Small Particle Events over the Southeastern Atlantic Ocean

MARCUS BATISTA, Paquita Zuidema, Yang Wang, University of Miami

     Abstract Number: 118
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
New particle formation (NPF) is a process consisting of the formation of thermodynamically stable clusters from trace gas molecules, followed by their growth to larger sizes. Globally, NPF contributes significantly to cloud condensation nuclei (CCN) that may further influence cloud formation. Many studies have showed that NPF over the open ocean is unlikely due to the relatively high concentrations of sea spray aerosols, which act as condensation and coagulation sinks. However, recent studies indicate that NPF could occur regularly following the passage of cold fronts [1].

Using data from the Department of Energy`s Layered Atlantic Smoke Interactions with Clouds campaign, we examine aerosol, precursor gas, and meteorological conditions associated with NPF at Ascension Island (ASI; 8S, 14.5W) in the remote southeastern Atlantic marine boundary layer, from June 2016 to October 2017. Based on aerosol size distribution data, we do not observe any events featuring continuous aerosol growth from 10 nm to larger sizes (i.e., the “banana curve”), a typical indicator of NPF. However, we observe episodes where aerosols sized from 15 to 30 nm are transported to ASI, which we refer to as small particle events (SPE). We identified 50 SPE, of which 68% are associated with post-cold frontal open-cell systems. This agrees with recent findings regarding NPF over the open ocean [1].

We find a relatively strong correlation (R = 0.74) between monthly concentrations of particles with sizes below 30 nm and monthly cumulative precipitation, indicating the removal of pre-existing particles as a mechanism for NPF. An association between SPE and vertical velocity, measured by the Doppler Lidar, suggests that recently formed particles were vertically transported to the surface from the upper boundary layer. These vertically transported small particles can grow to sizes relevant to CCN, further contributing to CCN and aerosol-cloud interactions over the Southeastern Atlantic Ocean.

[1] Zheng et al. 2021. Nature Communications, 12(1), p.527.