This paper discussed the urban heat island (UHI) intensity and local air quality by using observational data of project of the System of Air Quality Forecasting and Research (SAFAR) over Delhi during the month of May and December 2013. It is found that UHI magnitudes ~2.2 °C and ~1.5 °C are formed at the evening traffic hours during May and December respectively. Also, intensity of UHI < 0 °C over daytime is referred as Urban Cool Island (UCI) during May and December. The diurnal PM 2.5 concentration shows a bimodal pattern with peaks at morning and evening traffic hours during May and December. The planetary boundary layer height (PBLH) values show higher in magnitude during the daytime and lower in magnitude during the night-time. Whereas, the Ground Heat Flux values are lower during the daytime and higher during the night-time. The wind speed shows lower values during the UHI and higher magnitudes during the UCI formation hours. Concentration of PM 2.5 and wind speed shows a strong negative correlation during May (r = -0.56, p = 0.002) and December (r = -0.57, p = 0.001) at C V Raman (CVR) site, however, high values in the concentration of PM 2.5 during the low wind speed favour the condition for the formation of UCI. The regression analysis indicated that PM 2.5 plays a significant role in the daytime cooling and nighttime warming over the urban areas during the low wind speed condition.
The urban heat island (UHI) effect is simply defined as the temperature difference between an urban and the surrounding rural regions. This impacts to release in additional heat from the urban environment which is a major contribution to the urban heat island [
The basic heat transfer and energy conservation processes, such as conduction, convection and radiation play their characteristic roles in this heat exchange. The structures on ground level, such as walls and roof facets, irrigated gardens, non-irrigated green spaces, lawns and paved areas among others, capture solar radiation to different extents. These natural and man-made structures continuously absorb and store this radiation in the form of heat energy from sunrise till late afternoon. Afterwards, the sun starts setting a side and the environment starts cooling down. The heat energy stored in structures is then released to the environment. The method and quantity of heat released by the urban structures, however, depend on other controllable factors such as the sky view factor and building material. In a typical urban area, massive construction material is placed within a very small space that captures high intensity of solar radiation. The ability of heat release by long-wave radiation in cities is low due to decreased sky view which results in high heat storage in building structures. It is also believed that air pollutants, in particular aerosols that are abundant over polluted urban areas, can absorb and re-radiate long wave radiation and inhibit the corresponding radiative surface cooling producing a pseudo-greenhouse effect, which is responsible for causing UHI [
PM2.5, which refers to particulate matter (PM) in air that is less than 2.5 μm in aerodynamic diameter, is a key pollutant affecting radiation balance and a main factor in decreasing air quality [
Dispersion and transport of lower atmospheric pollutant depend largely on the local planetary boundary layer height (PBLH) structure. Thus, PBLH has been used as a key in weather, climate, and air quality models to determine turbulence mixing, vertical diffusion, convective transport, and atmospheric pollutant deposition [
The UHI has been the focus in many studies [
In this study we mainly focus on the hourly Meteorological data of air temperatures and surface concentrations of PM2.5, collected during the December 2013 and 2015 from the two stations, first Sir C V Raman (CVR) Industrial Training Institute (28.73˚N, 77.20˚E), which is located at north from the center of the city. This is completely urbanized area and surrounded majorly with significant infrastructure such as buildings for the residential as well the commercial purposes with a lot of roads and traffic and a little amount of trees and vegetative grass lands as in
(IITMD) Delhi branch (28.64˚N, 77.17˚E), which is located at the center of the city. This is majorly surrounded with reserve forest and natural green vegetative grass lands as shown from
The Mesoscale model used in this study is the Advanced Research WRF (ARW) model version 3.7. Our model simulation covers the PBLH and ground heat flux for the months of May and December 2013. The simulations were run at a three interactive domains with spatial resolution of 9 km, 3 km and 1 km which covers part of North India (22˚N - 34˚N - 71˚E - 84˚E), second Domain covers complete Delhi and outer boundaries (25˚N - 32˚N - 73˚E - 80˚E) region and Third domain covers Delhi (27˚N - 30˚N - 75˚E - 78˚E). The vertical grid contained 32 full sigma levels from the surface up to 50-hPa. Approximately eight of these levels were below 1 km, thereby providing a fine vertical resolution within the planetary boundary layer. The terrain and land use data for the entire 3 domain were taken from 30 s data available from the United States Geological Survey (USGS). The National Centre for Environmental Prediction (NCEP) Final Analysis data (FNL) available at 6 hourly intervals on a 1˚ × 1˚ resolution are used as initial and boundary conditions. The model uses two-way nested boun- dary conditions and they are updated every 6 hours.
The main physics schemes tested and used in the model are microphysics scheme from [
A simulation of the particular intricacies of such an environment is heavily influenced by its depiction of the planetary boundary layer (PBL)―that portion of the lower troposphere directly affected by the earth’s surface via troposphere- surface exchanges of heat, moisture, and momentum on sub hourly time scales [
From
during the month of December 2013, that UHI intensity shows a peak late in the evening around 20:00 hrs with a magnitude of ~1.5˚C, thereafter it decreases till the sunrise in the morning. During the daytime the magnitude in the intensity of UHI continuously increases with the morning UCI to the night time UHI effect. The PM2.5 concentrations over the CVR site shows a bimodal pattern in both the month May and December with the morning peak around 10:00 hrs and the second peak in the evening peak around 20:00 hrs, the peaks in the PM2.5 concentration hours are related to the morning and evening traffic hours [
The diurnal variation in the wind speed during May and December 2013 over the CVR and IITMD sites are seen in
The UHI formation during the evening traffic hours is due to increase in the concentration of PM2.5 with decrease in the height of the PBLH and higher in the magnitude of the ground heat flux and lower wind speed. Before sunrise the concentration in the PM2.5 are lower in the magnitude and with the decrease in the PBLH and lower in the ground heat flux values and lower in the wind speed, which leads to the formation of the Urban Cool Island (UCI).
Figures 4(a)-(d) show regression analysis to explore the correlation between the UHI and its two possible dependent variables during May 2013. The first variable is the concentration of PM2.5, here
0.61, p = 0.001) and IITMD (r = 0.65, p =0.001) sites. The second variable is the wind speed, from
Figures 5(a)-(d) show the regression analysis to explore the correlation between the UHI and its two possible dependent variables during December 2015. The first variable is the concentration of PM2.5, here
0.002) and IITMD (r = −0.27, p = 0.005) site respectively. From
The formation of UHI is mainly favoured with the low wind speed and high concentration of PM2.5 with a positive correlation between the UHI and PM2.5 concentrations. Whereas, a negative correlation between the wind speed and concentrations of PM2.5 [
We performed analysis of site-specific air quality and weather data from the project SAFAR over Delhi during May and December 2013. UHI of ~2.2˚C and ~0.5˚C observed over nighttime with a peak around 20:00 hrs during May and December respectively. The PM2.5 concentration shows a bimodal pattern with first peak at 10:00 hrs and the second peak at 20:00 hrs. The peaks in the diurnal concentration of PM2.5 correspond to morning and evening traffic hours during May and December. The evening peak in the concentration of PM2.5, ground heat flux values, low PBLH and wind speed leads to the formation of UHI. On the other hand, during daytime, peak in the concentration of PM2.5 during the morning traffic hours, high PBLH, low intensity of Ground heat flux and less wind speed leads to a peak in the intensity of UHI < 0˚C leading to the formation of UCI during May and December.
During May and December, the concentration of PM2.5 shows a positive correlation with UHI intensity at CVR site (r = 0.61, p = 0.001; r = 0.57, p = 0.001), and IITMD site (r = 0.65, p = 0.001l; r = 0.40, p = 0.003) and negative correlation of wind speed correlation with UHI intensity at the CVR (r = −0.40, p = 0.001; r = −0.40, p = 0.002) and IITMD (r = −0.41, p = 0.001; r = −0.27, p = 0.005). Also, concentration of PM2.5 and wind speed shows a strong negative correlation at CVR (r = −0.56, p = 0.002; r = −0.57, p = 0.001) and IITMD (r = −0.51, p = 0.002; r = −0.56, p = 0.001). The results from the regression analysis of wind speed and PM2.5 indicated the significant role played by the PM2.5 levels in the daytime cooling and nighttime warming over the urban areas which moderated the diurnal UHI during the low wind speed condition.
The authors are thankful to Director, IITM for his constant support and encouragement. The authors are all thankful to Ministry of Earth Science for providing funds for SAFAR network. The authors also wish to thank SAFAR team who helped in the collection of data.
Aslam, M.Y., Krishna, K.R., Beig, G., Tinmaker, M.I.R. and Chate, D.M. (2017) Seasonal Variation of Urban Heat Island and Its Impact on Air-Quality Using SAFAR Observations at Delhi, India. American Journal of Climate Change, 6, 294-305. https://doi.org/10.4236/ajcc.2017.62015