Role of Coarse and Fine Mode Aerosols in MODIS AOD Retrieval: a case study over southern India

Abstract

Abstract. In the present study we compare the MODIS (Moderate Resolution Imaging Spectroradiometer) derived aerosol optical depth (AOD) data with that obtained from operating sky-radiometer at a remote rural location in southern India (Gadanki, 13.45 • N, 79.18 • E) from April 2008 to March 2011. While the comparison between total (coarse mode + fine mode) AODs shows correlation coefficient (R) value of about 0.71 for Terra and 0.77 for Aqua, if one separates the AOD into fine and coarse mode, the comparison becomes very poor, particularly for fine mode with an R value of 0.44 for both Terra and Aqua. The coarse mode AOD derived from MODIS and sky-radiometer compare better with an R value of 0.74 for Terra and 0.66 for Aqua. The seasonal variation is also well captured by both ground-based and satellite measurements. It is shown that both the total AOD and fine mode AOD are significantly underestimated with slope of regression line 0.75 and 0.35 respectively, whereas the coarse mode AOD is overestimated with a slope value of 1.28 for Terra. Similar results are found for Aqua where the slope of the regression line for total AOD and fine mode AOD are 0.72 and 0.27 whereas 0.95 for coarse mode. The fine mode fraction derived from MODIS data is less than one-half of that derived from the sky-radiometer data. Based on these observations and comparison of single scattering albedo observed using sky-radiometer with that of MODIS aerosol models, we argue that the selection of aerosol types used in the MODIS retrieval algorithm may not be appropriate particularly in the case of southern India. Instead of selecting a moderately absorbing aerosol model (as being done currently in the MODIS retrieval) a more absorbing aerosol model could be a better fit for the fine mode aerosols, while reverse is true for the coarse mode aerosols, where instead of using "dust aerosols" which is relatively absorbing type, usage of coarse sea-salt particles which is less absorbing is more appropriate. However, not all the differences could be accounted based on aerosol model, other factors like errors in retrieval of surface reflectance may also be significant in causing underestimation of AOD by MODIS.

Aerosol chemical characterization and role of carbonaceous aerosol on radiative effect over Varanasi in central Indo-Gangetic Plain

“…The AOD retrievals on the days with PM 10 samples were used for DARE estimates. The overall error in AOD measurement is estimated to bẽ 5.5% at 500 nm and~8.4% at 1020 nm (Suman et al, 2014). The vertical profiles of aerosol extinction coefficients from the Cloud-Aerosol Lidar and Infrared Satellite Observation (CALIPSO; Winker et al, 2009) are also used during the period April to July 2011 over Varanasi to examine the vertical distribution of aerosols and its impact on DARE and atmospheric heating rates (Srivastava and Ramachandran, 2013).…”

Phase-wise analysis of the COVID-19 lockdown impact on aerosol, radiation and trace gases and associated chemistry in a tropical rural environment

“…This underestimation has been attributed to the usage of less absorbing type aerosol model in the MODIS AOD retrieval ( Kiran Kumar et al, 2013 ). Further, Sai Suman et al (2014) indicated that a more absorbing aerosol model could replicate the fine mode aerosols while less absorbing model with coarse sea-salt particles as appropriate for coarse mode aerosols towards reducing the uncertainty in MODIS retrievals over Southern India.

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Multi-Year Analysis of Aerosol Properties Retrieved from the Ångström Parameters for Different Spectral Ranges over Pune

“…AOD 400 nm also has seasonal variation but not as systematic as has been observed for AOD 1020 nm . The difference between the AOD 400 nm and AOD 1020 nm become low during June, July and August and high during November, December, March and April suggesting changes in columnar aerosol size distribution (Suman et al, 2014).…”