Ozone Chemistry on Greasy Glass Surfaces Affects the Levels of Volatile Organic Compounds in Indoor Environments.

The chemistry of ozone (O3) on indoor surfaces leads to secondary pollution, aggravating the air quality in indoor environments. Here, we assess the heterogeneous chemistry of gaseous O3 with glass plates after being 1 month in two different kitchens where Chinese and Western styles of cooking were applied, respectively. The uptake coefficients of O3 on the authentic glass plates were measured in the dark and under UV light irradiation typical for indoor environments (320 nm < λ < 400 nm) at different relative humidities. The gas-phase product compounds formed upon reactions of O3 with the glass plates were evaluated in real time by a proton-transfer-reaction quadrupole-interface time-of-flight mass spectrometer. We observed typical aldehydes formed by the O3 reactions with the unsaturated fatty acid constituents of cooking oils. The formation of decanal, 6-methyl-5-hepten-2-one (6-MHO), and 4-oxopentanal (4-OPA) was also observed. The employed dynamic mass balance model shows that the estimated mixing ratios of hexanal, octanal, nonanal, decanal, undecanal, 6-MHO, and 4-OPA due to O3 chemistry with authentic grime-coated kitchen glass surfaces are higher in the kitchen where Chinese food was cooked compared to that where Western food was cooked. These results show that O3 chemistry on greasy glass surfaces leads to enhanced VOC levels in indoor environments.

Resolving the Formation Mechanism of HONO via Ammonia-Promoted Photosensitized Conversion of Monomeric NO2 on Urban Glass Surfaces.

Understanding the formation processes of nitrous acid (HONO) is crucial due to its role as a primary source of hydroxyl radicals (OH) in the urban atmosphere and its involvement in haze events. In this study, we propose a new pathway for HONO formation via the UVA-light-promoted photosensitized conversion of nitrogen dioxide (NO2) in the presence of ammonia (NH3) and polycyclic aromatic hydrocarbons (PAHs, common compounds in urban grime). This new mechanism differs from the traditional mechanism, as it does not require the formation of the NO2 dimer. Instead, the enhanced electronic interaction between the UVA-light excited triplet state of PAHs and NO2-H2O/NO2-NH3-H2O significantly reduces the energy barrier and facilitates the exothermic formation of HONO from monomeric NO2. Furthermore, the performed experiments confirmed our theoretical findings and revealed that the synergistic effect from light-excited PAHs and NH3 boosts the HONO formation with determined HONO fluxes of 3.6 × 1010 molecules cm-2 s-1 at 60% relative humidity (RH) higher than any previously reported HONO fluxes. Intriguingly, light-induced NO2 to HONO conversion yield on authentic urban grime in presence of NH3 is unprecedented 130% at 60% RH due to the role of NH3 acting as a hydrogen carrier, facilitating the transfer of hydrogen from H2O to NO2. These results show that NH3-assisted UVA-light-induced NO2 to HONO conversion on urban surfaces can be a dominant source of HONO in the metropolitan area.

Heterogeneous chemistry of ozone with floor cleaning agent: Implications of secondary VOCs in the indoor environment.

Human daily activities such as cooking, and cleaning can affect the indoor air quality by releasing primary emitted volatile organic compounds (VOCs), as well as by the secondary product compounds formed through reactions with ozone (O3) and hydroxyl radicals (OH). However, our knowledge about the formation processes of the secondary VOCs is still incomplete. We performed real-time measurements of primary VOCs released by commercial floor-cleaning detergent and the secondary product compounds formed by heterogeneous reaction of O3 with the constituents of the cleaning agent by use of high-resolution mass spectrometry. We measured the uptake coefficients of O3 on the cleaning detergent at different relative humidities in dark and under different light intensities (320 nm < λ < 400 nm) relevant for the indoor environment. On the basis of the detected compounds we developed tentative reaction mechanisms describing the formation of the secondary VOCs. Intriguingly, under light irradiation the formation of valeraldehyde was observed based on the photosensitized chemistry of acetophenone which is a constituent of the cleaning agent. Finally, we modeled the observed mixing ratios of three aldehydes, glyoxal, methylglyoxal, and 4-oxopentanal with respect to real-life indoor environment. The results suggest that secondary VOCs initiated by ozone chemistry can additionally impact the indoor air pollution.

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.

Daytime SO2 chemistry on ubiquitous urban surfaces as a source of organic sulfur compounds in ambient air.

The reactions of sulfur dioxide (SO2) with surface-bound compounds on atmospheric aerosols lead to the formation of organic sulfur (OS) compounds, thereby affecting the air quality and climate. Here, we show that the heterogeneous reaction of SO2 with authentic urban grime under near-ultraviolet sunlight irradiation leads to a large suite of various organic compounds including OS released in the gas phase. Calculations indicate that at the core area of Guangzhou, building surface uptake of SO2 is 15 times larger than uptake of SO2 on aerosol surfaces, yielding ~20 ng m-3 of OS that represents an important fraction of the observed OS compounds (60 to 200 ng m-3) in ambient aerosols of Chinese megacities. This chemical pathway occurring during daytime can contribute to the observed fraction of OS compounds in aerosols and improve the understanding of haze formation and urban air pollution.

Unveiling the pH-Dependent Yields of H2O2 and OH by Aqueous-Phase Ozonolysis of m-Cresol in the Atmosphere.

Hydrogen peroxide (H2O2) and hydroxyl radical (OH) are important oxidants in the atmospheric aqueous phase such as cloud droplets and deliquescent aerosol particles, playing a significant role in the chemical transformation of organic and inorganic pollutants in the atmosphere. Atmospheric aqueous-phase chemistry has been considered to be a source of H2O2 and OH. However, our understanding of the mechanisms of their formation in atmospheric waters is still incomplete. Here, we show that the aqueous-phase reaction of dissolved ozone (O3) with substituted phenols such as m-cresol represents an important source of H2O2 and OH exhibiting pH-dependent yields. Intriguingly, the formation of H2O2 through the ring-opening mechanism is strongly promoted under lower pH conditions (pH 2.5-3.5), while higher pH favors the ring-retaining pathways yielding OH. The rate constant of the reaction of O3 with m-cresol increases with increasing pH. The reaction products formed during the ozonolysis of m-cresol are analyzed by an Orbitrap mass spectrometer, and reaction pathways are suggested based on the identified product compounds. This study indicates that aqueous-phase ozonolysis of phenolic compounds might be an alternative source of H2O2 and OH in the cloud, rain, and liquid water of aerosol particles; thus, it should be considered in future model studies.

Physical and chemical characterization of urban grime: An impact on the NO2 uptake coefficients and N-containing product compounds.

Urban grime represents an important environmental surface for heterogeneous reactions in urban environment. Here, we assess the physical and chemical properties of urban grime collected during six consecutive months in downtown of Guangzhou, China. There is a significant variation of the uptake coefficients of NO2 on the urban grime as a function of the relative humidity (RH). In absence of water molecules (0% RH), the light-induced uptake coefficients of NO2 on urban grime samples collected during six months are very similar in order of ≈10-6. At 80% RH, depending on the sampling month the light-induced uptake coefficient of NO2 can reach one order of magnitude higher values (1.5 × 10-5, at 80% RH) compared to those uptakes at 0% RH. In presence of 80% RH, there are strong correlations between the measured NO2 uptakes and the concentrations of the water soluble carbon, soluble anions, polycyclic aromatic hydrocarbons and n-alkanes depicted in the urban grime. These correlations, demonstrate that surface adsorbed water on urban grime play an important role for the uptakes of NO2. The heterogeneous conversion of NO2 on two-month old urban grime under sunlight irradiation (68 W m-2, 300 nm < λ < 400 nm) at 60% RH leads to the formation of unprecedented HONO surface flux of 4.7 × 1010 molecules cm-2 s-1 which is higher than all previously observed HONO fluxes, thereby affecting the oxidation capacity of the urban atmosphere. During the heterogeneous chemistry of NO2 with urban grime, the unsaturated and N-containing organic compounds are released in the gas phase which can affect the air quality in the urban environment.

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.

Effect of Inorganic Salts on N-Containing Organic Compounds Formed by Heterogeneous Reaction of NO2 with Oleic Acid.

Fatty acids are ubiquitous constituents of grime on urban and indoor surfaces and they represent important surfactants on organic aerosol particles in the atmosphere. Here, we assess the heterogeneous processing of NO2 on films consisting of pure oleic acid (OA) or a mixture of OA and representative salts for urban grime and aerosol particles, namely Na2SO4 and NaNO3. The uptake coefficients of NO2 on OA under light irradiation (300 nm < λ < 400 nm) decreased with increasing relative humidity (RH), from (1.4 ± 0.1) × 10-6 at 0% RH to (7.1 ± 1.6) × 10-7 at 90% RH. The uptake process of NO2 on OA gives HONO as a reaction product, and the highest HONO production was observed upon the heterogeneous reaction of NO2 with OA in the presence of nitrate (NO3-) ions. The formation of gaseous nitroaromatic compounds was also enhanced in the presence of NO3- ions upon light-induced heterogeneous processing of NO2 with OA, as revealed by membrane inlet single-photon ionization time-of-flight mass spectrometry (MI-SPI-TOFMS). These results suggest that inorganic salts can affect the heterogeneous conversion of gaseous NO2 on fatty acids and enhance the formation of HONO and other N-containing organic compounds in the atmosphere.

Ionic Strength Effect Triggers Brown Carbon Formation through Heterogeneous Ozone Processing of Ortho-Vanillin.

Methoxyphenols are an important class of compounds emerging from biomass combustion, and their reactions with ozone can generate secondary organic aerosols in the atmosphere. Here, we use a vertical wetted wall flow tube reactor to evaluate the effect of ionic strength on the heterogeneous reaction of gas-phase ozone (O3) with a liquid film of o-vanillin (o-VL) (2-hydroxy-3-methoxybenzaldehyde), as a proxy for methoxyphenols. Typical for moderately acidic aerosols, at fixed pH = 5.6, the uptake coefficients (γ) of O3 on o-VL ([o-VL] = 1 × 10-5 mol L-1) increase from γ = (1.9 ± 0.1) × 10-7 in the absence of Na2SO4 to γ = (6.8 ± 0.3) × 10-7 at I = 0.2 mol L-1, and then, it decreases again. The addition of NO3- ions only slightly decreases the uptakes of O3. Ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) reveals that the formation of multicore aromatic compounds is favored upon heterogeneous O3 reaction with o-VL, in the presence of SO42- and NO3- ions. The addition of NO3- ions favors the formation of nitrooxy (-ONO2) or oxygenated nitrooxy group of organonitrates, which are components of brown carbon that can affect both climate and air quality.

Unexpectedly High Indoor HONO Concentrations Associated with Photochemical NO2 Transformation on Glass Windows.

Nitrous acid (HONO) is an important gaseous pollutant contributing to indoor air pollution because it causes adverse health effects and is the main source of hydroxyl radicals (OH). Here, we present direct measurements of HONO produced through light-induced heterogeneous reactions of NO2 with grime adsorbed on glass window. The uptake coefficients of NO2 [γ(NO2)] on the glass plates from the kitchen increased markedly from (2.3 ± 0.1) × 10-6 at 0% RH to (4.1 ± 0.5) × 10-6 at 90% RH. We report a significant quantity of daytime HONO produced in the kitchen, compared to the living room and bedroom. Kinetic modeling suggests that phase state and bulk diffusivity play important roles in the NO2 uptake; the best fit to the measured uptake coefficients is obtained with fixed diffusion coefficients. Photon scattering may be occurring at the surface of the films, leading to enhanced photon-excitation rates of polycyclic aromatic hydrocarbons. By taking these effects into account, the results from this study indicate that the HONO yields obtained in this study can explain the missing HONO in the photochemical models describing the indoor air chemistry.

Light-Enhanced Heterogeneous Conversion of NO2 to HONO on Solid Films Consisting of Fluorene and Fluorene/Na2SO4: An Impact on Urban and Indoor Atmosphere.

Polycyclic aromatic hydrocarbons (PAHs) as constituents of urban grime and indoor surfaces can impact the photochemical conversion of nitrogen dioxide (NO2) to nitrous acid (HONO) thereby impacting the oxidation capacity of the atmosphere. In this study we investigate the effect of relative humidity (RH%), light intensity, and NO2 concentrations on uptake coefficients (γ) of NO2 on solid film consisting of fluorene (FL) and a mixture of FL and Na2SO4 as a proxy for urban and indoor grime at ambient pressure and temperature. γ(NO2) on solid FL increased markedly from (5.7 ± 1.7) × 10-7 at 0% RH to (4.6 ± 1.0) × 10-6 at 90% RH. The NO2 to HONO conversion yield, (ΔHONO/ΔNO2)%, increases with RH from 40% at 0% RH up to 80% at 60-90% RH, indicating that the water molecules favor the formation of HONO up to 60% RH. These results suggest that the heterogeneous photochemical reaction of NO2 on FL and FL/Na2SO4 can be an important source of HONO in the urban environment and indoor atmosphere and should be considered in photochemical models.