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This paper presents characterization of aerosols over the Indochina peninsular regions of Southeast Asia during pre-monsoon season from satellite and ground-based radiometric observations. Our analysis focuses on the seasonal peak period in aerosol loading and biomass burning, prior to the onset of the Asian summer monsoon, as observed in the inter-annual variations of Aerosol Optical Depth (AOD) and fire count data from MODIS. Multi-year (2007-2011) analysis of spaceborne lidar measurements, from CALIOP, indicates presence of aerosols mostly within boundary layer, however extending to elevated altitudes to approx. 4 km over northern regions of Indochina, encompassing Myanmar, northern Thailand and southern China. In addition, a strong gradient in aerosol loading and vertical distribution is observed from the relatively clean equatorial conditions to heavy smoke-laden northern regions (greater aerosol extinction and smaller depolarization ratio). Based on column-integrated ground-based measurements from four AERONET locations distributed over Thailand, the regional aerosol loading is found to be significantly absorbing with spectral single scattering albedo (SSA) below 0.91+/-0.02 in the 440-1020 nm range, with lowest seasonal mean SSA (most absorbing aerosol) over the northern location of Chiang Mai (SSA approx. 0.85) during pre-monsoon season. The smoke-laden aerosol loading is found to exhibit a significant diurnal pattern with higher AOD departures during early morning observations relative to late afternoon conditions (peak difference of more than 15% amplitude). Finally, satellite-based aerosol radiative impact is assessed using CERES shortwave Top-of-Atmosphere flux, in conjunction with MODIS AOD. Overall, a consistency in the aerosol-induced solar absorption characteristic is found among selected regions from ground-based sunphotometer-derived spectral SSA retrievals and satellite-based radiative forcing analysis.

This paper presents column integrated aerosol optical properties including aerosol optical depth (AOD), Angstrom wavelength exponent (α), single scattering albedo (SSA), and size distribution from ground-based measurements at four sites in Thailand: Chiang Mai (18.。N, 98.。 E), Ubon Ratchathani (15.25。N, 104.87。 E), Nakhon Pathom (13.82。N, 100.04。E), and Songkhla (7.2。N, 100.60。E). Results show a marked seasonal trend in AOD at 500 nm for the first three stations, with the monthly average maxima of 0.92, 0.78 and 0.61 for Chiang Mai, Ubon Ratchathani and Nakhon Pathom, respectively. These maxima occur in the dry season (November-April). Minimum values for these stations were recorded during the wet season (May-October). A similar pattern is exhibited in the α for the three stations, with maxima in the dry season and minima in the wet season. The lowest SSA values occur at Chiang Mai, which means this station has the highest absorption, with the highest SSA values occurring at Songkhla which corresponds to the lowest absorption. The southern station at Songkhla differs from the other three as it has less local pollution sources and is subjected to the influence of the tropical maritime environment. AOD at Songkhla maintains a low and more constant value year round with the maximum monthly average AOD of 0.27 and the minimum of 0.16. Diurnal changes in AOD at the four stations are discussed and related to various external variables.

Aerosols are small particles suspended in the atmosphere. They may have profound effects on human health. Their effects on the physical environment can also be of importance as they have the ability to both scatter and absorb incident solar radiation. The ability of aerosols to deplete solar radiation can be quantified in terms of aerosol optical depth (AOD). AOD can be retrieved from the MODIS satellite. However, AOD from this satellite has a wide range of uncertainty, depending on environments and climatic zones. In this work, AOD retrieved from MODIS was compared to that obtained from ground-based measurements at four sites in the tropical environment of Thailand. These are Chiang Mai (18.78 °N, 98.98 °E), Ubon Ratchathani (15.25 °N, 104.87 °E), Nakhon Pathom (13.82 °N, 100.04 °E) and Songkhla (7.2 °N, 100.60 °E). AOD at these sites was measured by using Cimel sunphotometers. The AOD data from these sites over a period of 2-5 years was used in the comparison. It was found that the discrepancy in terms of root mean squared difference between the daily AOD retrieved from MODIS and that of the ground-based measurements was in the range of 33.8%-53.7%.