Fig. 4. Monthly contribution of anthropogenic aerosols to the total ADRF at surface over Kanpur (in percentage) for 5 years. The annual mean (±SD) value is written over each box.

surface ADRF are 64.1%, 65.3%, 64.6%, 63.7%, and 60.2% for 2001, 2002, 2003, 2004, and 2005, respectively. The large SD indicates the high variability within a year. In 2005, the proportion of anthropogenic aerosols to total aerosol surface forcing is lowest on annual scale. Further analysis based on seasons reveals that the proportion in the pre-monsoon season is lowest (37%) in 2004. In the years 2003 and 2005, the proportions are very low (<42%) in the monsoon season also. In the winter season, the anthropogenic contribution is uniform in all 5 years (88-89%), suggesting insignificant inter-annual variation.

The monthly mean spatial distributions of anthropogenic surface and atmospheric forcing over the IGB are illustrated in Fig. 5(a). The mean annual clear-sky TOA, surface, and atmospheric anthropogenic forcing in IGB are +0 ± 6.8, -12.6 ± 6.7, and +12.6 ± 6.9Wm~2, respectively. The major difference in the spatial distributions of the anthropogenic surface forcing with total surface forcing is the persistent high values of anthropogenic forcing in the eastern IGB throughout the year except July-August, when monsoonal rain removes major fraction of aerosols from the atmosphere. However, the anthropogenic surface forcing over the urban centers like Kanpur and Delhi are still high (>-15Wm-2), which shows that even in the monsoon season, the washout of aerosols by monsoonal rain is not so effective as compared to the eastern IGB. Immediately after the monsoon season, anthropogenic surface forcing starts building

Fig. 5a. Monthly mean estimates of clear-sky anthropogenic aerosol (A) surface (top 12 panels) and (B) atmospheric (bottom 12 panels) forcing over IGB. Note that the color scales for the two sets of figures given are different.

60 e

^ Western ^ Central ^ Eastern

^ Western ^ Central ^ Eastern

75 80 85 Longitude (E)

Fig. 5b. Longitudinal variation of anthropogenic surface forcing in the IGB during the winter (WIN), pre-monsoon (PRM), monsoon (MON), and post-monsoon (POM) seasons. The vertical bars through each point represent the standard deviation for 2001-2005.

up along the major urban locations and continued until January. The anthropogenic atmospheric forcing remains high (>+12Wm~2) over the major urban locations throughout the year. However, during the winter due to the low-level inversions, a pool of high anthropogenic atmospheric forcing (>+24Wm~2) exists over the entire IGB. The highest anthropogenic contribution to the observed aerosol forcing in the IGB comes from the seven mega cities (Fig. 1). The longitudinal variation of anthropogenic surface forcing (Fig. 5(b)) shows reverse trend as that of surface ADRF. The anthropogenic surface forcing is less than -15Wm-2 for most of the places in the western and central IGB, but increases sharply toward the east of that for the winter and pre-monsoon seasons. Anthropogenic surface forcing is least during the pre-monsoon season in the western IGB, east of which, the forcing values for monsoon and post-monsoon seasons catch up with each other and the pre-monsoon forcing dips below -15Wm-2 to catch up with the wintertime forcing values. The strong spatial heterogeneity in anthropogenic contribution to ADRF in the IGB is due to complex nature of aerosol loading in the region, resulting from the mixing of anthropogenic and natural aerosols. Past studies (Ref. 5 and the references therein) have shown that the natural dusts are transported every year to the IGB, but their effect on the ADRF is dominant mostly in the western and central parts of the basin. The eastern part remains relatively unaffected from the dust storms.

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