Formation of environmentally persistent free radicals from the heterogeneous reaction of ozone and polycyclic aromatic compounds

Abstract

In the 1980s long-lived radical species were identified in cigarette tar. Since then, environmentally persistent free radicals (EPFRs) have been observed in ambient particulate matter, and have been generated in particulate matter generated from internal combustion engines. For the first time, we measure in situ the formation and decay of EPFRs through the heterogeneous reaction of ozone and several polycyclic aromatic compounds (PAC). Solid anthracene (ANT), pyrene (PY), benzo[a]pyrene (BAP), benzo[ghi]perylene (BGHIP), 1,4-naphthoquinone (1,4NQ), and 9,10-anthraquinone (AQ) were reacted with gas-phase ozone in a flow system placed in the active cavity of an electron paramagnetic resonance (EPR) spectrometer, and the formation of radicals was measured on the timescale of tens of minutes at ambient levels of ozone down to 30 ppb. For most substrates the net radical production is initially rapid, slows at intermediate times, and is followed by a slow decay. For oxidized solid BAP, radical signal persists for many days in the absence of ozone. To evaluate the effect of substrate phase, the solid PAHs were also dissolved in squalane, an organic oil inert to ozone, which yielded a much higher maximum radical concentration and faster radical decay when exposed to ozone. With higher mobility, reactants were apparently able to more easily diffuse and react with each other, yielding the higher radical concentrations. The EPR spectra exhibit three radicals types, two of which have been assigned to semiquinone species and one to a PAH-derived, carbon-centered radical. Although our system uses levels of PAC not typically found in the environment it is worth noting that the amounts of radical formed, on the order of 10(18) radicals per g, are comparable to those observed in ambient particulate matter.

Quantification of environmentally persistent free radicals and reactive oxygen species in atmospheric aerosol particles

“…Maximum values of ∼ 7×10 11 spins µg −1 were reached during sunny days, indicating that photochemistry may be related to EPFR production. For example, heterogeneous reactions of photo-oxidants including O 3 and OH with soot or PAH may contribute to the formation of long-lived radicals (Shiraiwa et al, 2011;Borrowman et al, 2015). Radical concentrations were as low as 6.3 × 10 10 spins µg −1 during rain events, most likely due to low production of EPFR and scavenging by precipitation.…”

“…The formation of stable radicals can also be induced by heterogeneous and multiphase chemistry of organic aerosols. Heterogeneous ozonolysis of aerosol particles such as polycyclic aromatic hydrocarbons (PAH) and pollen proteins can lead to the formation of long-lived reactive oxygen intermediates (ROI) (Shiraiwa et al, 2011Reinmuth-Selzle et al, 2014;Borrowman et al, 2015;Kampf et al, 2015;Berkemeier et al, 2016).…”

Ozone in urban China: Impact on mortalities and approaches for establishing indoor guideline concentrations

“…When indoors, humans are exposed not only to ozone, but also to products of ozone‐initiated chemistry . Some of these products (eg, hydroxyl radicals, hydrogen peroxide, organic peroxides, formaldehyde, dicarbonyls, secondary organic aerosols) potentially contribute to the total mortality associated with long‐term exposure to ozone as evaluated using measured or modeled outdoor ozone concentrations.…”

Influence of Titanium Species on the Ordered Mesoporous Structure of V‐Zr Catalysts for Naphthalene Oxidation with Hydrogen Peroxide

“…Naphthalene is the most stable PAHs, which is known as a major component of the coal and tar‐based industries and a highly toxic xenobiotic compound . Up to now, biodegradation, photocatalysis, ozone oxidation or aerobic catalytic oxidation has been employed to eliminate naphthalene. Aerobic catalytic oxidation such as gas‐phase or liquid‐phase oxidation has attracted extensive attentions due to its high catalytic performance.…”