**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−<sup>2</sup> at TOA and from 6.1 to 23.6 Wm−<sup>2</sup> within the atmosphere. Aerosol radiative forcing at TOA is found to be maximum of about 0.2±2.5 Wm−<sup>2</sup> during premonsoon, followed by -1.3±0.5, -2.7±1.6, and -3.1±1.3 Wm−<sup>2</sup> during monsoon, winter and postmonsoon, respectively. Forcing within the atmosphere is maximum of about 23.6±5.5 Wm−<sup>2</sup> during premonsoon, followed by 12.5±3.9, 7.4±1.8, and 6.1±1.8 Wm−<sup>2</sup> during monsoon, postmonsoon and winter, respectively. Annual mean aerosol forcing at Mt. Abu is found to be 8.7±3.4 Wm−<sup>2</sup> which is lower than other urban

**Figure 10.** Scatter plot of monthly averaged AOD obtained from Microtops observations and OPAC model simulations. The solid line is the 1:1 line. The dashed line is the best-fitted line with a slope of 0.90, indicating that the OPAC underestimates AOD by 10%.

regions (mean forcing, 50 Wm−2) and hill-top regions (mean forcing, 31 Wm−2) in the Indian subcontinent [14]. 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−<sup>2</sup> 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.

**Figure 11.** Seven days back trajectory analysis of aerosol parcels coming to Mt. Abu (location indicated by star) during (a) premonsoon and (b) winter. The color bar represents the height of the air parcels during their travel from source regions to the measurement site. Aerosols are mainly coming from desert region at higher altitude during premonsoon and from IGP within the boundary layer height (2 km)

Natural vs Anthropogenic Background Aerosol Contribution to the Radiation Budget over Indian Thar Desert 103

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

during winter.

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 22 Will-be-set-by-IN-TECH

**Figure 10.** Scatter plot of monthly averaged AOD obtained from Microtops observations and OPAC model simulations. The solid line is the 1:1 line. The dashed line is the best-fitted line with a slope of

regions (mean forcing, 50 Wm−2) and hill-top regions (mean forcing, 31 Wm−2) in the Indian subcontinent [14]. 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−<sup>2</sup> 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.

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

0.90, indicating that the OPAC underestimates AOD by 10%.

Abu.

**Figure 11.** Seven days back trajectory analysis of aerosol parcels coming to Mt. Abu (location indicated by star) during (a) premonsoon and (b) winter. The color bar represents the height of the air parcels during their travel from source regions to the measurement site. Aerosols are mainly coming from desert region at higher altitude during premonsoon and from IGP within the boundary layer height (2 km) during winter.

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


**Table 2.** Seasonal variation of aerosol radiative forcing over Mt. Abu

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 *μ*m as the upper limit of aerosol radius. In the present study dust is considered as natural aerosols, which in reality can be larger than this cut off limit over arid region, especially during premonsoon. Therefore, the contribution of natural forcing could be underestimated due to these large dust aerosols even though their AOD is very low. The comparison between OPAC and Microtops AOD indicates that this underestimation is not more that 10% (Figure 10).

**Figure 12.** Seasonal variation of the contribution of natural and anthropogenic forcing to the total

and thereby, both natural and anthropogenic forcings contribute equally.

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

Natural vs Anthropogenic Background Aerosol Contribution to the Radiation Budget over Indian Thar Desert 105

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

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

atmospheric radiative forcing over Mt. Abu.

significantly.

**8. Conclusions**

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

**Figure 12.** Seasonal variation of the contribution of natural and anthropogenic forcing to the total atmospheric radiative forcing over Mt. Abu.

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.

## **8. Conclusions**

24 Will-be-set-by-IN-TECH

Season Aerosol Radiative Forcing

Winter -2.7 -8.8 6.1 Premonsoon 0.2 -23.4 23.6 Monsoon -1.3 -13.8 12.5 Postmonsoon -3.2 -10.6 7.4

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

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 *μ*m as the upper limit of aerosol radius. In the present study dust is considered as natural aerosols, which in reality can be larger than this cut off limit over arid region, especially during premonsoon. Therefore, the contribution of natural forcing could be underestimated due to these large dust aerosols even though their AOD is very low. The comparison between OPAC and Microtops AOD indicates that this underestimation is not more that 10% (Figure 10).

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

**Table 2.** Seasonal variation of aerosol radiative forcing over Mt. Abu

forcing than that during winter and postmonsoon.

**7.2. Contribution of natural and anthropogenic aerosols**

TOA Surface Atmosphere

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

#### 26 Will-be-set-by-IN-TECH 106 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics Natural vs Anthropogenic Background Aerosol Contribution to the Radiation Budget over Indian Thar Desert <sup>27</sup>

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−<sup>2</sup> during premonsoon and minimum of about 6.1±1.8 Wm−<sup>2</sup> 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.

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## **Acknowledgment**

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.
