Aerosol Optical Properties at Four Sites in Thailand

Abstract

This paper presents column integrated aerosol optical properties including aerosol optical depth (AOD), Angstrom wavelength exponent (), single scattering albedo (SSA), and size distribution from ground-based measurements at four sites in Thailand: 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). 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  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.

Comparison of UV index from Ozone Monitoring Instrument (OMI) with multi‐channel filter radiometers at four sites in the tropics: effects of aerosols and clouds

“…The later reason can also explain the overestimations of the OMI UV index under clear sky conditions. The highest bias is found at Chiang Mai which is an urban area, while the lowest bias occurs at Songkhla, the coastal area with low aerosol loading (Janjai et al ., ). It is also noticed that the RMSE for the case of all sky conditions is higher than that of clear sky conditions.…”

“…The result also indicates that the difference between the ground‐based and satellite‐estimated values for Songkhla is less than that for the other sites. This is maybe because of the fact that Songkhla has less aerosol loading, as compared to the other sites (Janjai et al ., ). Table shows the confidence interval of the measurement and the satellite datasets for the case of all sky conditions.…”

“…This result indicates that the relative errors of Songkhla are significantly different from those of Chiang Mai and Ubon Ratchathani. This difference in the relative error may be due to the fact that Chiang Mai and Ubon Ratchathani have higher aerosol loads than those of Songkhla (Janjai et al ., ), and OMI algorithm has shortcomings in resolving surface UV irradiance under high urban aerosol loads, thus creating higher relative errors for Chiang Mai and Ubon Ratchathani.…”

AERONET-based models of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

“…Both sites primarily sample nearby agricultural burning, and forest burning from elsewhere in the region. In contrast, the site in the city of Chiang Mai (north-western Thailand) was not used as, while close to biomass burning source regions, it also has a significant urban contribution to the aerosol, coupled with the local topography leading to a "trapping" of pollutants (Janjai et al, 2012;Gautam et al, 2013). An exception is central and eastern Russia, where two sites (Tomsk 22 and Yakutsk) were included because the sites individually have a comparatively low data volume.…”

AERONET-based microphysical and optical properties of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

“…Both sites primarily sample nearby agricultural burning, and forest burning from elsewhere in the region. In contrast, the site in the city of Chiang Mai (north-western Thailand) was not used as, while close to biomass burning source regions, it also has a significant urban contribution to the aerosol, coupled with the local topography leading to a "trapping" of pollutants (Gautam et al, 2012;Janjai et al, 2012). Another point to note is the omission of sites from the Sahel; although AERONET contains several long-term sites in this region, the dry season biomass burning peak here (November-February) due to burning of grassland, cropland, and shrubs coincides with strong dust activity, such that the aerosol is normally a mixture of biomass burning smoke and dust (Pandithurai et al, 2001;Roberts et al, 2009 Johnson et al (2008) and Eck et al (2010) found cases where these smoke aerosols were highly absorbing, with midvisible SSA as low as 0.76, although the more common cases of mixed smoke and dust in this region are less absorbing.…”