Natural and Anthropogenically Influenced Isoprene Oxidation in Southeastern United States and Central Amazon

Abstract

Anthropogenic emissions alter secondary organic aerosol (SOA) formation chemistry from naturally emitted isoprene. We use correlations of tracers and tracer ratios to provide new perspectives on sulfate, NO x, and particle acidity influencing isoprene-derived SOA in two isoprene-rich forested environments representing clean to polluted conditionswet and dry seasons in central Amazonia and Southeastern U.S. summer. We used a semivolatile thermal desorption aerosol gas chromatograph (SV-TAG) and filter samplers to measure SOA tracers indicative of isoprene/HO2 (2-methyltetrols, C5-alkene triols, 2-methyltetrol organosulfates) and isoprene/NO x (2-methylglyceric acid, 2-methylglyceric acid organosulfate) pathways. Summed concentrations of these tracers correlated with particulate sulfate spanning three orders of magnitude, suggesting that 1 μg m–3 reduction in sulfate corresponds with at least ∼0.5 μg m–3 reduction in isoprene-derived SOA. We also find that isoprene/NO x pathway SOA mass primarily comprises organosulfates, ∼97% in the Amazon and ∼55% in Southeastern United States. We infer under natural conditions in high isoprene emission regions that preindustrial aerosol sulfate was almost exclusively isoprene-derived organosulfates, which are traditionally thought of as representative of an anthropogenic influence. We further report the first field observations showing that particle acidity correlates positively with 2-methylglyceric acid partitioning to the gas phase and negatively with the ratio of 2-methyltetrols to C5-alkene triols.

Future changes in isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) under the shared socioeconomic pathways: the importance of explicit chemistry

“…3.1), the inclusion of the recent findings from the chamber study (D'Ambro et al, 2019) improved the model simulation. One possible explanation is different IEPOX-SOA molecular components between the Southeast US and the Amazon, however the reported composition is similar for both locations (Yee et al, 2020) . This new approach also changed IEPOX as shown in Sect 3.1.…”

Secondary Organic Aerosol Formation from Isoprene: Selected Research, Historic Account and State of the Art

“…With respect to the choice of an extraction solvent, acetonitrile is to be preferred above methanol, because of the risk of methylation artefacts in acidic medium, e.g., [32]. An online measurement technique, applied successfully in later studies and allowing the assessment of isoprene SOA, e.g., [33], is thermal desorption aerosol gas chromatography (TAG)/MS, where in situ derivatization using N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) has been implemented [34].…”

Measurement report: Vertical distribution of biogenic and anthropogenic secondary organic aerosols in the urban boundary layer over Beijing during late summer

“…Some studies also suggested that the loading of 2-MTs increased with the enhancement of aerosol acidity (Surratt et al, 2007), and the relative humidity can affect the ratio of 2-MTs to C 5 -alkene triols (Surratt et al, 2010). Recent studies suggested the ratio of 2-MTs to C 5alkene triols decreased with aerosol acidity (Yee et al, 2020), and C 5 -alkene triols were likely formed from thermal degradation of 2-methyltetrol sulfates for GC/MS artifacts (Cui et al, 2018). Hence, it is difficult to explain the different ratios of 2-MTs to C 5 -alkene triols at three heights.…”