can be produced from industrial waste or natural resources, offer appealing properties for green roofs, such as the ability to lessen the impact of urban heat islands, superior thermal insulation, and fire resistance. This study assesses the advantages, challenges, and potential impact of geopolymer-based insulating materials on green construction practices.

Mitigating urban heat island effects in urban environments: strategies and tools

In the twenty-first century, urban heat islands (UHIs) have become a major problem for humanity as a consequence of urbanization and industrialization. The main causes of UHI are the vast amounts of heat generated by urban structures as they consume and re-radiate solar energy and anthropogenic heat sources. The two heat sources cause an urban area’s temperature to rise above its surroundings, a phenomenon known as Urban Heat Island (UHI). Many approaches, methods, models, and investigative tools have been implemented to study and analysis this phenomenon. In general, green areas in cities are thought to be an effective approach to mitigate urban heat island effects and bring comfort to residents. The improvement of microclimatic conditions in urban environments is mostly influenced by evapotranspiration. Most of the studies show a rising trend in the UHI, which is linked to decreased plant cover and land-use changes. The main objectives of this paper were to explain the concept, formation factors, and influential factors of UHI. In addition, the most common strategies and tools that are applied in mitigating rising temperatures in urban areas were reviewed and summarized. The finding of several studies showed that increasing urban vegetation areas in addition to optimizing their spatial distribution and configuration is an effective strategy to reduce the impact of UHI.

Role of reservoirs of urban heat island effect mitigation in human settlements: moderate climate zone

This paper presents the problem of the increasing negative impact of urban heat islands (UHI) on urban residents based on land surface temperature (LST). It is assumed that water bodies in the agglomeration remain cooler than the air and surrounding urban areas. The study aimed to determine the impact of water bodies and surrounding areas covered by trees on the temperature of an urban area and to minimise the impact of UHI on the life quality of people in the temperate climate zone at day temperatures 25°C (W day) and 29°C (H day). In the adopted research methodology, 167 reservoirs, larger than 1 ha, located within 300 m of urban areas, were analysed. Satellite thermal imagery, spatial land use data (Corine Land Cover), and local land characteristics were used. The average temperature of the reservoirs was appropriately at 4.69°C on W day and 1.9°C for H day lower than in residential areas. The average temperature of areas at a distance of 30 m from the reservoirs was 2.69°C higher onWand 0.32°C higher on H than the water of the reservoirs. The area covered by trees was 0.52°C lower on W day and 0.39°C lower on H day than the residential areas located at a distance of 300 m from the reservoir. On terrestrial areas, the lowest temperature was observed in the area covered by trees within 0–30 m from reservoirs both on warm and hot days. Based on the results of this study, UHI mitigation solutions can be suggested.

Impact of urban heat island on daily and sub-daily monsoon rainfall variabilities in East Asian megacities

Impact of urban heat island on daily and sub-daily monsoon rainfall variabilities in East Asian megacities

Application of Landsat-8 OLI/TIRS to assess the Urban Heat Island (UHI)

Urban Heat Island (UHI) phenomenon is a particular climate change impact in urban areas that can trigger adverse effects on environmental conditions. This effect is usually worsened by increasing anthropogenic activities. While the impact of UHI has mostly been studied in metropolitans around the world, few studies are presented investigating the outcome in mid-sized cities. This study aims to assess spatial distribution of Urban Heat Island (UHI) in Bogor city, Indonesia, by utilizing thermal data (Band-10) from Landsat-8 OLI/TIRS. The Landsat imagery was acquired in May 2021, in the beginning of the dry season. The result of study shows Land Surface Temperature (LST) values ranged from 26.23 to 33.76°C, with an average temperature of 31.29°C, while the range of NDVI was between 0.08 to 0.55. Further, the correlation between Land Surface Temperature (LST) and Normalized Difference Vegetation Index (NDVI) was calculated to investigate the relationship between vegetation density and surface temperature condition for examining the outlook and possibilities to minimize the impact of UHI through improving vegetal conditions. A negative correlation suggests that expanding vegetation coverage can reduce urban heat island impacts by 35%, and other factors influence the rest. The land surface temperature threshold to determine UHI is 32.01°C, so the UHI area was identified at ca. 13.12 km2 around the city centre.

ENVIRONMENTAL IMPACT ANALYSIS ON THEINCREASE IN TEMPERATURE AND URBAN HEAT ISLAND; A CASE STUDY OF GWAGWALADA, ABUJA

This research looks at the impact of urban heat island in Gwagwalada urban and it's the rural surrounding area, it was achieved through the following; i) the change detection of Gwagwalada, ii) identifying the relationship between the rise in temperature as well as vegetation, iii) examining the effect of UHI on the inhabitants of Gwagwalada, iv) determining if Gwagwalada urban generates more heat than its rural environs. Lands at 7 and 8 satellite imageries were used to do the classification, NDVI, and land surface temperature analysis. While air temperature data from POWER climate data were applied toward analyzing the UHIindex. Change detection of the study area shows that in 2020 the increase in urbanization affected the area while features like vegetation and water body reduced due to climate change and urban concentration. Land surface temperature also increased in 2020 due to various activities in the study area and the construction of surface imperious structures in the area. From the result of the air temperature with the calculation of the UHIindex, it shows that temperature increased in the study area and some of the adaptation and mitigation measures were considered.

A study of Urban Heat Island relating "Local Climate Zones" using Landsat Images- The Case of Kathmandu Valley

Local Climate Zone (LCZ) classification has been intensively used in classification of urban and rural landscapes in the cities, which includes urban temperature studies. The urban heat island (UHI) in Kathmandu valley (Kathmandu, Bhaktapur, and Lalitpur), has been analyzed and standardized, which mainly focused on the Local Climate Zones (LCZs). The LCZs has distribute the landscape into homogeneous types on the basis of structural type, land surface cover, materials used, and into the anthropogenic activities. Such standardized classification has improved the meaning of urban research and made it easier to compare results among cities around the world. Landsat images, Google Earth, and SAGA-GIS software were used for creating LCZ map for Kathmandu for both March 2013 and March 2019 Landsat 8 TM/ETM+/OLI imagery was used to estimate land surface temperature (LST) For the estimation of LST world urban database and access portal tools (WUDAPT) algorithm was used considering emissivity. The result thus shows that the difference within the built-up scheme is around 2-4 °C whereas the difference between Building and Land cover types on the comparison is around 5-10°C. The disparity in building the land cover types shows that the UHI impact is present in the Kathmandu valley.

Pengaruh Urban Geometri, Material, dan Vegetasi Terhadap Kenyamanan Termal Jalan Pangeran Diponegoro Yogyakarta Berbasis Simulasi

Yogyakarta city government plans to increase the number of tourists and make Pangeran Diponegoro street as a center for tourism and cultural development in the city of Yogyakarta. One way to realize these expectations is by increasing thermal comfort in the corridor space. However, the landuse on the south part of corridor has changed, so that it affect the changes in physical spatial condition which will have an impact on decreasing the quality of the thermal comfort on Pangeran Diponegoro street. The purpose of this research is to find a development guidelines that can reduce the impact of Urban Heat Island and find the most optimal design guidelines in increasing thermal comfort on corridor. The research method uses direct observation, then simulated modifies the physical condition of the corridor (building height, pavement material, and vegetation) using ENVI- met 3.1. The results showed that corridor existing had not yet reached a thermal comfort. The results of the geometry modification show that the height modification does not have an impact on increasing the thermal comfort of the corridor, while the albedo pavement material changes to 0.7 and the addition of vegetation as much as 75% succeeds in making the corridor comfortable.

Effects of Green Roof in Reducing Surface Temperature and Addressing Urban Heat Island in Tropical Climate of Malaysia

Urban heat island (UHI) in cities across Malaysia has worsened due to rapid development and construction. Previous research highlight temperature difference between Putrajaya and its suburban neighbouring area is as high as 5˚C. Green roofs in urban areas can be used as a green intervention strategy to reduce UHI impact by increasing air moisture and surface wetness through its plants, which consequently decreases surrounding air and surface temperature. Green roofs also provide additional insulation that reduces heat transfer into buildings, therefore reducing cooling demand and operational energy in buildings. Conventional building roofs in tropical climate have measured surface temperature up to 60˚C, compared to surrounding irrigated grass and water bodies that are measured as low as 15˚C. This paper compared surface temperatures of green roof and conventional non-green roof case studies in a tropical climate setting, to combat urban heat island effect. A triangulation technique was adopted, using a case study analysis with an in-depth structured interview and physical fieldwork investigation. The findings concluded that green roofs had significantly lower surface temperature compared to non-green flat roof. This paper provides evidence that supports green roof as a green intervention strategy in reducing urban heat island and encourage building designers to maximize under-utilized spaces of roofs to increase urban greenery coverage.

Urban heat island reduces annual building energy consumption and temperature related mortality in severe cold region of China

The urban heat island (UHI) phenomenon has gained increasing attention as it not only affects the energy consumption of buildings but also threatens the health of urban dwellers. In this study, we calculated the UHI intensity in major China cities across different climates based on the measured weather data of year 2019. The influences of UHI intensity on the building energy consumption and temperature-related mortalities were then quantified. Results show that the impact of UHI on urban dwellers is not always negative in terms of building energy consumption and temperature-related mortality. In most severe-cold-region cities in China, UHI reduces the annual building energy consumption caused by the cooling and heating load, because the amount of heating load reduced in winter is larger than the cooling load increased in summer. The annual temperature-related mortality is also reduced by UHI in certain cities. These reductions indicate that UHI has favourable effects in certain regions. Our findings indicate that detailed evaluation should be performed on a case-by-case basis to quantify the UHI effects on urban dwellers.

Impact of urban heat island on inorganic aerosol in the lower free troposphere: a case study in Hangzhou, China

Abstract. Urban heat island (UHI) and urban air pollution are two major environmental problems faced by many metropolises. The UHI affects air pollution by changing the local circulation and the chemical reaction environment, e.g. air temperature and relative humidity. In this study, the WRF-CMAQ (Weather Research and Forecasting Community Multiscale Air Quality) model was used to investigate the impact of an UHI on the vertical distribution of aerosol particles, especially secondary inorganic aerosol (SIA), taking the strong UHI in Hangzhou, China, as an example. The results show that during the day with the strongest UHI case of the 8 d simulation, the UHI effect resulted in a decrease in the PM2.5 concentrations in the boundary layer (BL) by about 33 %, accompanied by an increase in the lower free troposphere (LFT) by about 19 %. This is mostly attributed to the UHI circulation (UHIC) effect, which accounted for 91 % of the UHI-induced variations in PM2.5, rather than the UHI temperature or humidity effects, which contributed only 5 % and 4 %, respectively. The UHIC effect plays a dominant role, ranging from 72 % to 93 %, in UHI-induced PM2.5 variation in all eight UHI cases. The UHIC not only directly transports aerosol particles from ground level to the LFT but also redistributes aerosol precursors. During the strongest UHI case, about 80 % of the UHIC-induced increase in the aerosol particles in the LFT is due to direct transport of aerosol particles, whereas the other 20 % is due to secondary aerosol formation resulting from the transport of aerosol precursor gases. Of this 20 %, 91 % is contributed by SIA, especially ammonium nitrate aerosol formed from ammonia and nitric acid. In the atmosphere, ammonium nitrate is in equilibrium with ammonia and nitric acid, and the equilibrium depends on the ambient temperature. In the lower urban BL, the temperature is higher than in the LFT, and the ammonium nitrate equilibrium in the lower BL is more toward the gas phase than in the LFT; when these gases are transported by the UHIC into the colder LFT, the equilibrium shifts to the aerosol phase. Hence, the UHIC changes the vertical distribution of SIA, which may have potential implications on the radiation budget, cloud formation, and precipitation in the urban and surrounding areas.