Abstract
Abstract. Despite ongoing efforts, the vertical distribution of aerosols globally is poorly understood. This in turn leads to large uncertainties in the contributions of the direct and indirect aerosol forcing on climate. Using the Global Aerosol Synthesis and Science Project (GASSP) database – the largest synthesised collection of in situ aircraft measurements currently available, with more than 1000 flights from 37 campaigns from around the world – we investigate the vertical structure of submicron aerosols across a wide range of regions and environments. The application of this unique dataset to assess the vertical distributions of number size distribution and cloud condensation nuclei (CCN) in the global aerosol–climate model ECHAM-HAM reveals that the model underestimates accumulation-mode particles in the upper troposphere, especially in remote regions. The processes underlying this discrepancy are explored using different aerosol microphysical schemes and a process sensitivity analysis. These show that the biases are predominantly related to aerosol ageing and removal rather than emissions.
Highlights
Atmospheric aerosol particles play a crucial role in the global energy balance by interacting with long-wave (LW) and short-wave (SW) radiation both directly and indirectly through aerosol–cloud interactions (ACIs)
The indirect forcing in liquid clouds depends on the ability of aerosol particles to act as cloud condensation nuclei (CCN), which in turn depends on the hygroscopicity and size distribution at the altitude of cloud droplet activation, which is mostly around cloud base at altitudes of 1–3 km
We find that ECHAM-HAM represents the aerosol size distribution well in the boundary layer, but that it appears to underestimate accumulation-mode particles in the free-troposphere, which is reflected in the CCN distribution
Summary
Atmospheric aerosol particles play a crucial role in the global energy balance by interacting with long-wave (LW) and short-wave (SW) radiation both directly and indirectly through aerosol–cloud interactions (ACIs). In situ aircraft measurements provide important direct measurements of aerosol chemical composition, size distributions and radiative properties anywhere in the troposphere. These measurements have been used extensively to investigate the representation of black carbon (BC) in GCMs The Global Aerosol Synthesis and Science Project (GASSP) dataset (Reddington et al, 2017) brings together measurements from more than 1000 flights across 37 campaigns from around the world in a consistent, synthesised format. Using this combination of aircraft datasets, we are able to make more extensive evaluations of global climate models.
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