The Indo-Gangetic Plain (IGP) is one of the dominant sources of air pollution worldwide. During winter, the variations in planetary boundary layer (PBL) height, driven by a strong radiative thermal inversion, affect the regional air pollution dispersion. To date, measurements of aerosol-water vapour interactions, especially cloud condensation nuclei (CCN) activity, are limited in the Indian sub-continent, causing large uncertainties in the radiative forcing estimates of aerosol-cloud interactions. We present the results of a one-month field campaign (February-March 2018) in the megacity, Delhi, a significant polluter in the IGP. We measured the composition of fine particulate matter (PM1) and size-resolved CCN properties over a wide range of water vapour supersaturations. The analysis includes PBL modelling, backward trajectories, and fire spots to elucidate the influence of PBL and air mass origins on the aerosols. The aerosol properties depended strongly on the PBL height, and a simple power-law fit could parameterize the observed correlations of PM1 mass, aerosol particle number, and CCN number with PBL height, indicating PBL induced changes in aerosol accumulation. The low inorganic mass fractions, low aerosol hygroscopicity and high externally mixed weakly CCN-active particles under low PBL height (<100 m) indicated the influence of the PBL on aerosol aging processes. In contrast, aerosol properties did not depend strongly on air mass origins or wind direction, implying that the observed aerosol and CCN are from local emissions. An error function could parameterize the relationship between CCN number and supersaturation throughout the campaign.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or, or the life-cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso (200 km) and larger (500 km) scales, roughly four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.

There is a need to better characterize aerosol absorption due to its remarkable radiative effects on climate. In particular, we need to understand the separation of total absorption between the two main components, the black carbon (BC) and brown carbon (BrC), especially in places where the anthropic influence is little, such as in pristine regions like Central Amazonia. The mechanisms that control the formation and evolution of BC and BrC in tropical forests remain unclear. In this study, we have performed detailed measurements at the Amazon Tall Tower Observatory (ATTO) tower on aerosols collected in Nuclepore filters and analyzed them with high-resolution optical spectrometers with a wide spectral range (300 to 2500 nm). Thus, we determined the absorption characteristics of BrC as a function of wavelength. The results show that BrC absorption is spectrally significant below 660 nm and is maximum at wavelengths close to 370 nm. Combining the measured spectral dependency with MIE modeling of the BC contribution, we determined that the BrC accounts for 14.8% of the total absorption. A similar fraction of BrC to total absorption was obtained through a similar analysis of in-situ measurements. Elemental chemical analysis of the filters, cluster, and factor analysis shows that the BrC is associated with airborne dust. The different methods to quantify BC and BrC are consistent and show similar results. This study will allow the quantification of the role of BrC and BC in aerosol absorption of radiation in Amazonia.