In this study, the aerosol optical properties and vertical distributions in major biomass-burning emission area of northern Indochina were investigated using ground-based remote sensing (i.e., four Sun-sky radiometers and one lidar) during the Seven South East Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles & Interactions Experiment conducted during spring 2014. Despite the high spatial variability of the aerosol optical depth (AOD; which at 500 nm ranged from 0.75 to 1.37 depending on the site), the temporal variation of the daily AOD demonstrated a consistent pattern among the observed sites, suggesting the presence of widespread smoke haze over the region. Smoke particles were characterized as small (Ångström exponent at 440-870 nm of 1.72 and fine mode fraction of 0.96), strongly absorbing (single-scattering albedo at 440 nm of 0.88), mixture of black and brown carbon particles (absorption Ångström exponent at 440-870 nm of 1.5) suspended within the planetary boundary layer (PBL). Smoke plumes driven by the PBL dynamics in the mountainous region reached as high as 5 km above sea level; these plumes subsequently spread out by westerly winds over northern Vietnam, southern China, and the neighboring South China Sea. Moreover, the analysis of diurnal variability of aerosol loading and optical properties as well as vertical profile in relation to PBL development, fire intensity, and aerosol mixing showed that various sites exhibited different variability based on meteorological conditions, fuel type, site elevation, and proximity to biomass-burning sources. These local factors influence the aerosol characteristics in the region and distinguish northern Indochina smoke from other biomass-burning regions in the world.

The objectives of 7-SEAS/BASELInE (Seven SouthEast Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles & Interactions Experiment) campaigns in spring 2013-2015 were to synergize measurements from uniquely distributed ground-based networks (e.g., AERONET, MPLNET) and sophisticated platforms (e.g., SMARTLabs, regional contributing instruments), along with satellite observations/retrievals and regional atmospheric transport/chemical models to establish a critically needed database, and to advance our understanding of biomass-burning aerosols and trace gases in Southeast Asia (SEA). We present a satellite-surface perspective of 7-SEAS/BASELInE and highlight scientific findings concerning: (1) regional meteorology of moisture fields conducive to the production and maintenance of low-level stratiform clouds over land, (2) atmospheric composition in a biomass-burning environment, particularly tracers/markers to serve as important indicators for assessing the state and evolution of atmospheric constituents, (3) applications of remote sensing to air quality and impact on radiative energetics, examining the effect of diurnal variability of boundary-layer height on aerosol loading, (4) aerosol hygroscopicity and ground-based cloud radar measurements in aerosol-cloud processes by advanced cloud ensemble models, and (5) implications of air quality, in terms of toxicity of nanoparticles and trace gases, to human health. This volume is the third 7-SEAS special issue (after Atmospheric Research, vol. 122, 2013; and Atmospheric Environment, vol. 78, 2013) and includes 27 papers published, with emphasis on air quality and aerosol-cloud effects on the environment. BASELInE observations of stratiform clouds over SEA are unique, such clouds are embedded in a heavy aerosol-laden environment and feature characteristically greater stability over land than over ocean, with minimal radar surface clutter at a high vertical spatial resolution. To * Corresponding author.Tel.: +13016146188; Fax: +13016146307 E-mail address: si-chee.tsay@nasa.gov ** Corresponding author.Tel./Fax: +886-3-4254069 E-mail address: nhlin@cc.ncu.edu.tw Tsay et al., Aerosol and Air Quality Research, 16: 2581-2602, 2016 2582 facilitate an improved understanding of regional aerosol-cloud effects, we envision that future BASELInE-like measurement/modeling needs fall into two categories: (1) efficient yet critical in-situ profiling of the boundary layer for validating remote-sensing/retrievals and for initializing regional transport/chemical and cloud ensemble models, and (2) fully utilizing the high observing frequencies of geostationary satellites for resolving the diurnal cycle of the boundarylayer height as it affects the loading of biomass-burning aerosols, air quality and radiative energetics.

[1] The Ozone Monitoring Instrument (OMI), on board the NASA EOS Aura spacecraft since July 2004, provides a global view of surface spectral ultraviolet (UV) irradiance at 305, 310, 324, and 380 nm; erythemal dose rate both at overpass time and local noontime; and erythemal daily dose. Previous studies have shown comparisons of the OMI erythemal UV irradiances and ground-based UV measurements in areas of midlatitude and high latitudes, predominantly in the Northern Hemisphere. In this study the noontime erythemal UV dose rates retrieved from OMI and measured from broadband instruments at four sites in Thailand were compared. The comparisons show a positive bias for the OMI data with respect to the ground-based measurements. The differences between the two data sets were 30%-60% for all data and were 10%-40% for cloudless data. The differences for the cleanest site showed better agreement than those for the more urban sites. Using the Libradtran radiative transfer model, we show that aerosol is responsible for much of the positive bias in polluted areas. Since absorbing aerosol is not taken into account in the OMI surface UV algorithm, aerosol absorption correction factors have been introduced as a function of aerosol absorption optical thickness provided by OMI to improve the OMI UV data for urban and maritime sites. The differences between the corrected erythemal UV data and the ground-based data were reduced to less than 20%.

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.

ABSTRACT:The solar ultraviolet (UV) index from Ozone Monitoring Instrument (OMI) onboard NASA EOS/Aura satellite is compared with that derived from ground-based multi-channel filter radiometers at four sites in the tropical environment of Thailand. The sites include Chiang Mai (18.78 • N, 98.98Nakhon Pathom (13.82• N, 100.04• E) and Songkhla (7.2 • N, 100.6• E). At these sites, aerosol optical depth is monitored using sunphotometers and cloud images are taken by sky cameras. The comparison results clearly show the overestimation of OMI UV index because of influences by aerosols and clouds. The differences between the two datasets lie between 38.1 and 60.7% for all sky conditions, and reduce to 22.5-50.3% for clear sky conditions.