Isoprene–Chlorine Oxidation in the Presence of NO_x and Implications for Urban Atmospheric Chemistry

Abstract

Fine particulate matter (PM2.5) is a key indicator of urban air quality. Secondary organic aerosol (SOA) contributes substantially to the PM2.5 concentration. Discrepancies between modeling and field measurements of SOA indicate missing sources and formation mechanisms. Recent studies report elevated concentrations of reactive chlorine species in inland and urban regions, which increase the oxidative capacity of the atmosphere and serve as sources for SOA and particulate chlorides. Chlorine-initiated oxidation of isoprene, the most abundant nonmethane hydrocarbon, is known to produce SOA under pristine conditions, but the effects of anthropogenic influences in the form of nitrogen oxides (NO x ) remain unexplored. Here, we investigate chlorine–isoprene reactions under low- and high-NO x conditions inside an environmental chamber. Organic chlorides including C5H11ClO3, C5H9ClO3, and C5H9ClO4 are observed as major gas- and particle-phase products. Modeling and experimental results show that the secondary OH–isoprene chemistry is significantly enhanced under high-NO x conditions, accounting for up to 40% of all isoprene oxidized and leading to the suppression of organic chloride formation. Chlorine-initiated oxidation of isoprene could serve as a source for multifunctional (chlorinated) organic oxidation products and SOA in both pristine and anthropogenically influenced environments.

High Chlorine Concentrations in an Unconventional Oil and Gas Development Region and Impacts on Atmospheric Chemistry

“…In addition to molecular chlorine and nitryl chloride, we observed a variety of organic species in the gas and particle phases, including oxygenated organics (compounds with the molecular formula C x H y O z , abbreviated as CHO); chlorinated organics (C x H y Cl i O z , abbreviated as CHClO); and nitrogen-containing organics (C x H y N i O z , abbreviated as CHNO). Specifically, we observed organochlorines including C 2 H 3 ClO 2 , C 3 H 5 ClO 2 , C 4 H 5 ClO 3 , and C 6 H 9 ClO 5 , which were previously measured from environmental chamber reactions of alkanes with chlorine, and C 4 H 7 ClO 3 , C 5 H 9 ClO 2 , C 5 H 9 ClO 3 , C 5 H 9 ClO 4 , C 5 H 7 ClO 3 , C 5 H 11 ClO 3 , C 5 H 8 Cl 2 O 3 , and C 5 H 10 Cl 2 O 2 , which were measured from the reactions of isoprene with chlorine under varying levels of NO x . , As seen in Figures and S8, almost all of the species show similar behavior during phases 2 and 3 of the campaign, where their gas-phase concentrations generally only increase around 7 AM (sunrise), peak around 11 AM to 3 PM, and begin to decline around 6 PM (as solar radiance declines before sunset at 8 PM). The particle phase (FIGAERO) data are shown in the shaded gray areas in Figures and S8.…”

“…Reactive chlorine species such as molecular chlorine (Cl 2 ) and nitryl chloride (ClNO 2 ) photolyze in the atmosphere to form chlorine radical (Cl), a potent atmospheric oxidant that impacts air quality by oxidizing VOCs, contributing to the formation of ozone and secondary organic aerosol, and affecting radical budgets. – Cl radical oxidizes a wide range of VOCs, often more rapidly than hydroxyl radical (OH), – and forms a different set of products than other oxidants. , These include organochlorine species (generally formed when Cl adds to a double bond), which may have substantial health impacts. Furthermore, observations and models show that Cl radicals alter OH and O 3 concentrations. – …”

Exploring the Formation of High Levels of Hydroxyl Dicarboxylic Acids at an Urban Background Site in South China

“…Studies have demonstrated the importance of chlorine chemistry in coastal areas (Peng et al., 2022; Xue et al., 2015). However, chlorine‐initiated oxidations of unsaturated aliphatic VOCs including isoprene may not lead to OHDCA formation due to the addition of chlorine atom(s) into the main products (Guo et al., 2020; Wang et al., 2022). The ratio of toluene to benzene (T/B) was calculated to examine this effect, as the reaction between benzene and chlorine is very slow (Xue et al., 2015).…”

Effects of environmental exposure to iron powder on healthy and elastase-exposed mice

“…Chlorine, when combined with certain metallic components in PM, can lead to the formation of highly reactive chlorine radicals and further enhance oxidative stress in cells. This combined action of metallic components and chlorine can contribute to the development of respiratory inflammation and other health effects in individuals exposed to polluted air 44 .…”