Limited effect of anthropogenic nitrogen oxides on Secondary Organic Aerosol formation

Abstract

Abstract. Globally, secondary organic aerosol (SOA) is mostly formed from emissions of biogenic volatile organic compounds (VOCs) by vegetation, but 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 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. 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 two compared to Aerosol Mass Spectrometer (AMS) measurements from 13 aircraft-based field campaigns across different region 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 to 5.6, 6.4 to 12.0 and 0.9 to 2.8 % for global, the southeast US and the Amazon NOx perturbations, respectively. The fact that SOA formation is almost unaffected by changes in NOx can be largely attributed 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.

Regional Similarities and NOx‐Related Increases in Biogenic Secondary Organic Aerosol in Summertime Southeastern United States

“…For example, some field studies have shown that a 1 μg/m 3 decrease in sulfate can lead to a 0.2–0.42 μg/m 3 decrease in isoprene SOA (Blanchard et al, ; Budisulistiorini et al, ; Pye et al, ; Shrivastava et al, ; Xu et al, , ). Similarly, decreases in NO x have been shown to decrease biogenic SOA formation but there are also studies that have shown increases of biogenic SOA in some regimes with decreased NO x (Table ; de Sa et al, ; Edwards et al, ; Kroll et al, ; Lane et al, ; Liu et al, ; Matsui et al, ; Ng et al, ; Pye et al, , , ; Rollins et al, ; Wildt et al, ; Xu et al, , ; Zhang et al, ; Zheng et al, ). Since these effects of NO x have been shown by laboratory studies to affect biogenic SOA formation by changing oxidation pathways and ultimate products (Atkinson et al, ; Hoyle et al, ; Kroll et al, ; Presto et al, ; Shrivastava et al, ; Surratt et al, ; Ziemann & Atkinson, ), it is important to quantify them in atmospheric field studies.…”

“…Chemical transport models capture some aspects of the influence of NO x on organic aerosols. Zheng et al () used an updated SOA scheme in the global NCAR (National Center for Atmospheric Research) Community Atmospheric Model version 4 with chemistry (CAM4‐chem) with a four‐product volatility basis set scheme with NO x ‐dependent SOA yields and aging parameterizations and predicted only 6–12% biogenic SOA decreases in the southeastern United States for 50% NO x reductions. Pye et al () updated the comprehensive coupled gas and aerosol processes in CMAQ5.1 with SAPRC07tic (State Air Pollution Research Center mechanism update, https://www.airqualitymodeling.org/index.php/CMAQ_v5.1_SAPRC07tic_AE6i; Hutzell et al, ; Lin et al, ; Xie et al, ).…”

“…These model schemes incorporate the results of laboratory experiments that show that NO x affects biogenic SOA formation in at least three different, competing, and counteracting ways. First, NO x can reduce O 3 formation in the high‐NO x regime but under low‐NO x conditions increasing NO x will result in increases in OH radicals and O 3 (Seinfeld & Pandis, ), both of which result in higher SOA yields (Zheng et al, ). Second, high concentrations of nighttime nitrate radical (NO 3 ) increased SOA formation from isoprene in chamber experiments (Ng et al, ) by forming organonitrates (Ng et al, ).…”

“…Second, high concentrations of nighttime nitrate radical (NO 3 ) increased SOA formation from isoprene in chamber experiments (Ng et al, ) by forming organonitrates (Ng et al, ). Third, reaction of NO with organo‐peroxy radicals (RO 2 ) in the high NO x regime can lower SOA yields due to more volatile products compared to reaction with hydroperoxy radicals (HO 2 ; Kroll et al, ; Kroll & Seinfeld, ; Zheng et al, ; Ziemann & Atkinson, ). However, the role of NO x in systems that undergo autoxidation, particularly monoterpenes (Ehn et al, ), has not been elucidated.…”

Secondary organic aerosol (SOA) yields from NO<sub>3</sub> radical + isoprene based on nighttime aircraft power plant plume transects

“…Hoyle et al (2007) found an increase in global SOA production from 35 to 53 Tg yr −1 since preindustrial times, resulting in an increase in global annual mean SOA mass loading of 51 %, attributable in part to changing NO x emissions. Zheng et al (2015) found only moderate SOA reductions from a 50 % reduction in NO emissions: 0.9 %-5.6 % for global NO x or 6.4 %-12.0 % for southeastern US NO x , which they attributed to buffering by alternate chemical pathways and offsetting tendencies in the biogenic vs. anthropogenic SOA components. In contrast, Pye et al (2015) find a 9 % reduction in total organic aerosol in Centreville, AL for only 25 % reduction in NO x emissions.…”