Below-cloud scavenging of aerosol by rain: a review of numerical modelling approaches and sensitivity simulations with mineral dust in the Met Office's Unified Model

Abstract

Abstract. Theoretical models of the below-cloud scavenging (BCS) of aerosol by rain yield scavenging rates that are 1–2 orders of magnitude smaller than observations and associated empirical schemes for submicron-sized aerosol. Even when augmented with processes which may explain this disparity, such as phoresis and rear capture in the raindrop wake, the theoretical BCS rates remain an order of magnitude less than observations. Despite this disparity, both theoretical and empirical BCS schemes remain in wide use within numerical aerosol models. BCS is an important sink for atmospheric aerosol, in particular for insoluble aerosol such as mineral dust, which is less likely to be scavenged by in-cloud processes than purely soluble aerosol. In this paper, various widely used theoretical and empirical BCS models are detailed and then applied to mineral dust in climate simulations with the Met Office's Unified Model in order the gauge the sensitivity of aerosol removal to the choice of BCS scheme. We show that the simulated accumulation-mode dust lifetime ranges from 5.4 d in using an empirical BCS scheme based on observations to 43.8 d using a theoretical scheme, while the coarse-mode dust lifetime ranges from 0.9 to 4 d, which highlights the high sensitivity of dust concentrations to BCS scheme. We also show that neglecting the processes of rear capture and phoresis may overestimate submicron-sized dust burdens by 83 %, while accounting for modal widths and mode merging in modal aerosol models alongside BCS is important for accurately reproducing observed aerosol size distributions and burdens. This study provides a new parameterisation for the rear capture of aerosol by rain and is the first to explicitly incorporate the rear-capture mechanism in climate model simulations. Additionally, we answer many outstanding questions pertaining to the numerical modelling of BCS of aerosol by rain and provide a computationally inexpensive BCS algorithm that can be readily incorporated into other aerosol models.

Intra-event evolution of elemental and ionic concentrations in wet deposition in an urban environment

“…Several possible explanations are considered for these observations: this could be due to either an effect of "over-concentration" of falling raindrops or a release of aerosols due to their evaporation (Huff and Stout, 1964;Baechmann et al, 1996a, b;Gong et al, 2011); alternatively, it could be due to an increase in scavenging efficiency due to the reduction of droplet size distribution, implying a larger effective surface of capture (e.g. Jones et al, 2022) as well as to a local emission phenomenon (Karşı et al, 2018).…”

An optimisation method to improve modelling of wet deposition in atmospheric transport models: applied to FLEXPART v10.4

“…Theoretical wet scavenging models can take these processes into account but are known to underestimate the scavenging rate by 1-2 orders of magnitude for accumulation-mode particles (Wang et al, 2010(Wang et al, , 2011. This offset can be reduced by taking into account other so-called phoretic effects (thermophoresis, diffusiophoresis and electric effects; Sportisse, 2007;Jones et al, 2022) and the rear capture effect (Jones et al, 2022). Most ATMs, however, use empirical models for scavenging, which ignore these more complex microphysical processes and instead use fits of measured scavenging rates to certain empirical functions.…”