Impact of biomass burning on aerosol properties over tropical urban region of Hyderabad, India



[1] Biomass burning is identified as a major source of atmospheric pollution giving rise to the release of large quantities of gaseous emissions and particulate matter. The present study aims at analyzing the impacts of biomass burning on aerosol loading over urban area of Hyderabad, India using synchronous measurements of Aerosol Optical Depth (AOD), solar irradiance in different wavelength bands, aerosol particle size distribution measurements and black carbon (BC) aerosol mass concentration. Temporal variation of AOD and aerosol index (AI) correlated with occurrence of forest fires as derived from DMSP-OLS and MODIS satellite data. BC values showed good correlation with total aerosol number density and showed patterns correlating with wind direction. Radiative forcing estimated from synchronous measurements of AOD and ground reaching broadband solar irradiance suggested −12.5W/m 2 reduction per 0.1 increase in AOD. Diffuse to direct ratio of solar irradiance showed variations correlating with changes in aerosol optical depth.

1. Introduction

[2] Aerosols are tiny, suspended solid particles or liquid droplets that enter the atmosphere from either natural or anthropogenic sources such as forest fires, fossil fuel burning etc. Natural sources of aerosols are from volcanic eruptions, dust storms, ocean waves, and forest fires. Industrial sector comprising of power plants, oil refineries, and other manufacturing units also produce aerosols into the atmosphere. Other anthropogenic sources are automobiles; refuse burning, and home furnaces and fireplaces. Fine aerosols are important for the earth's climate, by scattering and absorbing sunlight and by modifying cloud characteristics. Furthermore, they have a negative impact on human health, as they contribute to respiratory and cardio-pulmonary diseases and mortality. Although carbonaceous particles are a major component of the fine aerosol, there is a large uncertainty about the importance of biogenic and anthropogenic emission sources of these particles.

[3] Each year more than 100 million tons of smoke aerosols are released into the atmosphere from biomass burning out of which 80% is in the tropical regions [Hao and Liu, 1994]. These sub-micron smoke particles, composed primarily of oxidized organic materials, are efficient in scattering and absorbing sunlight. There are two major radiative effects of biomass burning aerosols. The first, called the “direct” radiative effect, refers to the scattering and absorption of incoming solar radiation by smoke aerosols [Christopher et al., 2000]. The second effect, called the “indirect” radiative effect, refers to the interaction of smoke aerosols with clouds [Kaufman and Fraser, 1997]. The radiative effect of smoke aerosols on regional and global climate is yet to be understood due to several reasons among which chemical compositions and spatial distributions are critical [Hansen et al., 1997a, 1997b].

[4] Urban areas have always been known to be a major source of particulate pollution, which is expected to continue to increase due to world population growth and increasing industrialization and energy use, especially in developing countries.

[5] The most obvious effects are the contributions to unsightly smog and the visible deterioration of building materials. Further, biomass burning episodes such as agriculture residue burning, controlled burning as well as forest fires occurring near the cities carry particulate matter over to the cities and contribute to added aerosol loading in the urban environments [Niemi et al., 2005].

[6] The prime objective of the present study is to analyze the impacts of biomass burning around Hyderabad and its environs on the aerosol loading in the city. Aerosol Optical Depth (AOD) over Hyderabad has been continuously measured and comparative analysis with concurrent meteorological parameters, wind back-trajectories and fire locations given by satellite data has been done to explain the variations in AOD.

2. Study Area

[7] The study area of Hyderabad is located at 17° 10′ to 17° 50′N latitude and 78° 10′ to 78° 50′E longitude, which is the fifth largest city in India. Population of the city according to 2001 census is 3,449,878, which is purely urbanized. The climate of the study area is of semi-arid type with total rainfall of ∼700mm occurring mostly during monsoon season corresponding to June–October. The measurements on aerosol spectral optical depth (AOD) have been carried out in the premises National Remote Sensing Agency (NRSA) campus located at Balanagar (17°.28′N and 78°.26′E) located well within the urban center [Latha et al., 2004]. Wind direction during February predominantly northerly with continental air mass conditions over the study area and forest fires active over the region during this period.

3. Data Sets and Methodology

[8] MICROTOPS-II sunphotometer was used to measure aerosol optical depth (AOD) at different wavelengths viz., 380, 440, 500, 675, 870 and 1020nm [Leckner, 1978]. Aerosol number densities in different size ranges from 0.30 to 20 μm were measured using GRIMM aerosol spectrometer model 1–108 [Le Canut et al., 1996]. Continuous and near-real-time measurements of the mass concentration of black carbon aerosol (BC) were carried out using Aethalometer; model AE-21 of Magee Scientific, USA [Cooke et al., 1997; Borak et al., 2003]. The instrument aspirates ambient air from an altitude of ∼3 m above the ground using its inlet tube and its pump. The Multi-Filter Rotating Shadow band Radiometer (MFRSR) at wavelengths of 415.9, 496.6, 622.4, 670.2, 868.3, and 938.5 nanometers (FWHM ∼10 nm) was used for measurements of total, diffuse horizontal, and direct normal solar irradiances [Harrison et al., 1994]. These instruments provide synchronous observations of aerosol optical depth, black carbon aerosol mass concentration, aerosol particle number density and ground reaching solar irradiance in different spectral bands required for understanding urban air quality and studying their interrelation.

[9] Daily data sets of DMSP-OLS for February 2006 were processed for generating nighttime fire products over the Indian region. DMSP operates in sun-synchronous orbits with nighttime overpasses ranging from about 7 pm to 10 pm local time with a swath width is 3000 km. The OLS is an oscillating scan radiometer with two spectral bands. The visible band pass straddles the visible and near-infrared portion of the spectrum (0.5 to 0.9 μm) and the thermal band pass covers the 10.5 to 12.5 μm region [Elvidge et al., 1997]. The low light sensing capabilities of the OLS at night permit the measurement of radiances down to 10−9 watts/cm2/sr. Fires present at the Earth's surface at the time of the nighttime overpass of the DMSP are readily detected in the visible band data. Daytime MODIS data over the region were processed for forest using the thermal bands data [Giglio et al., 2003].

4. Results and Discussion

[10] Figure 1 shows the Julian day variation of aerosol optical depth at 500nm during February 2006. It can be noticed from Figure 1 that high Aerosol Optical Depth(AOD) values occurred on 45, 46, 47, 49, 50, 56, 57, 58 Julian days. Figure 2 shows the Variation of AOD at 500 nm on normal day and high aerosol loading day. The AOD values were 20% higher on burning days compared to normal days suggesting additional source causing high aerosol loading over urban region of Hyderabad. Significant variations in AOD were observed during the course of the day with higher values during forenoon hours. The surface temperatures are low during forenoon hours and convective activity sets in by afternoon hours due to solar heating causing breakup of inversions resulting in ventilation of aerosol particles. In order to identify the possible sources for such high aerosol loading, satellite data sets from Aqua - MODIS and DMSP-OLS nighttime data were analysed for occurrence of forest fires over the region (Figure 3). MODIS and nighttime DMSP-OLS data sets showed occurrence of forest fires towards north of the study area. NOAA Hysplit model runs suggested occurrence of northerly winds over the study area. Aerosol particles number density estimated using GRIMM also showed corresponding high values on these days with increase in black carbon aerosol mass concentration (Figure 4). The diurnal variation of black carbon aerosol (BC) shows a gradual build up during early morning hours and a sharp peak occurs between 6:00 and 9:00 LT almost an hour after the local sunrise. This arises from the combined effects of (i) the well-known fumigation effect in the boundary layer, which brings-in aerosols from the nocturnal residual layer shortly after the sunrise [Stull, 1998; Babu and Krishna Moorthy, 2002] and (ii) the morning build up of local anthropogenic activities in the urban area from where the wind is still directed. The increase in BC concentration during certain days over and above the normal diurnal trends are attributed to additional source coming from forest fires over the region as inferred from satellite data sets(Figure 3). Aerosol particle size distribution estimated from GRIMM analyser suggested high accumulation mode particle loading (<1.0 μm) on days with biomass burning activity north of the study area. The increase in accumulation mode particle loading coincided with high values of black carbon aerosol mass concentration suggesting that the forest fires occurring in the region are causing additional aerosol loading. Earlier reports on chemical analysis of aerosol samples collected through High Volume Sampler in the study area showed K+ ion concentrations [Kulshrestha et al., 2004]. The source of K+ ions is biomass burning. Similar observations were made earlier studies on aerosol concentration over Finland [Niemi et al., 2005]. Figure 5 shows the scatter plot of aerosol optical depth at 500 nm vs broad band (400–1100nm) solar irradiance (PAR) measured using MFRSR-7. The slope of the scatter plot provides an account of radiative forcing efficiency of the aerosols. The analysis of the Figure 5 suggests that 0.1 increases in AOD results in reduction of −12.5 W/m2 ground reaching solar irradiance. The radiative forcing of biomass burning aerosols during periods of forest fires estimated to be −15.4 W/m2. The increase in aerosol loading resulted in higher diffuse to direct ratio (DDR) of solar irradiance. The Diffuse to Direct Ratio (DDR) is an indicator of the prevailing atmospheric conditions [Kaskaoutis and Kambezidis, 2006]. The increase in aerosol loading with higher black carbon aerosol concentration results in attenuation of ground reaching solar irradiance. In order to assess the impact of Aerosol Optical Depth (AOD) on spectral solar radiation, scatter plot of AOD at different wavelengths and ratio of diffuse to direct solar irradiance in different MFRSR spectral bands were analysed. Figure 6 suggests that the Diffuse to Direct Ratio (DDR) on normal days shows lower values compared to high aerosol loading conditions associated with biomass burning. Similar observations were reported in variable pollutions conditions over urban areas [Kaskaoutis and Kambezidis, 2006]. The high concentration of fine-mode particles seems to also have modified the diffuse-to-direct-beam irradiance ratio (DDR) in different spectral bands as shown in Figure 6. This figure refers to the same SZA (40°) for the normal and biomass burning days and significant differences in the curves are attributed to the aerosol loading and their different optical properties. The DDR ratio at a specific wavelength as a function of the AOD has already been used for the discrimination of biomass burning particles in the Mediterranean, as the more absorbing aerosols exhibit lower DDR values [Meloni et al., 2005]. It is obvious that the DDR for the turbid day suggested a marked deviation with very high values especially at the shorter wavelengths. The changes (increase) in DDR at shorter wavelengths are more intense than those at longer due to enhanced values of the diffuse radiation. TOMS aerosol index (AI) also showed high values coinciding with the spatial occurrence of forest fires over the region. Results of the study suggests that integration of spatial information derived from satellite data together with ground observations helps in understanding the variable aerosol loading over urban areas.

Figure 1.

Julian day variation of aerosol optical depth at 500 nm during February 2006.

Figure 2.

Variation of AOD at 500 nm on normal day and high aerosol loading day.

Figure 3.

Aqua MODIS & DMSP-OLS derived fire locations overlaid on MODIS False Color Composite (FCC).

Figure 4.

Julian Day variation of black carbon aerosol mass concentration during February 2006.

Figure 5.

Scatter plot of broadband solar irradiance (400–1100 nm) measured from MFRSR with Aerosol Optical Depth at 500 nm.

Figure 6.

Variation of Diffuse to Direct Ratio (DDR) with Aerosol Optical Depth at different wavelengths on normal and high aerosol conditions.

5. Conclusions

[11] Impacts of biomass burning on aerosol properties over urban region of Hyderabad were analysed using ground based instrumentation and satellite data sets. The results of the study suggested that:

[12] 1) Aerosol optical depth (AOD) values were observed to be high on certain days suggesting additional aerosol loading due to anthropogenic disturbances. Occurrence of forest fires towards north of the study area correlated with high aerosol optical depth values with corresponding increase in accumulation mode particles and black carbon aerosol mass concentration.

[13] 2) Black carbon (BC) mass concentration positively correlated with aerosol total number density.

[14] 3) Radiative forcing estimated using synchronous measurements of PAR and aerosol optical depth at 500nm suggested forcing efficiency of −12.5 W/m2 for 0.1 increases in aerosol optical depth.

[15] 4) Diffuse to direct ratio estimated using MFRSR suggested positive correlation with aerosol optical depth.

[16] 5) Aerosol Index showed positive correlation with aerosol optical depth and correlation of the multi-data sets suggested that urban areas are influenced by biomass burning in addition to anthropogenic vehicular pollution during fire season.


[17] The authors thank Director, NRSA and Dy.Director (RS&GIS-AA) for necessary help at various stages and ISRO-GBP for funding the project.