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Current knowledge in the aerosol trends over northern India

Heterogeneity in aerosol characteristics over the Indo-Gangetic Basin

Satellite-based estimation of PM2.5 over India

Absorption enhancement by black carbon (BC) cored polydisperse aerosols under hygroscopic conditions

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IASTA e-Bulltein |  Vol. 1 No.2 Home
Fig. 1: Inter-annual variability and trend of the AOD500 daily values over Kanpur for 2001-2010. The slope of the regression analysis along with the % difference and the P value are given. Black for AOD500, green for a(440-870), blue for a(380-500) and red for a(675-870). [Source: Kaskaoutis et al., ERL, 2012]

1.2 Evidence of declining aerosol trend over IGP during pre-monsoon/monsoon seasons
Except the general increasing aerosol trend over India, MODIS (Kaskaoutis et al., 2011) and MISR (Dey and Di Girolamo, 2011) observations firstly revealed a declining AOD trend over IGP during late pre-monsoon and monsoon months (May to September) during the 2000s. However, in the majority of the cases, this declining trend is not statistically significant due to large intra-annual fluctuations in AOD over IGP. However, it’s presence attracted the scientific interest to be examined further. Analysis of the AOD trends using Kanpur AERONET data (2001-2010) verified this feature, revealing an AOD decrease of -0.01 per year in June, when the dust activity is at its maximum (Kaskaoutis et al., 2012a). The analysis of the size distribution over Kanpur showed a general decrease in coarse-mode fraction associated with variable dust activity during the 2000s, verified the results of attenuation in dust activity during the second half (2005-2010) of the previous decade. Thus, the AOD decrease seems to be attributed to weakness of dust activity in the northern part of India during the last decade. GOCART model simulations over south Asia revealed a pronounced decreasing trend in dust AOD and in dust contribution to the total AOD over south Asia during 2000-2007, which are in general agreement with the ground-based and satellite observations. However, much more analysis and longer dataset are required for establishing this evidence.
1.3 Reasons for the AOD declining trend
Since the main aerosol type over northern India in late pre-monsoon and monsoon seasons is dust transported mainly from the Thar desert, the fluctuations in dust emission, transport and dust-aerosol lifetime are of considerable importance in order to explain the AOD variability over this region. Kanpur (Fig. 1) as well as Delhi aerosol dataseries revealed considerable high AODs during monsoon of 2002 and late pre-monsoon of 2003, so the interest was focused on examining these periods. A synergy of satellite and ground-based radiometric observations, along with chemical transport modeling and NCEP/NCAR reanalysis data, was used for the assessment of the influence of the weather conditions of summer/monsoon 2002 and 2003 on aerosol properties over northern India (Kaskaoutis et al., 2012b). The meteorology data showed prevalence of westerlies under anti-cyclonic circulation and subsidence favoring the accumulation of aerosols. Subsequently, the anomalous and prolonged dry conditions favored heavy aerosol buildup as indicated by strong positive anomalies (20-80%) of MODIS AOD (Fig. 2). Ground-based sun photometer observations at Delhi and Kanpur also revealed enhanced
presence of desert-dust aerosols during July 2002 and May-June 2003, characterized by large AOD and significantly low Angstrom exponent values. The analysis suggested a cause-and-effect association between the deficit of monsoon rainfall, and increased dust activity as well as prolonged aerosol lifetime that influences the dynamics and persistence of the spatio-temporal aerosol loading, and associated optical properties over northern India. The role of rainfall in aerosol properties and variations is very crucial during the monsoon season. On the other hand, the increase in aerosol loading over northern India may also affect precipitation and hydrological cycle (Lau et al., 2006). However, it is difficult to quantify the influence of rainfall in AOD trends over Kanpur, since anthropogenic emissions and dust transport play a significant role in influencing aerosol loading and properties. In synopsis, the deficit of rainfall during monsoon of 2002 and late pre-monsoon of 2003 caused an increase in dust activity and atmospheric aerosol lifetime over northern India strongly influencing the AOD trends over IGP during the last decade.
Figure 2: (a) Monthly normalized rainfall anomaly (%) for July 2002 and for May-June 2003 based on the monthly rainfall climatology of TRMM 3B43 V6 during the period 1998-2009, (b) percentage deviation of the Terra-MODIS AOD550 for July 2002 and for May-June 2003 from the monthly mean value during the period 2000-2009.
[Source: Kaskaoutis et al. JGR, 2012]
© 2013 Indian Aerosol Science and Technology Association