Photoenhanced Uptake of NO₂ and HONO Formation on Real Urban Grime

Nitrous acid (HONO) is one of the most important photochemical precursors of the hydroxyl radical in the sunlit urban atmosphere. The sources of HONO, however, are still poorly characterized, yet there is a disagreement between the field observations and the model results. Here, we show that light-induced NO2 heterogeneous chemistry on authentic urban grime can make an important contribution to the total HONO levels in the urban atmosphere. The obtained results indicate that the effective uptake coefficients of NO2 on urban grime in the presence of ultraviolet light [2.6 × 1015 photons cm–2 s–1 (300 nm < λ < 400 nm)] increased markedly from (1.1 ± 0.2) × 10–6 at 0% relative humidity (RH) to (5.8 ± 0.7) × 10–6 at 90% RH, exhibiting the following linear correlation with RH: γ­(NO2) = (7.4 ± 3.3) × 10–7 + (5.5 ± 0.6) × 10–8 × RH%. The flux densities of HONO mediated by light-induced heterogeneous conversion of NO2 (46 ppb) on urban grime were enhanced by ∼1 order of magnitude from (2.3 ± 0.2) × 109 molecules cm–2 s–1 at 0% RH to (1.5 ± 0.01) × 1010 molecules cm–2 s–1 at 90% RH. This study promotes light-induced NO2 chemistry on urban grime being an important source of HONO and suggests that further experiments be performed in the future

Interfacial Ozone Oxidation Chemistry at a Riverine Surface Microlayer as a Source of Nitrogen Organic Compounds

Nitrogen (N)-containing organic compounds, including “brown carbon” (BrC), represent an important fraction of organic aerosols. However, little is known about the processes of formation of the secondarily formed N-containing organics in the atmosphere. Here, we investigated the formation of gas-phase organic compounds, including N-containing organics, through interfacial oxidation chemistry of gaseous O3 with an authentic riverine surface microlayer (SML) by using a high-resolution quadrupole Orbitrap mass spectrometer coupled to a commercial secondary electrospray ionization source. The resulting hierarchical cluster diagram obtained for real-time observation for 60 min shows the occurrence of 677 ions in positive mode. The level of N-containing organics, including BrC compounds (e.g., imidazoles), formed during the heterogeneous processing of O3 on the SML in the dark and under ultraviolet–visible light irradiation, was on average 20.7% among all samples. Many of the detected N-containing compounds comprise a CN bond, suggesting that they are potentially toxic compounds that also affect urban air quality. Overall, this study provides evidence that interfacial ozone oxidation chemistry at the riverine SML plays an important role as an additional source of air pollution in urban environments, which can affect both human health and the absorption properties of urban aerosols

Inorganic Ions Enhance the Number of Product Compounds through Heterogeneous Processing of Gaseous NO₂ on an Aqueous Layer of Acetosyringone

Methoxyphenols represent important pollutants that can participate in the formation of secondary organic aerosols (SOAs) through chemical reactions with atmospheric oxidants. In this study, we determine the influence of ionic strength, pH, and temperature on the heterogeneous reaction of NO2 with an aqueous film consisting of acetosyringone (ACS), as a proxy for methoxyphenols. The uptake coefficient of NO2 (50 ppb) on ACS (1 × 10–5 mol L–1) is γ = (9.3 ± 0.09) × 10–8 at pH 5, and increases by one order of magnitude to γ = (8.6 ± 0.5) × 10–7 at pH 11. The lifetime of ACS due to its reaction with NO2 is largely affected by the presence of nitrate ions and sulfate ions encountered in aqueous aerosols. The analysis performed by membrane inlet single-photon ionization-time-of-flight mass spectrometry (MI-SPI-TOFMS) reveals an increase in the number of product compounds and a change of their chemical composition upon addition of nitrate ions and sulfate ions to the aqueous thin layer consisting of ACS. These outcomes indicate that inorganic ions can play an important role during the heterogeneous oxidation processes in aqueous aerosol particles

The Effect of Human Occupancy on Indoor Air Quality through Real-Time Measurements of Key Pollutants

The primarily emitted compounds by human presence, e.g., skin and volatile organic compounds (VOCs) in breath, can react with typical indoor air oxidants, ozone (O3), and hydroxyl radicals (OH), leading to secondary organic compounds. Nevertheless, our understanding about the formation processes of the compounds through reactions of indoor air oxidants with primary emitted pollutants is still incomplete. In this study we performed real-time measurements of nitrous acid (HONO), nitrogen oxides (NOx = NO + NO2), O3, and VOCs to investigate the contribution of human presence and human activity, e.g., mopping the floor, to secondary organic compounds. During human occupancy a significant increase was observed of 1-butene, isoprene, and d-limonene exhaled by the four adults in the room and an increase of methyl vinyl ketone/methacrolein, methylglyoxal, and 3-methylfuran, formed as secondary compounds through reactions of OH radicals with isoprene. Intriguingly, the level of some compounds (e.g., m/z 126, 6-methyl-5-hepten-2-one, m/z 152, dihydrocarvone, and m/z 194, geranyl acetone) formed through reactions of O3 with the primary compounds was higher in the presence of four adults than during the period of mopping the floor with commercial detergent. These results indicate that human presence can additionally degrade the indoor air quality

Evolution of Indoor Cooking Emissions Captured by Using Secondary Electrospray Ionization High-Resolution Mass Spectrometry

Cooking emissions represent a major source of air pollution in the indoor environment and exhibit adverse health effects caused by particulate matter together with volatile organic compounds (VOCs). A multitude of unknown compounds are released during cooking, some of which play important roles as precursors of more hazardous secondary organic aerosols in indoor air. Here, we applied secondary electrospray ionization high-resolution mass spectrometry for real-time measurements of VOCs and particles from cooking peanut oil in the presence of 300 ppbv nitrogen oxides (NOx) generated by a gas stove in an indoor environment. More than 600 compounds have been found during and after cooking, including N-heterocyclic compounds, O-heterocyclic compounds, aldehydes, fatty acids, and oxidation products. Approximately 200 compounds appeared after cooking and were hence secondarily formed products. The most abundant compound was 9-oxononanoic acid (C9H16O3), which is likely the product formed during the heterogeneous hydroxyl (OH) radical oxidation of oleic acid (C18H34O2) or linoleic acid (C18H32O2). Real-time detection of an important number of organic compounds in indoor air poses a challenge to indoor air quality and models, which do not account for this extremely large range of compounds