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h i g h l i g h t s < Multi-year, international efforts for studying aerosol and climate impacts across Southeast Asia. < Aerosol and cloud properties measured from both ground-and satellite-based platforms. < Investigation of aerosolecloud interactions and planning for future experiments.

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Aerosol optical thickness (τaer) is a fundamental parameter for analyzing aerosol loading and associated radiative effects. The τaer can constrain many inversion algorithms using passive/active sensor measurements to retrieve other aerosol properties and/or the abundance of trace gases. In the next wave of spectroradiometric observations from geostationary platforms, we envision that a strategically distributed network of robust, well‐calibrated ground‐based spectroradiometers will comprehensively complement spaceborne measurements in spectral and temporal domains. Spectral τaer can be accurately obtained from direct‐Sun measurements based on the Langley calibration method, which allows for the analysis of distinct spectral features of the calibration results. In this study, we present a spectral τaer retrieval algorithm for an in‐house developed, field deployable spectroradiometer instrument covering wavelengths from ultraviolet to near‐infrared (UV‐Vis‐NIR). The spectral total optical thickness obtained from the Langley calibration method is partitioned into molecular and particulate components by utilizing a least squares method. The resulting high temporal‐resolution τaer and Ångström Exponent can be used effectively for cloud screening. The new algorithm was applied to month‐long measurements acquired from the rooftop at National Aeronautics and Space Administration Goddard Space Flight Center's Building 33. The retrieved τaer demonstrated excellent agreement with those from well‐calibrated Aerosol Robotic Network Sun photometers at all overlapping wavelengths (correlation coefficients higher than 0.98). In addition, empirical stray light corrections considerably improved τaer retrievals at short wavelengths in the UV. The continuous spectrum of τaer from UV‐Vis‐NIR spectroradiometers is expected to provide more informative constraints for retrieval of additional aerosol properties such as refractive indices, size, and bulk vertical distribution.

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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.

Two methods designed to parameterize mesoscale ascent in a three-dimensional numerical cloud model via near-surface momentum and heat fluxes are presented and compared to the commonly used technique of an initial perturbation placed within the model initial condition. The flux techniques use a continuously reinforced thermal or convergent low-level wind field to produce upward vertical motion on the order of 10 cm s Ϫ1, by which deep, moist convection can be initiated. The sensitivity of the convective response to the type, strength, and size of the forcing is evaluated using numerical simulations of a conditionally unstable environment with weak unidirectional shear. Precipitation-free cloud processes are used to further simplify the model response to the forcing. The three methods tested produce an initial convective response, but only the momentum and heat flux methods are able to produce sustained deep convection that approximately resembles isolated multicellular convection. Cell regeneration periods, defined as the elapsed time between subsequent vertical velocity maxima passing through a constant level in the updraft region above the source, vary from 8 to 25 min, depending on the forcing type, magnitude, and geometry.

The spring 2015 deployment of a suite of instrumentation at Doi Ang Khang (DAK) in northwestern Thailand enabled the characterization of air masses containing smoke aerosols from burning predominantly in Myanmar. Aerosol Robotic Network (AERONET) Sun photometer data were used to validate Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 'Deep Blue' aerosol optical depth (AOD) retrievals; MODIS Terra and Aqua provided results of similar quality, with correlation coefficients of 0.93-0.94 and similar agreement within expected uncertainties to globalaverage performance. Scattering and absorption measurements were used to compare surface and total column aerosol single scatter albedo (SSA); while the two were well-correlated, and showed consistent positive relationships with moisture (increasing SSA through the season as surface relative humidity and total columnar water vapor increased), insitu surface-level SSA was nevertheless significantly lower by 0.12-0.17. This could be related to vertical heterogeneity and/or instrumental issues. DAK is at ~1,500 m above sea level in heterogeneous terrain, and the resulting strong diurnal variability in planetary boundary layer depth above the site leads to high temporal variability in both surface and column measurements, and acts as a controlling factor to the ratio between surface particulate matter (PM) levels and column AOD. In contrast, while some hygroscopic effects were observed relating to aerosol particle size and Ångström exponent, relative humidity variations appear to be less important for the PM:AOD ratio here.