Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.

The large-scale emissions of airborne particulates from burning of agricultural residues particularly over the upper Indo-Gangetic Plain (IGP) have often been associated with frequent formation of haze, adverse health impacts, and modification in aerosol climatology and thereby aerosol impact on regional climate. In this study, short-term variations in aerosol climatology during extreme biomass burning emissions over the IGP were investigated. Size-segregated particulate concentration was initially measured and submicron particles (PM_1.1) were found to dominate particulate mass within the fine mode (PM_2.1). Particulate-bound water-soluble ions were mainly secondary in nature and primarily composed of sulfate and nitrate. There was evidence of gaseous NH₃ dominating neutralization of acidic aerosol species (SO₄²⁻) in submicron particles, in contrast to crustal-dominating neutralization in coarser particulates. Diurnal variation in black carbon (BC) mass ratio was primarily influenced by regional meteorology, while gradual increase in BC concentration was consistent with the increase in Delta-C, referring to biomass burning emissions. The influence of biomass burning emissions was established using specific organic (levoglucosan), inorganic (K⁺ and NH₄⁺), and satellite-based (UV aerosol index, UVAI) tracers. Levoglucosan was the most abundant species within submicron particles (649±177 ng m⁻³), with a very high ratio (> 50) to other anhydrosugars, indicating exclusive emissions from burning of agriculture residues. Spatiotemporal distribution of aerosol and a few trace gases (CO and NO₂) was evaluated using both spaceborne active and passive sensors. A significant increase in columnar aerosol loading (aerosol optical depth, AOD: 0.98) was evident, with the presence of absorbing aerosols (UVAI > 1.5) having low aerosol layer height ( ∼ 1.5 km). A strong intraseasonality in the aerosol cross-sectional altitudinal profile was even noted from CALIPSO, referring to the dominance of smoke and polluted continental aerosols across the IGP. A possible transport mechanism of biomass smoke was established using cluster analysis and concentration-weighted air mass back trajectories. Short-wave aerosol radiative forcing (ARF) was further simulated considering intraseasonality in aerosol properties, which resulted in a considerable increase in atmospheric ARF (135 W m⁻²) and heating rate (4.3 K day⁻¹) during extreme biomass burning emissions compared to the non-dominating period (56 W m⁻², 1.8 K day⁻¹). Our analysis will be useful to improve understanding of short-term variation in aerosol chemistry over the IGP and to reduce uncertainties in regional aerosol–climate models.

This review synthesizes the existing knowledge on the characteristics of PM_2.5 at sites under the direct influence of industrial emissions, with a specific focus on their morphology, size distributions and chemical composition. Results from online and off-line analytical techniques indicate a high temporal and spatial variability of mass size distribution and chemical composition depending on the type of industrial processes, the sampling distances and frequencies, and the meteorological conditions. Tracers of specific activities have been identified in a number of studies and may help to provide estimates of the relative contribution of pollutant sources from heavily industrialized areas.