Average values of size distribution parameters obtained by fitting lognormal curves to the measured aerosol number distribution over Mt. Abu
In recent times, atmospheric aerosols are receiving increasing attention as they directly affect the Earth’s radiation balance by altering incoming shortwave solar radiation that can cause positive (heating) or negative (cooling) radiative forcing depending on their scattering and absorption properties, the reflectivity of the underlying surface [10, 24] and the position of aerosols with respect to the global cloud coverage [8, 88]. Aerosols also affect the outgoing longwave radiation by absorption, emission and scattering. Presently, effects of radiative forcing of atmospheric aerosols on climate is a subject of great concern to atmospheric researchers. An accurate quantification of the aerosol direct radiative forcing is critical for the interpretation of existing climate records and also for the projection of future climate change [11, 47]. Significant amount of atmospheric radiative forcing causes high atmospheric heating due to strong absorption of solar radiation which can change the regional atmospheric stability and may alter the large scale circulation and the hydrological cycle, enough so, apparently, to account for observed temperature and precipitation changes in China and India [1, 46, 62, 70]. Therefore, the effect of aerosols on the radiation budget in terms of radiative forcing calculations is challenging and demanding, especially on the regional scale for the exclusive understanding of climate change.
The uncertainties involved in the climate models are mainly due to optical properties of aerosols on the regional scale, specially underestimated absorption of solar radiation by aerosols, both, naturally and anthropogenically produced , their residence time [57, 58], etc, which arise mostly due to lack of observations. Black carbon (BC) or soot and dust aerosols are playing the leading role in aerosol interaction with the solar radiation due to their strong absorption properties. BC comes into the atmosphere during combustion of fossil fuels, principally, diesel and coal, and from biomass burning. BC demands large attention due to its strong absorption of incoming solar radiation and produces positive radiative forcing which is sometimes comparable to the forcing of the green-house gas methane .
Therefore, underestimation of BC can introduce large uncertainty in the climate models. On the other hand, dust, mainly coming from arid regions, is generally known for scattering of solar radiation. However, dust also has a strong absorption in the UV and infrared regimes and therefore, can influence radiative forcing not only in the shortwave region but also in the longwave region. Hence, the study of dust aerosols is equally important. In addition, long-range transported dust aerosols can enhance the atmospheric radiative forcing in the presence of soot aerosols [14, 54].
South-East Asia, with its fast growing urbanization and industrialization, is one of the major hot-spot regions on the global aerosol map. A study of historical records from different locations on the globe reported an increasing trend of BC emissions in South and Central Asia . In addition, dust aerosols are also transported from the Middle-East region to over South-East Asia. A mixture of locally produced anthropogenic aerosols with natural aerosols like mineral dust and seasalt, reported over this hot spot region [42, 60–62] aids in the warming of the atmosphere. There were several campaigns of ship-, land- and air-borne measurements over Indian subcontinent and surrounding marine regions to investigate the regional effects of anthropogenic aerosols [32, 48, 75, etc.]. In-situ measurements during the Indian Ocean Experiment (INDOEX) and several campaigns under Indian Space Research Organisation – Geosphere Biosphere Programme (ISRO–GBP) found that the sources of the anthropogenic aerosols are biomass burning and fossil-fuel combustions [33, 61]. The second phase of the ISRO-GBP land campaign during winter conducted in the Indo-Gangetic Plain (IGP), a hot-spot region over India, reported significant anthropogenic aerosol loading in the atmosphere coming from industries and vehicular emissions [15, 18, 50]. Satellite-based observations suggested that significant amount of dust is also transported over IGP from Thar Desert located in western India during premonsoon (March to May) [16, 17, 54]. This transported dust helps to sustain the hot-spot over IGP maintaining the large background aerosol loading. Majority of the earlier research works focused on aerosol contribution, either locally produced anthropogenic aerosols or transported natural dust, to regional climate change over this hot-spot region. However, uncertainties in those results are found to be relatively large, especially in studies on transported dust, as the dust becomes aged by externally and internally mixing with locally produced pollutants.
This chapter investigates and quantifies the natural and anthropogenic contribution of background aerosols over western India where both the source regions, Thar Desert, source of natural dust, and IGP, hot spot region of anthropogenic aerosols, are present. The contributions of both types of aerosols are estimated for the years 2006 and 2007 from ground-based and satellite measurements of aerosol optical and physical properties. Ground-based observations have been conducted at Mt. Abu (24.65°N, 72.78°E, 1.7 km asl), the highest location in Aravalli mountains in western India. The main advantage of the location is its proximity to both, Thar Desert and IGP. Also, due to the high altitude, the observation site is less affected by the boundary layer aerosols. The hill-top background aerosols are significantly influenced by wind that carries aerosols from either Thar Desert or IGP and show strong seasonal variation. Therefore, the site becomes a unique location for the investigation of both, natural and anthropogenic aerosols. The present study investigates the seasonal variation of aerosol properties at Mt. Abu and estimates the contribution of both aerosols on the radiation budget during the four seasons – winter (Dec-Feb), premonsoon (Mar-May), monsoon (Jun-Aug), and postmonsoon (Sep-Nov).
2.1. Ground-based instruments
Aerosol Optical Depth (AOD) was measured using a hand-held Microtops II (Solar Light Co., Inc., USA)  at every five minutes interval during daytime from 0730 to 1600 hours. This instrument can measure AOD at five different wavelengths centered at 0.380, 0.440, 0.500, 0.675, 0.870
Aerosol Mass Concentration was measured using a 10-stage Quartz Crystal Microbalance (QCM) cascade impactor (model PC-2, California Measurements Inc., USA) and the aerosol size distribution at the ground level was determined. Aerosols were collected in 10 stages of the impactor with cut-off radii at 12.5, 6.25, 3.2, 1.6, 0.8, 0.4, 0.2, 0.1, 0.05 and 0.025
Absorbing aerosol mass concentrations at seven different wavelengths (centered at 0.37, 0.47, 0.52, 0.59, 0.66, 0.88 and 0.95
2.2. Space-borne measurements
AOD over Mt. Abu is also obtained from observations of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on-board Terra and Aqua satellites. Terra and Aqua spacecrafts pass over the equator at 10:30 and 13:30 Local Solar Time, respectively . Global images of the full disc are produced due to larger swath widths and instrument-scanning angle of 110° . MODIS has 36 channels spanning the spectral range from 0.41 to 14.4
Aerosol index (AI) is obtained from observations in the UV region (UV-1, 0.270 to 0.314
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) provides a new insight into the role of atmospheric aerosols and clouds in regulating the study of Earth’s climate change and air quality. It is a part of the A-train satellite constellation that includes Aqua, CloudSat, and Aura satellites. CALIPSO is in a sun-synchronous orbit at 705 km at an inclination of 98°and provides the vertical distribution of aerosols and clouds. It consists of three sensors: a Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), an Imaging Infrared Radiometer (IIR), and a moderate spatial resolution Wide Field-of-view Camera (WFC). CALIPSO passes over the equator at 13:31 local hours, one minute behind Aqua. The primary instrument, CALIOP, transmits linearly polarized laser light of 0.532
OPAC (Optical Properties of Aerosols and Clouds) model  is used to derive aerosol optical depth from the measured atmospheric aerosol chemical compositions obtained from literature [39, 40] at Mt. Abu. OPAC model contains two major parts: (1) a dataset of microphysical properties and the resulting optical properties of cloud and aerosol components at different wavelengths and for different humidity conditions, (2) a FORTRAN program that allows the user to extract data from this dataset, to calculate additional optical properties, and to calculate optical properties of mixtures of the stored clouds and aerosol components. In the present study, OPAC model has been used for obtaining the aerosol optical properties in shortwave region (0.25-4
Atmospheric radiative transfer code, named Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART)  developed at the University of California, Santa Barbara, is used to estimate aerosol radiative forcing over the study area. SBDART is a well established code for estimation of radiation flux in the shortwave (0.25-4.0
The ground surface cover is an important determinant of the overall radiation environment because spectral albedo of the surface which defines the ratio of upwelling to downwelling spectral irradiance at the surface determines upwelling irradiance from the surface. In SBDART there are five basic surface types, namely (1) ocean water , (2) lake water , (3) vegetation , (4) snow , and (5) sand . The spectral albedo describing a given surface is often well approximated by combinations of these basic surface types. Input parameters in SBDART allow the user to specify a mixed surface consisting of weighted combinations of water, snow, vegetation and sand. SBDART can compute the radiative effects of several lower and upper atmosphere aerosol types. In the lower atmosphere, typical rural, urban, or maritime conditions can be simulated using the standard aerosol models of Shettle & Fenn . SBDART gives the opportunity to specify up to five aerosol layers (i.e., at five different altitudes), with radiative characteristics that model fresh or aged volcanic, meteoric, and upper-tropospheric background aerosols.
The major inputs required to estimate the aerosol radiative effects for DISORT module in SBDART include spectral values of solar radiation incident on the atmosphere, spectral values of columnar AOD, SSA and angular phase function of the scattered radiation or asymmetry factor. The asymmetry factor is used to generate a scattering phase function through the Henyey-Greenstein approximation. The Henyey-Greenstein parameterization provides good accuracy when applied to radiative flux calculations [22, 84]. It can also compute radiation fluxes with less uncertainty from the aerosol optical properties at 0.55 micron wavelength obtained from satellite observations. Spectral values of AOD, SSA and asymmetry parameter are also obtained from OPAC using the chemical properties of the atmospheric aerosols. OPAC model derived aerosol optical parameters are obtained by varying the number concentration of individual components in small steps until the model derived parameters satisfactorily match the observed values. Another important input parameter that is required for accurate computation of the aerosol radiative effects over land is the surface reflectance [71, 90]. Radiative forcing is determined from the difference of the solar radiation with and without aerosols during clear-sky conditions in the short wave (0.25-4.0
3. Site location and meteorology
Major aerosol parameters have been monitored during 2006 and 2007 inside the campus of Physical Research Laboratory situated at Gurushikhar, Mt. Abu – the highest peak (1.7 Km asl) of Aravalli range in India. Topography of the Indian Peninsula, Himalayas and the Tibetan plateau are shown in Figure 1a. Arid (dashed line) and semi-arid (solid line) regions of Thar Desert in western India are shown in Figure 1b. More details on physical features of Thar Desert are described in literature . Mt. Abu is situated within the semi-arid region of Thar Desert. A picture of the campus is shown in Figure 1c, which is better known for the astronomy observatory. Aravalli mountains are located in between Thar Desert and IGP. Major part of these mountains on the western side is in the semi-arid region of Thar Desert while the north-east region of the mountains is in IGP. The highest location, Mt. Abu is situated in the south-west of the mountain range. The observatory being a prohibited hilltop area makes the measurement site anthropogenic free and hence, is a suitable place for background aerosol measurements in western India. The observatory is built on rocky mountainous terrain surrounded by forest and therefore, there is significantly less soil dust coming from the surface of the nearby mountain region. Being very close (300 Km) to Thar Desert, measurement site gives an opportunity to study desert dust. Freshly generated desert dust aerosols are transported within few hours to Mt. Abu and thereby are exposed to local pollutants minimally. Also, due to the high altitude, these aerosols are less influenced by the boundary layer aerosols that consist mostly of locally produced anthropogenic aerosols.
Diurnal variations of surface temperature and relative humidity (RH) at Mt. Abu during different seasons are shown in the top row of Figure 2. The vertical bars represent the ±1
3.1. Land surface properties
Underlying surface plays an important role in the aerosol radiative effects towards climate change [24, 27, etc.]. Aerosols over high surface reflectance (bright surface) can produce relatively higher positive radiative forcing than those over low surface reflectance (dark surface). Space-borne observations suggest that there is a strong seasonal variation of surface over western India. Figures 3a and 3b show images of land surface over western India during premonsoon and postmonsoon seasons, respectively, captured by MODIS-Terra satellite. The surface is very bright during premonsoon due to open bare land, while it is relatively dark during postmonsoon due to green vegetation born during monsoon rain. As a result, surface reflectance is maximum during premonsoon and minimum during postmonsoon.
In the present study, MODIS derived surface reflectance data over Mt. Abu is used in the estimations of radiative forcing. It is obtained from Nadir BRDF-Adjusted Reflectance 16-Day L3 Global 0.5 km SIN Grid product which is derived at the mean solar zenith angle of Terra overpasses for every successive 16-day period, calculating surface reflectance as if every pixel in the grid was viewed from nadir direction. Surface reflectance data available in seven wavelength bands of MODIS centered around 0.47, 0.56, 0.65, 0.86, 1.24, 1.64 and 2.13
4. Background aerosol optical and physical properties over Thar desert
4.1. Aerosol optical depth
The seasonal variation of AOD spectrum at the hilltop station over western India is shown in Figure 4. Vertical lines represent ±1
The spectral dependence of AOD is parameterized through Ångström exponent (
4.2. Aerosol mass concentration
Aerosol mass concentration measured separately in ten different sizes by Quartz Crystal Microbalance (QCM) cascade impactor has been classified into three different categories, viz., nucleation (radius<0.1
Figure 5 shows the seasonal variation of aerosol mass concentration (
In general, nucleation aerosols contribute least to the total aerosol mass concentration. This contribution was maximum during postmonsoon when the wind speed was almost calm and RH was relatively high. This atmospheric condition helps in gas-to-particle conversion and enhances the nucleation mode aerosols which explains the maximum mass of 6.4±1.1
The seasonal variation observed in the accumulation aerosols is similar to the nucleation aerosols. The accumulation aerosol mass concentration was minimum at 8.4±2.8
The coarse mode aerosols show a slightly different seasonal behaviour at Mt. Abu. During premonsoon, they mainly consist of dust aerosols transported from Thar desert and the mass concentration is maximum at 8.6±0.4
4.3. Aerosol number concentration
Aerosol number concentration is also obtained from the observed aerosol mass concentration from QCM observations for the hilltop area using appropriate mass density valid for semi-arid background atmosphere and prevailing relative humidity conditions [13, 26]. Figure 6 shows the typical aerosol size distributions for the four seasons. The vertical bars represent ±1
Premonsoon Monsoon Postmonsoon
|12000 0.019 2.0|
10000 0.018 1.9
15000 0.018 1.9
17000 0.020 1.9
|18 0.14 2.0|
22 0.19 1.8
50 0.13 1.9
60 0.12 1.9
|0.02 1.4 1.9|
0.01 2.2 1.8
0.01 1.7 1.8
0.08 1.1 1.8
Accumulation aerosols are mainly produced by the condensation growth and coagulation of nucleation aerosols. During winter accumulation mode aerosols number concentration (
During premonsoon, there is large transportation of mineral dust aerosols from Thar Desert which enhanced the abundance of coarse mode aerosols at the hill top area. The coarse mode radius was maximum at 2.2
4.4. Black carbon mass concentration
Black carbon (BC) produced due to incomplete combustion of carbon-based fuels [3, 31, 53, 86, etc] is the most efficient light absorbing aerosol component in the atmosphere. BC has major contribution to alter the radiative balance by absorbing the solar radiation in the visible spectrum. As a result, it cools the surface and warms the atmosphere [24, 38]. Arecent study of BC contribution to radiative forcing by Jacobson  showed that BC has a great contribution towards global warming and is the second most important component of global warming after CO2 and has a larger impact on direct radiative forcing than that of methane. As a result, in populated countries like China and India, the large production of BC aerosols has a large impact on the hydrological cycle and precipitation pattern [46, 61, 71]. In India the fraction of BC production from fossil fuel burning, open burning and biofuel combustion to the global emission is significantly large and hence, it is necessary to estimate radiative impact of different kinds of BC not only on global scale but also in the regional scale.
In recent years, global climate has received considerable attention due to increase in the percentage contribution of anthropogenic aerosols on the Earth’s radiation budget . BC particles exist mainly in the accumulation mode and can be transported over long distances  from source regions to far off pristine environment and perturb the climate of the latter, like that of Mt. Abu. The diurnal variation of BC mass concentration during different months over Mt. Abu is shown in Figure 7. Observations were not possible in July and August due to heavy rain. Minimum BC concentration was observed during monsoon (0.428±0.128
The diurnal variation of BC mass concentration does not show any significant morning and nocturnal peaks like other urban regions. However, increased BC was observed during the noon hours except during November and December. The reason for such an increase is during the day time the thermal convection becomes stronger and as a result, the pollutants at the foothill area rise up to the hilltop region and enhance the BC concentration. This day time enhancement was prominent during winter and postmonsoon because during these seasons there is a large difference between the day and night time temperatures. During November and December the night time BC concentration was larger by a factor of two. During these months the nearby villagers burn wood and fallen leaves to keep themselves warm thereby increasing the BC mass concentration. During January this nocturnal enhancement was not observed. The reason is that the boundary layer height is less than the station altitude and the night time BC that is produced cannot reach the hill top region due to weak thermal convection. During this period hill top region becomes pollutant free region.
5. Satellite observed aerosol properties over Mt. Abu region
5.1. Aerosol optical and physical properties
In the current satellite era, large databases are available to study aerosol properties from space, both in the regional and the global scale, that are essentially demanding. For the present study, Terra and a series of satellite sensors flying on the A-train platform provide the required data. MODIS on board Terra and Aqua provide aerosol parameters in the morning and afternoon. OMI on board Aura satellite provides AI. The joint information of AOD, Ångström exponent (
5.2. Aerosol vertical profile
Seasonal variation of aerosol vertical profiles over the study region is obtained from CALIPSO observations. Figure 9 shows the seasonal variation of aerosol extinction coefficient (km-1). The horizontal dotted line at 1.7 km represents the height of Mt. Abu. The extinction coefficient is directly proportional to the total aerosol loading. It is clearly seen from the figure that aerosol loading over Mt. Abu is minimum during winter and higher during other seasons. There is a peak found near 2.2 km altitude during monsoon which becomes weak during postmonsoon. Ganguly et al.  reported that this peak is due to seasalt aerosols transported from Arabian sea and chemical analysis also supports this result showing significantly high amount of seasalt present over Mt. Abu during monsoon . During premonsoon, there is a peak at 4.2 km which is due to the transported dust layer. MODIS and OMI observations also indicate significant amount of dust present in the atmosphere.
Near surface region also shows high extinction coefficient values. This could be due to locally produced anthropogenic aerosols. In the present study, the properties of aerosols at the hill-top region are considered and defined as the ‘background aerosols’. The vertical profile of aerosols indicate that these background aerosols are less influenced from these locally produced anthropogenic aerosols. Therefore, the aerosol properties observed over Mt. Abu are assumed to represent those of the background aerosols over semi-arid region of western India.
6. Natural and anthropogenic background aerosol properties
6.1. Estimation of natural and anthropogenic aerosols
The estimation of natural and anthropogenic aerosols over this background site is a challenging task because many aerosol compositions have both origins. For example, sulphates are mainly considered as anthropogenic components over urban regions as they from marine sources as di-methyl sulphate. On the other hand, BC is mainly anthropogenic, but it becomes natural when produced during natural forest fires. In the present study, dust and seasalt are considered as natural aerosols and BC, sulphate and nitrates as anthropogenic aerosols. BC is obtained from ground-based measurements using Aethalometer. Other aerosols like dust, sulphate and nitrates are obtained from the chemical analysis of aerosols samples collected over this hill-top region [39, 40]. These chemical compositions are used as input to the OPAC model to obtained aerosol optical properties and compared with measured values. OPAC model is also used to distinguish the natural and anthropogenic aerosols by separating the natural and anthropogenic components. A scatter plot of monthly averaged AODs obtained from Microtops observations and OPAC model is shown in Figure 10. The solid line represents the 1:1 line. Model derived and observed AODs are linearly varying with a slope of 0.90 and very close to the 1:1 line which indicates that the model derived AOD are very close to the observed values. However, the model is underestimating the AOD by about 10%. This is due to the cut-off radius of aerosols at 7.5 micron considered by the model, but in reality, aerosols are larger, especially over semi-arid regions, though their residence period is only for a few hours and their contribution towards optical depth is small.
6.2. Source identification of natural and anthropogenic aerosols
Seven days air parcel back trajectories are considered to identify the possible source regions of the natural and anthropogenic aerosols at Mt. Abu. The back-trajectories during premonsoon and winter are shown in Figure 11(a) and (b), respectively. Air parcels are mainly coming from IGP during winter and the heights of the trajectories are within 2 km. Ground based observations show that BC values at Mt. Abu are higher during winter and it is also clearly seen that there is long-range transportation of anthropogenic aerosols like BC from IGP within the boundary layer height. On the other side, air parcels are direction during premonsoon season. The heights are also greater than 3 km. Earlier chemical analyses report that dust concentration during this season is maximum of about 80% (in mass) of the total aerosols . Therefore, one can easily conclude by these trajectories that the source of these dust aerosols is the nearby desert region. The back-trajectory analysis indicates that there is significant contribution of IGP during winter enhancing anthropogenic aerosols and that by nearby arid region during premonsoon increasing natural dust aerosols.
7. Natural vs anthropogenic background aerosol radiative forcing
7.1. Seasonal variation of aerosol radiative forcing
Aerosol radiative forcing is estmated using SBDART model considering aerosol optical properties obtained from OPAC, aerosol vertical profile from CALIPSO and MODIS surface reflectance. Aerosol radiative forcings in different seasons are given in table 2. Aerosol radiative forcing is found to vary from -3.2 to +0.2 Wm-2 at TOA and from 6.1 to 23.6 Wm-2 within the atmosphere. Aerosol radiative forcing at TOA is found to be maximum of about 0.2±2.5Wm-2 during premonsoon, followed by -1.3±0.5, -2.7±1.6, and -3.1±1.3Wm-2 during monsoon, winter and postmonsoon, respectively. Forcing within the atmosphere is maximum of about 23.6±5.5 Wm-2 during premonsoon, followed by 12.5±3.9, 7.4±1.8, and 6.1±1.8 Wm-2 during monsoon, postmonsoon and winter, respectively. Annual mean aerosol forcing at Mt. Abu is found to be 8.7±3.4 Wm-2 which is lower than other urban regions (mean forcing, 50 Wm-2) and hill-top regions (mean forcing, 31 Wm-2) in the Indian subcontinent . For example, aerosol forcing over other hill-top regions like Pune, Kathmandu, Dibrugarh are about 33, 25 and 35.7 Wm-2, respectively. These hilly areas are mainly influenced by anthropogenic aerosols. However, maximum forcing over Mt. Abu is found of about 23.6 Wm-2 during premonsoon which is lower but comparable with their forcing values. This is due to the maximum natural dust loading in the atmosphere at Mt. Abu.
Aerosol radiative forcing mainly depends on the amount of aerosol loading and underlying surface. Also, the sign of forcing at TOA is influenced by the aerosol type. An increase of absorbing aerosol loading causes positive forcing at TOA. In addition, bright surface which reflects more solar radiation back to the space can cause positive TOA forcing. Radiative forcing at TOA changes its sign from negative to positive during premonsoon. This could be due to the combined effects of the relatively brighter land surface over western India and high dust loading in the atmosphere by frequently occurring dust storms over Thar Desert. TOA forcing becomes minimum during postmonsoon as land surface becomes darker by the growing forest area over western India after monsoonal rain. Atmospheric forcing is proportionally varying with amount of aerosol loading. Maximum atmospheric forcing is found during premonsoon due to the maximum dust loading in this season while minimum forcing is observed during winter since the boundary layer height becomes lower than observational site which makes the site a free tropospheric station over western India with minimum aerosol loading in the atmosphere. During monsoon, heavy rains wash out the aerosols from the atmosphere, though atmospheric forcing is observed to be significantly high. This is due to the existence of aerosol layer, as found in the CALIPSO observations, that consist of large abundance of seasalt aerosols transported from Arabian Sea. This layer reflects the solar radiation significantly to the space which also causes relatively positive TOA forcing than that during winter and postmonsoon.
7.2. Contribution of natural and anthropogenic aerosols
Mt. Abu experiences large variation in aerosol properties and hence in the radiation forcing. During premonsoon there is large transportation of natural dust aerosols from surrounding arid region by the strong westerly wind and during monsoon large amount of seasalt is transported from the Arabian sea by the southwesterly wind. Figure 12 shows the seasonal variation of contributions of natural and anthropogenic forcings to the total aerosol radiative forcing within the atmosphere over Mt. Abu. The contributions of anthropogenic radiative forcing are 52%, 40%, 33%, and 56% and those of natural forcing are 48%, 60%, 67%, and 44% during winter, premonsoon, monsoon, and postmonsoon, respectively. Natural forcing is dominating at Mt. Abu during premonsoon and monsoon, whereas, the contributions of anthropogenic and natural forcing during winter and postmonsoon are almost equal. It is to be noted that natural and anthropogenic aerosol radiative forcings are calculated on the basis of their optical properties derived from OPAC model and OPAC model considers 7.5
Due to the proximity of Mt. Abu to the Thar desert dust aerosols are transported to this hill-top region during premonsoon and hence natural forcing is higher. During monsoon also, natural forcing is higher due to the large amount of seasalt coming from over the Arabian sea and simultaneously, dust and boundary layer anthropogenic aerosols are washed out by the heavy rains. Chemical analysis also shows that during monsoon, anthropogenic compositions like non-seasalt potassium, ammonium and nitrate are relatively less and the natural compositions like seasalt are enhanced over Mt. Abu [40, 63]. During postmonsoon, there is less transportation of seasalt aerosols to the measurement site due to low wind speed and hence the natural forcing reduces and anthropogenic forcing increases. During winter, total aerosol loading is minimum as the measurement site becomes a free tropospheric station and thereby, both natural and anthropogenic forcings contribute equally.
Annual mean contributions of natural and anthropogenic forcing are about 55% and 45%, respectively. This indicates that anthropogenic aerosols are also significantly contributing to total radiative forcing within the atmosphere. This could be due to the close proximity of IGP which is a potential source of anthropogenic aerosols over semi-arid region. Therefore, it is concluded that western India is influenced by natural as well as anthropogenic aerosols significantly.
Western India is known for the presence of Thar Desert, which is a potential source of dust aerosols in the Indian subcontinent. Therefore, it is commonly believed that the atmosphere over western India is largely influenced by natural dust aerosols. With this motivation, the present study investigates the natural and anthropogenic contribution to the background aerosols and their radiative effects over western India. The optical and physical properties of aerosols over Mt. Abu, highest peak of the Aravalli mountains in western India are obtained from a variety of ground-based and satellite-borne instruments. Mt. Abu is situated in the semi-arid region of Thar Desert and is less influenced by the local anthropogenic aerosols. It is therefore, a unique site for the observation of background aerosols over semi-arid region. Also, the Aravalli mountains are located in between Thar Desert and IGP which has large abundance of anthropogenic aerosols. Therefore, there is a significant variation of aerosol properties over Mt. Abu during different seasons, namely, winter (DJF), premonsoon (MAM), monsoon (JJA) and postmonsoon (SON). Ground-based observations show that AOD is maximum during premonsoon due to the large dust loading in the atmosphere by frequently occurring dust storms over Thar desert and minimum during winter due to low boundary layer height. Space-borne observations suggest that natural dust aerosols are dominating during premonsoon while anthropogenic aerosols are dominating during winter over western India. An interesting observation of CALIPSO is a layer of transported seasalt aerosols during monsoon over western India coming from Arabian Sea. These aerosols increase the contribution of natural forcing to the total atmospheric radiative forcing. Atmospheric radiative forcing is found to be maximum of about 23.6±5.5 Wm-2 during premonsoon and minimum of about 6.1±1.8 Wm-2 during winter. Another interesting result is TOA forcing is positive due to the bright land surface over western India during premonsoon, while it is negative during other seasons. The contribution of natural aerosols is found to be higher during premonsoon and monsoon and that of anthropogenic aerosols is higher during postmonsoon. During winter, they contribute equally. The annual average of natural and anthropogenic contribution is about 55% and 45%, respectively, indicating that the anthropogenic effects are also very significant. Thus the background aerosols over western India are not only influenced by desert dust aerosols but also by seasalt coming from Arabian Sea and anthropogenic aerosols transported from IGP.
Author would like to thank his Ph. D. thesis supervisor, Prof. A. Jayaraman, for his guidance, constant help, inspiration and support during this research work initiated at PRL. Author would also like to acknowledge NOAA National Center for Environmental Prediction and Air Resources Laboratory for providing reanalysis data and online HYSPLIT model output for back-trajectories analysis. Author acknowledges Terra, Aqua, Aura and CALIPSO mission scientists and associated NASA personnel for the production of the data used in this research effort. Special thanks to Prof. J. P. Chen and Dr. U. Das for the valuable scientific discussions. This work is partially supported by NSC grant 100-2119-M-002 -023 -MY5, Taiwan.
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