Temperature-Dependent Oxidation of Hydroxylated Aldehydes by •OH, SO4•-, and NO3• Radicals in the Atmospheric Aqueous Phase.

T-dependent aqueous-phase rate constants were determined for the oxidation of the hydroxy aldehydes, glyceraldehyde, glycolaldehyde, and lactaldehyde, by the hydroxyl radicals (•OH), the sulfate radicals (SO4•-), and the nitrate radicals (NO3•). The obtained Arrhenius expressions for the oxidation by the •OH radical are: k(T,GLYCERALDEHYDE+OH•) = (3.3 ± 0.1) × 1010 × exp((-960 ± 80 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+OH•) = (4.3 ± 0.1) × 1011 × exp((-1740 ± 50 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+OH•) = (1.6 ± 0.1) × 1011 × exp((-1410 ± 180 K)/T)/L mol-1 s-1; for the SO4•- radical: k(T,GLYCERALDEHYDE+SO4•-) = (4.3 ± 0.1) × 109 × exp((-1400 ± 50 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+SO4•-) = (10.3 ± 0.3) × 109 × exp((-1730 ± 190 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+SO4•-) = (2.2 ± 0.1) × 109 × exp((-1030 ± 230 K)/T)/L mol-1 s-1; and for the NO3• radical: k(T,GLYCERALDEHYDE+NO3•) = (3.4 ± 0.2) × 1011 × exp((-3470 ± 460 K)/T)/L mol-1 s-1, k(T,GLYCOLALDEHYDE+NO3•) = (7.8 ± 0.2) × 1011 × exp((-3820 ± 240 K)/T)/L mol-1 s-1, k(T,LACTALDEHYDE+NO3•) = (4.3 ± 0.2) × 1010 × exp((-2750 ± 340 K)/T)/L mol-1 s-1, respectively. Targeted simulations of multiphase chemistry reveal that the oxidation by OH radicals in cloud droplets is important under remote and wildfire influenced continental conditions due to enhanced partitioning. There, the modeled average aqueous •OH concentration is 2.6 × 10-14 and 1.8 × 10-14 mol L-1, whereas it is 7.9 × 10-14 and 3.5 × 10-14 mol L-1 under wet particle conditions. During cloud periods, the aqueous-phase reactions by •OH contribute to the oxidation of glycolaldehyde, lactaldehyde, and glyceraldehyde by about 35 and 29%, 3 and 3%, and 47 and 37%, respectively.

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.

Ionic Strength Effect Alters the Heterogeneous Ozone Oxidation of Methoxyphenols in Going from Cloud Droplets to Aerosol Deliquescent Particles.

Methoxyphenols are one of the most abundant classes of biomarker tracers for atmospheric wood smoke pollution. The reactions of atmospheric oxidants (ozone, OH) with methoxyphenols can contribute to the formation of secondary organic aerosols (SOA). Here, for the first time, we use the well-established vertical wetted wall flow tube (VWWFT) reactor to assess the effect of ionic strength (I), pH, temperature, and ozone concentration on the reaction kinetics of ozone with acetosyringone (ACS), as a representative methoxyphenol compound. At fixed pH 3, typical for acidic atmospheric deliquescent particles, and at I = 0.9 M adjusted by Na2SO4, the uptake coefficient (γ) of O3 increases by 2 orders of magnitude from γ = (5.0 ± 0.8) × 10-8 on neat salt solution (Na2SO4) to γ = (6.0 ± 0.01) × 10-6 on a mixture of ACS and Na2SO4. The comparison of the uptake coefficients of O3 at different pH values indicates that the reaction kinetics strongly depends on the acidity of the phenolic group of ACS. The observed different reactivity of gas-phase ozone with ACS has implications for ozone uptake by the dilute aqueous phase of cloud droplets and by aerosol deliquescent particles loaded with inorganic salts, and it can affect the formation of SOA in the atmosphere.

A Novel Insight into the Ozone-Skin Lipid Oxidation Products Observed by Secondary Electrospray Ionization High-Resolution Mass Spectrometry.

Emissions of secondary products through reactions of oxidants, ozone (O3), and hydroxyl radical (·OH) with human skin lipids have become increasingly important in indoor environments. Here, we evaluate the secondary organic compounds formed through heterogeneous reactions of gaseous O3 with hand skin lipids by using a high-resolution quadrupole Orbitrap mass spectrometer coupled to a commercial secondary electrospray ionization (SESI) source. More than 600 ions were detected over a period of less than 40 min real-time measurements, among which 53 ions were characterized with a significant increasing trend in signal intensity at the presence of O3. Based on the detected ions, we suggest detailed reaction pathways initiated by ozone oxidation of squalene that results in primary and secondary ozonides; we noticed for the first time that these products may be further cleaved by direct reaction of nucleophilic ammonia (NH3), emitted from human skin. Finally, we estimate the fate of secondarily formed carbonyl compounds with respect to their gas-phase reactions with ·OH, O3, and NO3 and compared with their removal by air exchange rate (AER) with outdoors. The obtained results suggest that human presence is a source of an important number of organic compounds, which can significantly influence the air quality in indoor environments.

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.

Ionic-Strength Effects on the Reactive Uptake of Ozone on Aqueous Pyruvic Acid: Implications for Air-Sea Ozone Deposition.

A vertical wetted-wall flow-tube technique was used to explore the ionic strength effects at the air-water interface in mediating the sea-surface reaction between ozone (O3) and pyruvic acid (PA). The uptake coefficients of ozone on aqueous PA increase substantially with the concentrations of bromide (Br-) ions, clearly indicating that the dry deposition of ozone could be significantly enhanced due to the presence of carbonyl compounds such as PA at the bromide-rich sea surface. Based on the observed uptake coefficients, the estimated deposition velocity of ozone (100 ppb) for a nanomolar range of PA concentrations is ∼1 × 10-3 m s-1, which represents a significant contribution to the known deposition velocity of ozone at the sea surface. The analysis of reaction products by ultra-high-resolution Fourier transform-ion cyclotron resonance mass spectrometry suggests the formation of oligomers during both the dark and light-induced heterogeneous reactions between gaseous O3 and PA occurring at the surface of a dilute aqueous phase (representative of cloud droplets). The detected high-molecular-weight compounds are much more complex than the oligomeric species identified during the photolytic degradation of bulk aqueous PA alone.

Tracking photodegradation products and bond-cleavage reaction pathways of triclosan using ultra-high resolution mass spectrometry and stable carbon isotope analysis.

Triclosan (TCS) is an antimicrobial compound ubiquitously found in surface waters throughout the world. Although several studies have focused on the photochemical degradation of TCS, there is still limited knowledge about its environmental fate. In this study, we got molecular-level insights into the photochemical degradation of TCS. Significant stable carbon isotope fractionation was observed during photodegradation; different bond-cleavage reaction pathways under different photolytic conditions were characterized, using compound specific isotope analysis (CSIA). Photochemical modeling of TCS photodegradation showed that direct photolysis would be the main transformation pathway if pH > 7, even in presence of dissolved organic matter. Moreover, by use of ultrahigh resolution mass spectrometry, FT-ICR-MS, a broad and complex spectrum of organic by-products (some of which potentially toxic, as assessed by a quantitative structure-activity relationship approach) were identified. A detailed reaction mechanism was developed on the basis of the detected compounds. A possible sequence of steps leading to some of the detected product compounds in aqueous solution is suggested.

Assessing indoor gas phase oxidation capacity through real-time measurements of HONO and NOx in Guangzhou, China.

The hydroxyl radical (OH) is one of the most important oxidants controlling the oxidation capacity of the indoor atmosphere. One of the main OH sources indoors is the photolysis of nitrous acid (HONO). In this study, real-time measurements of HONO, nitrogen oxides (NOx) and ozone (O3) in an indoor environment in Guangzhou, China, were performed under two different conditions: (1) in the absence of any human activity and (2) in the presence of cooking. The maximum NOx and HONO levels drastically increased from 15 and 4 ppb in the absence of human activity to 135 and 40 ppb during the cooking event, respectively. The photon flux was determined for the sunlit room, which has a closed south-east oriented window. The photon flux was used to estimate the photolysis rate constants of NO2, J(NO2), and HONO, J(HONO), which span the range between 8 × 10-5 and 1.5 × 10-5 s-1 in the morning from 9:30 to 11:45, and 8.5 × 10-4 and 1.5 × 10-4 s-1 at noon, respectively. The OH concentrations calculated by photostationary state (PSS) approach, observed around noon, are very similar, i.e., 2.4 × 106 and 3.1 × 106 cm-3 in the absence of human activity and during cooking, respectively. These results suggest that under "high NOx" conditions (NOx higher than a few ppb) and with direct sunlight in the room, the NOx and HONO chemistry would be similar, independent of the geographic location of the indoor environment, which facilitates future modeling studies focused on indoor gas phase oxidation capacity.