Quantifying Timescales for Below-Cloud Removal of Particles by Precipitation
CHRISTOS STAMATIS, Chenyang Bi, Gabriel Isaacman-VanWertz, Virginia Tech
Abstract Number: 578
Working Group: Aerosols, Clouds and Climate
Abstract
Wet deposition is an important removal mechanism for aerosols that affects their atmospheric lifetimes. Studies involving global circulation models suggest that wet deposition is the dominant mechanism for particle removal in the atmosphere. Unfortunately, there are few direct observational constraints on the wet deposition of particles and their timescales of removal, leading to significant uncertainty regarding the impact of precipitation on aerosols. In this work we will present observationally constrained estimates of below-cloud wet deposition timescales for particles. Precipitation size distribution data from globally distributed sites in the Department of Energy Atmospheric Radiation Measurement (ARM) network provided highly detailed measurements of precipitation characteristics, collocated with measurements of particle size distributions. Direct observations of changes in particle size distributions during precipitation events are used to calculate size-dependent below-cloud scavenging coefficients. Using data from Lamont, Oklahoma (2017-2023), Manacapuru, Brazil (2014-2015), southern great plains (2016-2024), Gunnison, Colorado (2021-2023), eastern north Atlantic (2022-2024), and Cordoba, Argentina (2018-2019) removal rates are examined as a function of size, rain intensity and geography. Below-cloud scavenging shows weak dependence on geographical location and particle size, but is strongly influenced by rainfall rate (with little observable impact at rates below ~1 mm/hr). During precipitation events, particles are scavenged at an average rate of 10-100 hours, and can be parameterized as a function of particle size and rainfall rate. Accounting for rainfall rate and frequency across sites the overall below-cloud removal is 8-42 days, which is comparable to particle dry deposition. With global models suggesting wet deposition dominating particle removal our results indicate that in-cloud scavenging must be dominating below-cloud removal and therefore wet deposition in models must be evaluated in this new context.