Large hemispheric difference in nucleation mode aerosol concentrations in the lowermost stratosphere at mid- and high latitudes

Christina J Williamson,Thao Paul V Bui,Charles A Brock,Thomas B Ryerson,Jan Kazil,Gregory P Schill,Maximilian Dollner,Andrew Rollins,Ilann Bourgeois,Eric A Ray,Glenn S Diskin,Donald R Blake,Chelsea Thompson,Daniel M Murphy,Bernadett Weinzierl,Joshua P Digangi,Karl D Froyd,Agnieszka Kupc,Jeff Peischl

doi:10.5194/acp-21-9065-2021

Abstract

Abstract. The details of aerosol processes and size distributions in the stratosphere are important for both heterogeneous chemistry and aerosol–radiation interactions. Using in situ, global-scale measurements of the size distribution of particles with diameters > 3 nm from the NASA Atmospheric Tomography Mission (ATom), we identify a mode of aerosol smaller than 12 nm in the lowermost stratosphere (LMS) at mid- and high latitudes. This mode is substantial only in the Northern Hemisphere (NH) and was observed in all four seasons. We also observe elevated SO2, an important precursor for new particle formation (NPF) and growth, in the NH LMS. We use box modelling and thermodynamic calculations to show that NPF can occur in the LMS conditions observed on ATom. Aircraft emissions are shown as likely sources of this SO2, as well as a potential source of nucleation mode particles directly emitted by or formed in the plume of the engines. These nucleation mode particles have the potential to grow to larger sizes and to coagulate with larger aerosol, affecting heterogeneous chemistry and aerosol–radiation interactions. Understanding all sources and characteristics of stratospheric aerosols is important in the context of anthropogenic climate change as well as proposals for climate intervention via stratospheric sulfur injection. This analysis not only adds to the, currently sparse, observations of the global impact of aviation, but also introduces another aspect of climate influence, namely a size distribution shift of the background aerosol distribution in the LMS.

Highlights

Aerosols in the stratosphere have both radiative and chemical effects: (i) scattering or absorbing light which cools or warms the Earth and (ii) providing surfaces for heterogeneous chemical reactions
Concentrations of nucleation mode particles in the Northern Hemisphere (NH) lowermost stratosphere (LMS) are sometimes correlated with elevated SO2 in the NH LMS (Fig. 6), which is consistent with our understanding of SO2 as a precursor for new particle formation (NPF)
Within the scope of this study, the most likely cause of elevated numbers of nucleation mode aerosol in the NH LMS at mid- and high latitudes is aviation, through a combination of direction emission and nucleation in the exhaust plume, as well as NPF caused by elevated SO2 from aircraft in the background NH LMS

Summary

Introduction

Aerosols in the stratosphere have both radiative and chemical effects: (i) scattering or absorbing light which cools or warms the Earth and (ii) providing surfaces for heterogeneous chemical reactions. Total concentrations of aerosol and the related sinks for condensable vapours, clusters, and nucleation mode particles are generally higher in the PBL, so PBL NPF is mostly observed where concentrations of precursor vapours are high or under specific local conditions where condensation sinks are lower (Kerminen et al, 2018) It may be possible for lower concentrations of condensable vapours to cause NPF in the LMS because of these low sinks. We examine in situ observations of nucleation mode particles, as well as relevant gas-phase tracers and condensable species in the lowermost stratosphere in both hemispheres, to understand the prevalence, potential causes, and importance of NPF in the LMS. Back trajectories, thermodynamic calculations, and emissions estimates to understand how NPF can occur in the LMS, factors influencing the amount of NPF, and other potential sources of nucleation mode aerosol in this region

Methods

Observations and models suggest NPF occurs in the NH LMS

Findings

Conclusions