Limited effect of anthropogenic nitrogen oxides on secondary organic aerosol formation

Y Zheng,L Emmons,P Yu,A Hodzic,J.-F Lamarque,S Tilmes,N Unger,C Knote

doi:10.5194/acp-15-13487-2015

Abstract

Abstract. Globally, secondary organic aerosol (SOA) is mostly formed from emissions of biogenic volatile organic compounds (VOCs) by vegetation, but it can be modified by human activities as demonstrated in recent research. Specifically, nitrogen oxides (NOx = NO + NO2) have been shown to play a critical role in the chemical formation of low volatility compounds. We have updated the SOA scheme in the global NCAR (National Center for Atmospheric Research) Community Atmospheric Model version 4 with chemistry (CAM4-chem) by implementing a 4-product volatility basis set (VBS) scheme, including NOx-dependent SOA yields and aging parameterizations. Small differences are found for the no-aging VBS and 2-product schemes; large increases in SOA production and the SOA-to-OA ratio are found for the aging scheme. The predicted organic aerosol amounts capture both the magnitude and distribution of US surface annual mean measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network by 50 %, and the simulated vertical profiles are within a factor of 2 compared to aerosol mass spectrometer (AMS) measurements from 13 aircraft-based field campaigns across different regions and seasons. We then perform sensitivity experiments to examine how the SOA loading responds to a 50 % reduction in anthropogenic nitric oxide (NO) emissions in different regions. We find limited SOA reductions of 0.9–5.6, 6.4–12.0 and 0.9–2.8 % for global, southeast US and Amazon NOx perturbations, respectively. The fact that SOA formation is almost unaffected by changes in NOx can be largely attributed to a limited shift in chemical regime, to buffering in chemical pathways (low- and high-NOx pathways, O3 versus NO3-initiated oxidation) and to offsetting tendencies in the biogenic versus anthropogenic SOA responses.

Highlights

Organic aerosols (OAs) account for a substantial fraction of atmospheric fine particulate matter and can have significant impacts on both air quality (Huang et al, 2014; Zhang et al, 2007) and climate (Carslaw et al, 2010)
Aside from primary organic aerosols (POAs) that are directly emitted into the atmosphere, another major fraction of OA is composed of secondary organic aerosols (SOAs), which are formed through chemical transformation of anthropogenic and biogenic volatile organic compounds (AVOCs and BVOCs)
In the VBS_agAVOC run, anthropogenic SOA (ASOA) accounts for a larger fraction in Northern Hemisphere mid-latitudes than other simulations ranging from 30 to 50 % because in this scheme the aging process is only applied to ASOA

Summary

Introduction

Organic aerosols (OAs) account for a substantial fraction of atmospheric fine particulate matter and can have significant impacts on both air quality (Huang et al, 2014; Zhang et al, 2007) and climate (Carslaw et al, 2010). Aside from primary organic aerosols (POAs) that are directly emitted into the atmosphere, another major fraction of OA is composed of secondary organic aerosols (SOAs), which are formed through chemical transformation of anthropogenic and biogenic volatile organic compounds (AVOCs and BVOCs). AVOCs include aromatics, alkanes and alkenes of about 25, 44 and 38 TgC year−1, respec-. POA can re-evaporate upon dilution and participate in the chemical oxidation processes leading to the formation of SOA (Robinson et al, 2007)

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