Estimation of the anthropogenic fraction in ADRF

The mean annual SSA (at 0.5 ^m) for anthropogenic aerosols is 0.89 with the lowest value (0.86) in December. During this time (i.e. winter season), the anthropogenic aerosols contribute more than 80% to the AOD and more than 90% to the total aerosol mass. The relative contributions of the anthropogenic aerosols to the composite aerosol properties decrease from March onwards due to commencement of dusts in the basin. The optical properties of the anthropogenic aerosols and composite aerosols derived from the optical model are used as input to the SBDART22 to estimate the ADRF and the anthropogenic ADRF over Kanpur region first.23 Then from the estimates of composite (anthropogenic + natural) aerosol forcing (AF) and anthropogenic ADRF (AFX), we have followed the relation3:

where AEF is anthropogenic efficiency factor and AF is the anthropogenic fraction. AEF is defined as the ratio of anthropogenic aerosol forcing efficiency to composite aerosol forcing efficiency, forcing efficiency being known as the DRF per unit optical depth. Both the surface and atmospheric ADRF are used to obtain two sets of AEF values for each month.23

Next, the aerosol forcing efficiency calculated from the estimations is assumed to be the representative of IGB,12 and by multiplying the forcing efficiency with AOD from Moderate Resolution Imaging Spectroradiometer (MODIS) in 1° x 1° grid, ADRF over the IGB in spatial scale is obtained. Over the IGB, the nature of aerosol loading is represented as monthly frequency distribution in Fig. 2(a) with the monthly mean (± standard deviation, SD) AOD0.55 value written in corresponding histogram. The distribution is unimodal in November to May and September, bi-modal in June, August and October, and tri-modal in July. The spread of the unimodal distribution in the winter season is narrower than the other

1 0.1 OA 0.7 1 0.1 0.4 0.7 1 0.1 0.4 0.7 1 0.1 0.4 0.7 1 0.1

1 0.1 OA 0.7 1 0.1 0.4 0.7 1 0.1 0.4 0.7 1 0.1 0.4 0.7 1 0.1

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HISSAR

NAINITAL

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HISSAR

NAINITAL

Fig. 2. (a) Frequency distribution of AOD0.55 as derived from MODIS over IGB. Monthly mean (± standard deviation) AOD is shown at the top of each panel. (b) Comparisons of ADRF over few locations in IGB estimated using our model with the independent estimates.

months because during that period, aerosols are mostly anthropogenic and the nature of loading is uniform throughout the basin. In the summer months, dust activities start increasing and add to the anthropogenic burden, resulting in widening of the histograms and shifting of the peaks to the larger values. The spatial distribution of monsoon rainfall in the IGB is not uniform; at some parts it washes out the aerosols from the atmosphere through wet deposition, while in other parts aerosol loading remains high. This results in wide bi- and tri-modal distributions. In the post-monsoon season, the AOD distribution tends to readjust to unimodality and the frequencies of large AOD start decreasing. The ADRF estimated in the clear-sky condition using our model are compared with the independent estimates over some locations in the IGB to test the validity of our model (Fig. 2(b)), which shows that the expansion of our model is able to produce the ADRF within the current uncertainty levels. The cloudy-sky ADRF is calculated incorporating the cloud parameters from MODIS (for details see Ref. 23).

The critical part of Eq. (1) is AF, as no such measurements exist in IGB. We employ MODIS-aerosol fine mode fraction (AFMF) product to infer the AF. Earlier, researchers have utilized MODIS-AFMF data to assess the anthropogenic contribution over the oceanic region,3 as in general, anthropogenic aerosols dominate in the fine mode fraction. The model-derived AF was compared with the MODIS-AFMF product over Kanpur for the entire 5 year period and a statistically significant relationship (correlation of 0.96) was found at 95% confidence level:

The equation suggests that MODIS has a bias in determining the AF for this region, and thus using the above relation, AF was estimated for each 1° x 1° grid of IGB. Subsequently, the anthropogenic ADRF at the surface and the atmosphere were estimated for each grid using Eq. (1). The anthropogenic ADRF at the TOA was calculated from the anthropogenic surface and atmospheric ADRF values.

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