Trends in Aerosol Optical Depth

Of late, long-term changes (trends) in aerosols have gained increased interest due to their importance to regional and global climate. There have been several studies over the Indian region to address trends in aerosols using ground-based measurements using short-term, single-station data (Parameswaran et al. 1998; Moorthy et al. 1999; Satheesh et al. 2002; Kaskoutis et al. 2012) as well as long-term multi-station databases (Moorthy et al. 2013; Babu et al. 2013). Some investigators have also used satellite-derived products to study trends in aerosols (Ramachandran et al. 2012; Dey and Girolamo 2011). Aerosol optical depth (AOD) is a common data available on aerosols from several satellites regularly, and there are limitations arising from the large uncertainties involved in the satellite-retrieved AODs, especially over the landmass mainly due to the diverse surface reflectance and cloud contamination (Jethva et al. 2007). Studies by Zhang et al. (2005), Remer et al. (2005), Kahn et al. (2007), Levy et al. (2010), and several others have suggested that satellite data may have problems in long-term trend analysis due to the fact that noises and biases in the products can often be (mistakenly) interpreted as legitimate. Such studies are also beleaguered by the calibration drifts, which can often be (mistakenly) interpreted as trends in aerosols. Therefore, data from ground-based Sun photometers (preferably from a network), which can provide the most accurate AOD data, are the best choice for trend analysis. As on now, ARFINET is the only ground-based network with such long-term data.

The detailed regional synthesis of long-term data on AOD from the ARFINET has shown an increasing trend with a seasonal variability (Moorthy et al. 2013; Babu et al.

2013). The results are shown in Figs. 2, 3, and 4. Comparison of the turbidity coefficients in this study with those reported around late 1960s and early 1970s (Mani 1968) using Volz Sun photometer data indicates the extraordinary nature of the increase in aerosols during the last decades. The rate of increase is always high during the dry season (December-March) over the entire region, whereas the trends are rather inconsistent and weak during the pre-monsoon (April-May) and summer monsoon period (June-September) (Fig. 3). The variations in the spectral variation in AOD reveal the important contribution of anthropogenic sources on the increasing trend in AOD (Fig. 4, which reveals an increasing trend in the submicron aerosol abundance). The insignificant trend in AOD observed over the Indo-Gangetic Plain during the summer and pre-monsoon months is mainly ascribed to the competing effects of transport of dust and wash out of aerosols by the monsoon rainfall. Nonetheless, if this trend continues so, AOD at several locations would nearly double and approach unity in the next few decades. This can lead to an enhancement in aerosol-induced lower atmospheric warming by a factor of two. These observations indicate that trends in the aerosol forcing elements and their regional and global climate implications need to be better evaluated using global climate models.

Though not extensive, there are other studies on aerosol trends based on data from one or a few stations. Parameswaran et al. (1998) have reported an increasing trend in the AOD from 1989 to 1994. Satheesh et al. (2002) reported increase in AOD using two decades of ground-based Sun photometer measurements. Aerosols have an important role in determining solar radiation reaching the Earth’s surface. Padmakumari et al. (2007) have made an assessment of monthly mean surface-reaching solar radiation under all-sky conditions for several stations, distributed across the Indian region, for the period 1981-2004 (see Chap. 9). They have shown that all the stations showed a decline in S ranging from -0.17 to -1.44 W m² per year. The consequent annually averaged solar dimming over India for the period 1981-2004 is estimated to be -0.

Fig. 2 a Long-term trends in aerosol optical depth at 500 nm, derived from measurements at ARFINET stations where different colors and symbols differentiate the stations. b Long-term trend in regional mean aerosol optical depth at 500 nm (color figure online)

Fig. 3 Seasonal changes in the spatial variation in AOD trend over Indian region

86 W m ², while the seasonal mean values for winter, pre-monsoon, and monsoon seasons are pegged at -0.94, -1.04, and -0.74 W m² per year, respectively. Dey and Girolamo (2011) have used ten years (2000-2010) of observations from MISR to quantify seasonal linear trends of AOD and showed that many regions have statistically significant increasing trend in the range 0.1-0.4 in the last decade. Kaskoutis et al. (2012) discussed aerosol loading variability and trends at Kanpur using AERONET

Fig. 4 Long-term trend in the regional mean values of Angstrom wavelength exponent, an indicator of anthropogenic impact of aerosol column loading

data. Their studies show an increase in AOD and attributed to an increase in the seasonal/monthly averaged AOD during the winter and post-monsoon seasons. Aerosol sources during these seasons are dominated by anthropogenic emissions. In contrast, a weak decreasing trend is observed during pre-monsoon and monsoon seasons. Soni et al. (2012) have studied the variability in annually averaged irradiance (global and diffuse) and bright sunshine duration over 12 stations of solar radiation network of IMD for the period 1971-2005 (see Chap. 9). They have reported that the annually averaged all-sky global irradiance decreased between

0 W m^-2) per decade at these stations. Examining the seasonal and annual mean trends in AODs for the last decade using MODIS data over different locations in India, Ramachandran et al. 2012 have reported an increasing trend. Dani et al. (2012) have reported an increasing trend of 45 % per decade over Pune, based on Sun photometer-derived multispectral AOD measurements. Pathak et al. (2012) examined the variability in aerosol loading over the central region of the Indo-Gangetic Plains (IGP), during a decade (2001-10). An overall increase in AOD (ground-based radiometric measurements) was observed on yearly basis. Lu et al. (2013) presented the first inter-annual comparison of SO₂ emissions and those retrieved from satellite data [ozone monitoring instrument (OMI)] for Indian coal-fired power plants during 2005-2012. The results show that SO₂ emissions increased by 71 % during the study period.

Horizontal surface visibility is one of the simplest measures of local atmospheric pollution. Jaswal et al. (2013) have studied trends of morning poor visibility days (PVD, visibility <4 km) and afternoon good visibility days (GVD, visibility >10 km) based on 279 surface meteorological stations, well distributed over India, for the period 1961-2008. Their studies show that during the last 5 decades, all India averaged range of annual morning PVD has increased from 6.7 to 27.3 % days, while the range of afternoon GVD has decreased from 76.1 to 30.6 % days. Sreekanth (2013) has used the MODIS data to study trends in AOD over Bangalore.

They have shown an overall increasing trend, due to sustained increase in the seasonal averaged AOD during summer. Ramachandran and Kedia (2013) have used satellite-derived AOD to study inter-annual and regional variations in aerosols over six homogeneous rainfall zones in India for the month of July from 2000 to 2010 and reported that AOD over India in a drought year (2002) was higher when compared to normal monsoon years. Investigations using MODIS data for the period 2003-2012 over Delhi region show an increase in AODs by more than 25 % (Kumar 2014). Yearly mean Terra/Aqua AOD values have shown an increasing trend at a rate of 0.005/0.009 per year, respectively, with higher rates during winter (0.012/0.007 per year, respectively). An increasing trend in AOD is also reported by Soni et al. (2014) over Raniganj (7.31 %) in eastern and Korba (5.0 %) in southeast, and Godavari Valley (32 %) in the south coalfield region in India using MODIS data from 2000 to 2012 during winter and post-monsoon periods. Thus, overall, AOD over India shows an increasing trend, which is steady and stronger during dry season, and weak and inconsistent during the monsoon season.