Condensed-phase biogenic-anthropogenic interactions with implications for cold cloud formation.

Joseph C Charnawskas,Peter A Alpert,Ryan C Moffet,Thomas Berkemeier,Timothy B Onasch,Daniel A Knopf,Andrew T Lambe,Mary K Gilles,Manabu Shiraiwa,Rachel E O’Brien,Paola Massoli,Douglas R Worsnop,Paul Davidovits

doi:10.1039/c7fd00010c

Abstract

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA-soot biogenic-anthropogenic interactions and their impact on ice nucleation in relation to the particles' organic phase state. SOA particles were generated from the OH oxidation of naphthalene, α-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (Tg) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core-shell configuration (i.e. a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respective Tg and FDRH. α-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the α-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

Highlights

Aerosol particles play an important role in the radiative budget of the Earth by scattering and absorbing short wave and long wave radiation and modifying the radiative properties of clouds by acting as cloud condensation (CCN) nuclei and ice nuclei (IN).[1,2,3] glaciation of clouds plays a crucial role in precipitation and the hydrological cycle.[4]
Coalescence of deposited longifolene secondary organic aerosol (SOA) particles does occur upon mixing with sulfate due to the plasticizing effect of liquid water associated with the sulfate
We characterized SOA particles generated from the OH oxidation of naphthalene, a-pinene, and longifolene serving as surrogates of anthropogenic and biogenic SOA particles with or without the presence of sulfate and soot particles for their role in atmospheric ice nucleation under mixed-phase and cirrus cloud conditions

Summary

Introduction

Aerosol particles play an important role in the radiative budget of the Earth by scattering and absorbing short wave and long wave radiation and modifying the radiative properties of clouds by acting as cloud condensation (CCN) nuclei and IN.[1,2,3] glaciation of clouds plays a crucial role in precipitation and the hydrological cycle.[4] the magnitude of aerosol climate forcing remains highly uncertain. This knowledge is crucial to improve predictive understanding of our current climate, and to better establish cold cloud formation during pre-industrial times[6] where sulfate and anthropogenic emissions were much lower.[7]

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