Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

Abstract

Abstract. Isoprene-derived secondary organic aerosol (iSOA) is a significant contributor to organic carbon (OC) in some forested regions, such as tropical rainforests and the Southeast US. However, its contribution to organic aerosol in urban areas, with high levels of anthropogenic pollutants, is poorly understood. In this study we examined the formation of anthropogenic-influenced iSOA during summer in Beijing, China. Local isoprene emissions and high levels of anthropogenic pollutants, in particular NOx and particulate SO42−, led to the formation of iSOA under both high- and low-NO oxidation conditions, with significant heterogeneous transformations of isoprene-derived oxidation products to particulate organosulfates (OSs) and nitrooxy-organosulfates (NOSs). Ultra-pressure liquid chromatography coupled to high-resolution mass spectrometry was combined with a rapid automated data processing technique to quantify 31 proposed iSOA tracers in offline PM2.5 filter extracts. The co-elution of the inorganic ions in the extracts caused matrix effects that impacted two authentic standards differently. The average concentration of iSOA OSs and NOSs was 82.5 ng m−3, around three times higher than the observed concentrations of their oxygenated precursors (2-methyltetrols and 2-methylglyceric acid). OS formation was dependant on both photochemistry and sulfate available for reactive uptake as shown by a strong correlation with the product of ozone (O3) and particulate sulfate (SO42−). A greater proportion of high-NO OS products were observed in Beijing compared to previous studies in less polluted environments. The iSOA derived OSs and NOSs represented on average 0.62 % of the oxidised organic aerosol measured by aerosol mass spectrometry, but this increased to ~ 3 % on certain days. These results indicate for the first time that iSOA formation in urban Beijing is strongly controlled by anthropogenic emissions and results in extensive conversion to heterogeneous OS products.

Coupling a gas chromatograph simultaneously to a flame ionization detector and chemical ionization mass spectrometer for isomer-resolved measurements of particle-phase organic compounds

“…The limitations in these estimates will be discussed in detail later in this section, but these data nevertheless provide a useful, if uncertain, understanding of the prevalence of isomers in atmospherically-relevant samples. Data in (b) are from ambient atmospheres in a ponderosa pine forest in California measured by two-dimensional GC without derivatization (Worton et al, 2017), Southeastern U.S. by two-dimensional GC with derivatization (Zhang et al, 2018), and combined datasets from SOAs formation measured by liquid chromatography (Bryant et al, 2019;Dixon, 2020). The number, N, of formulas observed in each dataset is provided in the legend.…”

“…We compare these data to previously published studies using isomer-resolved analyses of SOA (Figure 3b). Ambient measurements from a range of environments show a qualitatively similar distribution of isomers per formula, though with somewhat lower averages of 2.7, 2.1, and 1.9, based on data collected from ambient air in a ponderosa pine forest in California measured by two-dimensional GC without derivatization (Worton et al, 2017), Southeastern U.S. by two-dimensional GC with derivatization (Zhang et al, 2018), and combined datasets from SOA formation measured by liquid chromatography (Bryant et al, 2019;Dixon, 2020). These data were collected by different instruments (two-dimensional GC with filter samples analyzed by thermal-desorption and derivatization, two-dimensional GC with filter samples analyzed by thermal desorption, and LC, respectively) that are likely not sensitive to the same compounds.…”

A Four Carbon Organonitrate as a Significant Product of Secondary Isoprene Chemistry

“…Oxidation initiated by NO 3 radicals leads to significant formation of organonitrates (ONs) which add to the ONs produced during daytime oxidation under high NO x conditions (Hamilton et al., 2021; Kiendler‐Scharr et al., 2016). ONs can act both as a reservoir and as a permanent sink of NO x (Kenagy et al., 2020; Kiendler‐Scharr et al., 2016), and can contribute to SOA formation (Bryant et al., 2020; Fry et al., 2018; Kiendler‐Scharr et al., 2016; Lee et al., 2016; Xu et al., 2021; Zaveri et al., 2020). Investigating isoprene‐originated ONs formation is necessary to understand isoprene's effects on atmospheric NO x , HO x and ozone formation (Li et al., 2019; Schwantes et al., 2020; Vasquez et al., 2020).…”

Using highly time-resolved online mass spectrometry to examine biogenic and anthropogenic contributions to organic aerosol in Beijing

“…15 These have been associated with high emissions of VOC from diverse sources 16,17 which lead to enhanced ozone production 18,19 and SOA formation 20 alongside regional transport which is an important source of semi-volatile organic compounds (SVOC), intermediate-volatility organic compounds (IVOC), POA and SOA in Beijing during the summertime. 21,22 Historically, source apportionment of OA has been limited to low timeresolution high molecular precision offline measurements of molecular tracers, focusing on the behaviour of specic VOC systems including isoprene, [23][24][25][26] apinene 27 and toluene. 28,29 More recently, highly time-resolved bulk composition measurements by aerosol mass spectrometry (AMS) and aerosol chemical speciation monitors (ACSM) have been used to apportion OA to broad source classi-cations including fossil fuels (FFOA), biomass burning (BBOA) and oxidised organic aerosol (OOA).…”

“…45 and 82) and C 4 H 8 O 3 , which can correspond to dihydroxycarbonyl compounds from IEPOX oxidation 83. Given that IEPOX in Beijing typically peaks in the aernoon under low-NO x conditions26 it is likely in the case of this factor that these ions are associated with the aromatic sources. This is particularly likely for C 4 H 8 O 3 which is one of the most dominant ions observed from oxidation of 1,2,4-trimethyl benzene 1,44.…”