The role of highly oxygenated organic molecules in the Boreal aerosol-cloud-climate system

Pontus Roldin,Hanna Vehkamäki,Emilie Öström,Theo Kurtén,Douglas R Worsnop,Tuukka Petäj̈ä ,Liqing Hao,Pekka Rantala,Lukas Pichelstorfer,Petri Clusius,Carlton Xavier,Liine Heikkinen,Nina Sarnela,Matti Rissanen ,Annele Virtanen,Jonas Elm,Mikael Ehn,Ilona Riipinen,Tinja Olenius,Noora Hyttinen,Markku Kulmala,Michael Boy

doi:10.1038/s41467-019-12338-8

Abstract

Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by ~10 % and causes a direct aerosol radiative forcing of −0.10 W/m2. In contrast, NPF reduces the number of CCN at updraft velocities < 0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m2. Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest.

Highlights

Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN)
In this work we have developed the first comprehensive peroxy radical autoxidation mechanism (PRAM) that describes the formation of Highly Oxygenated organic Molecules (HOM) from monoterpenes
We show that the combination of new particle formation (NPF) and particle growth by biogenic HOM SOA has a profound but complex impact on the aerosol-cloud-climate system over the Boreal forest

Summary

Introduction

Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We define HOM as molecules with at least six oxygen atoms formed from peroxy radical autoxidation of VOC23,26. PRAM has the potential to become a widely used mechanism for more realistic representations of HOM SOA formation in regional and global scale CTMs. We demonstrate a very good agreement between the modelled and measured gasphase HOM composition, secondary organic aerosol (SOA) mass concentrations and new particle formation (NPF) over the Boreal forest, and apply the new model to explore the climatic implications of HOM. While HOM is contributing to climate cooling by increasing the number of CCN and the magnitude of the direct aerosol radiative forcing, NPF can result in net climate warming by decreasing the direct aerosol radiative forcing and by supressing the cloud droplet number concentrations at low to moderate cloud updraft velocities

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Results

Conclusion