**1. Introduction**

46 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

95, Springer

vehicles; J. Air & Waste Manage. Assoc. 54(9):1138-50.

[34] Zielinska B, Sagebiel J, McDonald JD, Whitney K, Lawson DR (2004) Emission rates and comparative chemical composition from selected in-use diesel and gasoline fueled

[35] Hoshiko T, Nakajima F, Prueksasit T, Yamamoto K (2012): 4. Health risk of exposure to vehicular emissions in wind-stagnant street canyons, In: "Ventilating cities -Air-flow criteria for healthy and comfortable urban living-" (Eds.: Kato S and Hiyama K), pp.59-

> The climatic and environmental effects of atmospheric aerosols are the critical issues in global science community because aerosols, derived from variety of natural and man-made (or anthropogenic) emission sources, are well known to affect the air quality, human health and radiation budget [1]. While comparing the third and fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) as shown in Figure 1, the level of scientific understanding for the role of green house gases (GHGs) in projected temperature changes is higher relative to that of aerosols [2,3]. This is because of inadequate measurements of aerosols, their microphysical and optical properties and poor understanding of their role in the Earth's radiation budget.

> Aerosols influence the Earth's climate directly by scattering and absorbing the solar and terrestrial radiations and indirectly by modifying the cloud macro- and micro-physical properties [4]. The direct and indirect effects of atmospheric aerosols are shown in the schematics in Figure 2a and 2b, respectively. Varity of aerosols present in the atmosphere from natural and anthropogenic emission sources can influence our Earth's atmosphere directly by absorbing/scattering the incoming solar radiations (Figure 2a). It can also absorb and re-radiated the outgoing radiations emitted from the Earth. On the other hand, aerosols indirectly affect the climate system by acting as cloud condensation nuclei (CCN) and ice nuclei (IN) and thereby modify the cloud properties and their impacts depending upon the environment like polluted or un-polluted regions (Figure 2b). In a recent study, reported in [5], they have investigated the indirect aerosol effect during the successive contrasting monsoon seasons over Indian subcontinent. However, a different study reported in [6] was carried out to investigate the intensity and the spatial extent of the indirect effect over the Indo-Gangetic Basin (IGB) region.

© 2012 Srivastava et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Srivastava et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Aerosol Characteristics over the Indo-Gangetic Basin: Implications to Regional Climate 49

only few of them have fairly long-term data of aerosol microphysical properties [9-11]. Satellites are proved to be a good tool to understand the broad spatio-temporal characteristics of aerosols and associated effects from global to local scales [12-15]; but they are unable to provide an in-depth view of aerosol properties on a local scale and pose higher uncertainties as compared to the ground-based instruments [16]. In the IGB, aerosols of natural and anthropogenic origins mix with each other during dust loading season [17,18]. As a result, aerosol properties change, leading to even larger uncertainty in satellite retrievals [14,19] as this is not considered in the aerosol retrieval algorithm [20]. National Aeronautics and Space Agency (NASA) has setup ground-based aerosol monitoring network under the Aerosol Robotic Network (AERONET) program [21], in which automatic sun/sky radiometers are deployed at various places around the world. As per India, particularly in the northern part, is concerned, the routine measurements of aerosols under this network were started initially by the deployment of the sun/sky radiometer at Kanpur over the IGB region in year 2001 [22]. At a later stage, it was deployed at other places in the IGB, considering the region as crucial for aerosol measurements where significant aerosol loads of pollution mostly from the combustion of biomass, bio-fuel/fossil fuel emissions and the transported mineral dust have led to one of the largest regional TOA energy losses worldwide (For Example see [11,23-25]). The measurements by individual instruments can particularly be useful for the quantification of the regional impact of aerosols on the radiative energy balance. Provided a parallel individuation of aerosol types is possible, these local studies can also help to reduce the uncertainties on the effect of individual aerosol species, which is necessary because the direct radiative forcing by individual aerosol species is less certain than the total direct radiative forcing by all aerosols [3]. The complex mixture of aerosols over the IGB has been evaluated in the literature over the last decade starting with Indian Ocean Experiment (INDOEX) [26], and the research has been continued

Understanding and quantifying the aerosol effects are important in the IGB region due to several pathways have been hypothesized to explain the possible impacts of aerosols on the regional hydrological cycle. The region is of great research interest due to its unique topography surrounded by the Himalayas to the north, moderate hills to the south, Thar Desert and Arabian Sea in the west, and Bay of Bengal to the east. The IGB region is dominated by the urban/industrial aerosols [28-30], which demonstrate significant seasonal variability based on the complex mixture of these aerosols with the naturally produced aerosols, particularly during the pre-monsoon and monsoon seasons. In addition to the urban-industrial pollution, desert dust is one of the other major natural sources of the aerosols over the Ganga basin [24,27,31-36], transported frequently from the neighboring desert regions, mostly during pre-monsoon periods. Dust storms are often experienced in northern and northwestern part of India, including over different parts of the IGB region during the pre-monsoon season, when dust aerosols are transported by southwesterly summer winds from the western Thar Desert [37,38]. High dust loading over the IGB region during the pre-monsoon period has been established by remote sensing data [39,40]. These dust storms apparently deposited silty materials in the downwind directions, as observed

with the AERONET data [27 and references therein].

**Figure 1.** A comparison between III and IV assessment reports of IPCC for 2001 and 2007 (*Adopted from [2,3])*.

**Figure 2.** Schematics showing (a) Direct and (b) Indirect impacts of atmospheric aerosols.

The uncertainty in quantifying these impacts have no doubt improved over the years due to assimilation of observations (especially after global observations of aerosols by EOS-Terra started in 1999), but not up to the desired level, particularly at regional scale [3]. The Indian subcontinent is one such region, where heterogeneity in aerosol optical and microphysical properties over a wide range of spatial and temporal scales continues to hinder in improving the estimates of aerosol-induced climate forcing. Thus, it is important to improve aerosol characterization with high spatio-temporal resolutions; particularly over the IGB region, which supports nearly 70% of the country's population and is one of the highly polluted regions in the world. The problem is also critical due to lack of adequate long-term measurements of aerosol properties and large uncertainty in emission factors, leading to poor representation of aerosol distribution by General Circular Model (GCM) [7].

Although, aerosol properties have been measured at many sites in India in continuous and campaign modes (the *in-situ* observations have summarized in [8]) in the last two decades, only few of them have fairly long-term data of aerosol microphysical properties [9-11]. Satellites are proved to be a good tool to understand the broad spatio-temporal characteristics of aerosols and associated effects from global to local scales [12-15]; but they are unable to provide an in-depth view of aerosol properties on a local scale and pose higher uncertainties as compared to the ground-based instruments [16]. In the IGB, aerosols of natural and anthropogenic origins mix with each other during dust loading season [17,18]. As a result, aerosol properties change, leading to even larger uncertainty in satellite retrievals [14,19] as this is not considered in the aerosol retrieval algorithm [20]. National Aeronautics and Space Agency (NASA) has setup ground-based aerosol monitoring network under the Aerosol Robotic Network (AERONET) program [21], in which automatic sun/sky radiometers are deployed at various places around the world. As per India, particularly in the northern part, is concerned, the routine measurements of aerosols under this network were started initially by the deployment of the sun/sky radiometer at Kanpur over the IGB region in year 2001 [22]. At a later stage, it was deployed at other places in the IGB, considering the region as crucial for aerosol measurements where significant aerosol loads of pollution mostly from the combustion of biomass, bio-fuel/fossil fuel emissions and the transported mineral dust have led to one of the largest regional TOA energy losses worldwide (For Example see [11,23-25]). The measurements by individual instruments can particularly be useful for the quantification of the regional impact of aerosols on the radiative energy balance. Provided a parallel individuation of aerosol types is possible, these local studies can also help to reduce the uncertainties on the effect of individual aerosol species, which is necessary because the direct radiative forcing by individual aerosol species is less certain than the total direct radiative forcing by all aerosols [3]. The complex mixture of aerosols over the IGB has been evaluated in the literature over the last decade starting with Indian Ocean Experiment (INDOEX) [26], and the research has been continued with the AERONET data [27 and references therein].

48 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

**(a) (b)**

*[2,3])*.

**Figure 1.** A comparison between III and IV assessment reports of IPCC for 2001 and 2007 (*Adopted from* 

**IPCC III Assessment Report- 2001 IPCC IV Assessment Report- 2007**

**Figure 2.** Schematics showing (a) Direct and (b) Indirect impacts of atmospheric aerosols.

poor representation of aerosol distribution by General Circular Model (GCM) [7].

Although, aerosol properties have been measured at many sites in India in continuous and campaign modes (the *in-situ* observations have summarized in [8]) in the last two decades,

The uncertainty in quantifying these impacts have no doubt improved over the years due to assimilation of observations (especially after global observations of aerosols by EOS-Terra started in 1999), but not up to the desired level, particularly at regional scale [3]. The Indian subcontinent is one such region, where heterogeneity in aerosol optical and microphysical properties over a wide range of spatial and temporal scales continues to hinder in improving the estimates of aerosol-induced climate forcing. Thus, it is important to improve aerosol characterization with high spatio-temporal resolutions; particularly over the IGB region, which supports nearly 70% of the country's population and is one of the highly polluted regions in the world. The problem is also critical due to lack of adequate long-term measurements of aerosol properties and large uncertainty in emission factors, leading to Understanding and quantifying the aerosol effects are important in the IGB region due to several pathways have been hypothesized to explain the possible impacts of aerosols on the regional hydrological cycle. The region is of great research interest due to its unique topography surrounded by the Himalayas to the north, moderate hills to the south, Thar Desert and Arabian Sea in the west, and Bay of Bengal to the east. The IGB region is dominated by the urban/industrial aerosols [28-30], which demonstrate significant seasonal variability based on the complex mixture of these aerosols with the naturally produced aerosols, particularly during the pre-monsoon and monsoon seasons. In addition to the urban-industrial pollution, desert dust is one of the other major natural sources of the aerosols over the Ganga basin [24,27,31-36], transported frequently from the neighboring desert regions, mostly during pre-monsoon periods. Dust storms are often experienced in northern and northwestern part of India, including over different parts of the IGB region during the pre-monsoon season, when dust aerosols are transported by southwesterly summer winds from the western Thar Desert [37,38]. High dust loading over the IGB region during the pre-monsoon period has been established by remote sensing data [39,40]. These dust storms apparently deposited silty materials in the downwind directions, as observed

on the quartzite ridges in the Delhi area [41]. The wind also carries heavy metals to the IGB during the summer season [42] along with the dusts, causing severe air pollution and degradation in the visibility. On contrary, the spatial distribution of aerosols (in terms of AOD) during the winter season also revealed high aerosol loading over the IGB and its outflow to the northern Bay of Bengal due to high anthropogenic emission sources, which was observed by satellite [8,12,13,15] and ground-based measurements [22,43-46].

Aerosol Characteristics over the Indo-Gangetic Basin: Implications to Regional Climate 51

and by Vindhyan and Satpura range of mountains in the south. Due to its unique topography, this region can be summarized as a type of region, where, both anthropogenic and natural, aerosols show distinct seasonal characteristics and mixing [13,17,22,25,28,30,47]. General seasonal abundance shows that the winter months are dominated by the fine-mode aerosols, produced by various anthropogenic sources from the IGB region, and pre-monsoon or summer months are dominated by the coarse-mode mineral dust, primarily from the Thar Desert region in the western Rajasthan and its frequent transportation over the IGB region. Further details regarding geography, climate, regional sources and emissions of these aerosols over the IGB as well as over the other Indian region, however, can be found in [15]. This region also provides favorable climate for the agricultural activities due to its fertile soils and abundant water supply from the southwest monsoon and the rivers originating from the Himalayan glaciers such as the Ganges. Consequently, the cultivable land forms a major fraction of the total geographical area in the IGP region (∼76%) as compared to the rest of India (∼50%)

Synoptic meteorology (e.g. wind pattern, air temperature and specific humidity) over the IGB region along with its surroundings is shown in Figure 4 for (a) winter and (b) summer seasons for the period of 2007-2008. The European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis monthly data of weather parameters such as wind, air temperature and specific humidity at 850 hPa pressure level were used to study the synoptic meteorological conditions over the region. In both the figures, winds are shown with arrows pointing towards the wind direction, where length of arrows defines the magnitude of wind speed (in ms−1), line contour represents air temperature (in oC) and shaded color contour represents specific humidity (in kg kg-1), showing in dark blue color for low and red color for high magnitude of specific humidity. Results reveal that the IGB region during the winter period is relatively drier than during the summer. The persistence of low temperature and the westerlies (with low intensity) can be seen over the region during the winter whereas during the summer, relatively high temperature with intense southwesterly winds was observed to dominate. These winds are found to pass through arid regions of the western India (particularly from the Thar Desert) and bring dry air masses over the region

The general aerosol characteristics over the entire IGB region are shown in Figure 5 as mean AOD values at 550 nm for (a) winter and (b) summer seasons for the period of 2007-2008 in color codes, obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS). Large spatial heterogeneity in AOD can readily be noticed over the IGB region during both winter and summer periods, which has also been confirmed through various ground-based measurements, discussed in the later section of this chapter. Relatively large magnitude of AOD was observed throughout the IGB region during the summer, which is mainly due to frequent occurrence of dust storms over the Thar Desert region that caused large amount of dust particles to be transported over the station (showing the highest AOD). However, large

**2.2. Synoptic conditions and aerosol characteristics over IGB** 

(http://dacnet.nic.in/).

[24,27,48].
