Unveiling the effect of O2 on the photochemical reaction of NO2 with polycyclic aromatic hydrocarbons

The photochemical reaction of NO2 with organics may be a source of atmospheric HONO during the daytime. Here, the conversion of NO2 to HONO on polycyclic aromatic hydrocarbons (PAHs) under solar irradiation under aerobic and anaerobic conditions was investigated using a flow tube reactor coupled to a NOx analyzer. O2 played an inhibition role in NO2 uptake and HONO formation on PAHs, as shown by 7%-45% and 15%-52% decrease in NO2 uptake coefficient (γ) and HONO yield (YHONO), respectively. The negative effect of O2 on the reaction between NO2 and PAHs should be attributed to three reasons. First, O2 could compete with NO2 for the available sites on PAHs. Second, the quenching of the triple excited state of PAHs (3PAHs*) by O2 inhibited the NO2 uptake. Third, NO3- formed under aerobic conditions reduced the conversion efficiency of NO2 to HONO. The environmental implications suggested that the NO2 uptake on PAHs could contribute to a HONO source strength of 10-120 ppt h-1 in the atmosphere.

Formation of nitrogen-containing gas phase products from the heterogeneous (photo)reaction of NO2 with gallic acid

Heterogeneous reaction of gas phase NO2 with atmospheric humic-like substances (HULIS) is potentially an important source of volatile organic compounds (VOCs) including nitrogen (N)-containing compounds, a class of brown carbon of emerging importance. However, the role of ubiquitous water-soluble aerosol components in this multiphase chemistry, namely nitrate and iron ions, remains largely unexplored. Here, we used secondary electrospray ionization ultrahigh-resolution mass spectrometry for real-time measurements of VOCs formed during the heterogeneous reaction of gas phase NO2 with a solution containing gallic acid (GA) as a proxy of HULIS at pH 5 relevant for moderately acidic aerosol particles. Results showed that the number of detected N-containing organic compounds largely increased from 4 during the NO2 reaction with GA in the absence of nitrate and iron ions to 55 in the presence of nitrate and iron ions. The N-containing compounds have reduced nitrogen functional groups, namely amines, imines and imides. These results suggest that the number of N-containing compounds is significantly higher in deliquescent aerosol particles due to the influence of relatively higher ionic strength from nitrate ions and complexation/redox reactivity of iron cations compared to that in the dilute aqueous phase representative of cloud, fog, and rain water.

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.

Heterogeneous photochemical uptake of NO2 on the soil surface as an important ground-level HONO source

Nitrous acid (HONO) production from the heterogeneous photochemical reaction of NO₂ on several Chinese soils was performed in a cylindrical reactor at atmospheric pressure. The NO₂ uptake coefficient (γ) and HONO yield (Y_HONO) on different soils were (0.42-5.16) × 10^-5 and 6.3%-69.6%, respectively. Although the photo-enhanced uptake of NO₂ on different soils was observed, light could either enhance or inhibit the conversion efficiency of NO₂ to HONO, depending on the properties of the soils. Soils with lower pH generally had larger γ and Y_HONO. Soil organics played a key role in HONO formation through the photochemical uptake of NO₂ on soil surfaces. The γ showed a positive correlation with irradiation and temperature, while it exhibited a negative relationship with relative humidity (RH). Y_HONO inversely depended on the soil mass (0.32-3.25 mg cm^-2), and it positively relied on the irradiance and RH (7%-22%). There was a maximum value for Y_HONO at 298 K. Based on the experimental results, HONO source strengths from heterogeneous photochemical reaction of NO₂ on the soil surfaces were estimated to be 0.2-2.7 ppb h^-1 for a mixing layer height of 100 m, which could account for the missing daytime HONO sources in most areas.

Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property

Heterogeneous photochemical reaction of O₃ with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O₃ analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O₃ on HA. The initial uptake coefficient (γ_i) and the steady-state uptake coefficient (γ_ss) of O₃ under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γ_i and γ_ss on HA varied in the range of 0.76-2.77 × 10^-5 and 1.50-9.55 × 10^-6, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O₃ concentration, O₂ content, temperature, relative humidity (RH) and HA solution pH. Both γ_i and γ_ss showed linear dependence on the total irradiance (0-2.07 × 10¹⁶ photons/(cm²⋅s)) of the light source, and increased with the HA mass (0-3.2 μg/cm²), temperature (278-298 K) and HA solution pH (4.0-9.6). However, they showed negative correlations with the initial O₃ concentration and O₂ content. The γ_i remained constant in the RH range of 7%-60%, while γ_ss exhibited the maximum value at RH = 20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH₂, CH₃, aromatic CC, OH, CO, COOH and COO^-. HA aged by O₃ exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600 nm wavelength (AAE_300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.

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.

Development and application of a twin open-top chambers method to measure soil HONO emission in the North China Plain

HONO (nitrous acid) is a crucial precursor for tropospheric OH radicals, and its sources are not well understood. In the past decade, soil was proven to be a potential source for HONO. However, more field measurements of soil HONO emission flux are needed to explore the mechanism and its impact on regional air quality. Here, we developed a system based on twin open-top chambers (OTCs) and wet chemical methods to measure HONO emission flux from agricultural soil in the North China Plain (NCP). The performance of the OTC system was tested under laboratory and field measurement conditions. The results showed that the system could reflect the strength (>90%) and variation of gas emission with an average residence time of 4-5 min. The greenhouse effect and chemical reaction interference in the chamber was proven to have no significant influence on the HONO flux measurement. Field measurement revealed that agricultural soil before fertilization was an important source of HONO. The emission flux showed radiation-dependent or temperature-dependent variation, with a peak of 3.21 ng m^-2 s^-1 at noontime that could account for approximately 67 pptv h^-1 of the missing HONO source under an assumed mixing layer height of 300 m. Fertilization substantially accelerated HONO emission, which was rationally attributed to biological processes including nitrification. Considering the high fertilization rate in the NCP and other similar regions in China, HONO emission from agricultural soil likely has enormous impact on regional photochemistry and air quality, suggesting that more research should be conducted on this aspect.

Significant HONO formation by the photolysis of nitrates in the presence of humic acids

The generation of HONO and NO₂ by the photolysis of nitrates in the presence of humic acids (HA) was measured under various conditions. The photolysis experiments of HA, KNO₃ and KNO₃/HA under simulated sunlight was carried out by a flow tube reactor at ambient temperature and pressure. HONO and NO₂ were major products by the photolysis of KNO₃. By contrast, the photolysis of HA and KNO₃/HA mainly generated HONO. HA significantly enhanced the formation of HONO during the photolysis process of KNO₃. With increasing the KNO₃ mass, the HONO formation rate (R_HONO) on KNO₃/HA increased while the photolysis rate normalized by the KNO₃ mass exhibited an opposite trend. R_HONO on KNO₃/HA linearly increased with irradiation intensity (88-262 W/m²) and relative humidity (7-70%), whereas it linearly decreased with the pH (pH = 2-12). In addition, the reaction paths of the HONO formation by the photolysis of nitrates in the presence of HA were proposed according to experimental results. Finally, atmospheric implications of the enhanced HONO formation by the photolysis of nitrates in the presence of HA were discussed.

Enhanced photochemical conversion of NO2 to HONO on humic acids in the presence of benzophenone

The photochemical conversion of NO₂ to HONO on humic acids (HA) in the presence of benzophenone (BP) was investigated using a flow tube reactor coupled to a NO_x analyzer at ambient pressure. BP significantly enhanced the reduction of NO₂ to HONO on HA under simulated sunlight, as shown by the increase of NO₂ uptake coefficient (γ) and HONO yield with the mass ratio of BP to HA. The γ and HONO yield on the mixtures of HA and BP obviously depended on the environmental conditions. Both γ and HONO yield increased with the increase of irradiation intensity and temperature, whereas they decreased with pH. The γ exhibited a negative dependence on the NO₂ concentration, which had slight influences on the HONO yield. There were maximum values for the γ and HONO yield at relative humidity (RH) of 22%. Finally, atmospheric implications about the photochemical reaction of NO₂ and HA in the presence of photosensitive species were discussed.

Evidence for Quinone Redox Chemistry Mediating Daytime and Nighttime NO2-to-HONO Conversion on Soil Surfaces

Humic acid (HA) is thought to promote NO₂ conversion to nitrous acid (HONO) on soil surfaces during the day. However, it has proven difficult to identify the reactive sites in natural HA substrates. The mechanism of NO₂ reduction on soil surrogates composed of HA and clay minerals was studied by use of a coated-wall flow reactor and cavity-enhanced spectroscopy. Conversion of NO₂ to HONO in the dark was found to be significant and correlated to the abundance of C-O moieties in HA determined from the X-ray photoelectron spectra of the C 1s region. Twice as much HONO was formed when NO₂ reacted with HA that was photoreduced by irradiation with UV-visible light compared to the dark reaction; photochemical reactivity was correlated to the abundance of C═O moieties rather than C-O groups. Bulk electrolysis was used to generate HA in a defined reduction state. Electrochemically reduced HA enhanced NO₂-to-HONO conversion by a factor of 2 relative to non-reduced HA. Our findings suggest that hydroquinones and benzoquinones, which are interchangeable via redox equilibria, contribute to both thermal and photochemical HONO formation. This conclusion is supported by experiments that studied NO₂ reactivity on mineral surfaces coated with the model quinone, juglone. Results provide further evidence that redox-active sites on soil surfaces drive ground-level NO₂-to-nitrite conversion in the atmospheric boundary layer throughout the day, while amphoteric mineral surfaces promote the release of nitrite formed as gaseous HONO.

Heterogeneous reaction of NO2 with soot at different relative humidity

The influences of relative humidity (RH) on the heterogeneous reaction of NO₂ with soot were investigated by a coated wall flow tube reactor at ambient pressure. The initial uptake coefficient (γ _initial) of NO₂ showed a significant decrease with increasing RH from 7 to 70%. The γ _initial on "fuel-rich" and "fuel-lean" soot at RH = 7% was (2.59 ± 0.20) × 10^-5 and (5.92 ± 0.34) × 10^-6, respectively, and it decreased to (5.49 ± 0.83) × 10^-6 and (7.16 ± 0.73) × 10^-7 at RH = 70%, respectively. Nevertheless, the HONO yields were almost independent of RH, with average values of (72 ± 3)% for the fuel-rich soot and (60 ± 2)% for the fuel-lean soot. The Langmuir-Hinshelwood mechanism was used to demonstrate the negative role of RH in the heterogeneous uptake of NO₂ on soot. The species containing nitrogen formed on soot can undergo hydrolysis to produce carboxylic species or alcohols at high RH, accompanied by the release of little gas-phase HONO and NO.

Photolysis of Particulate Nitrate as a Source of HONO and NOx

Photolysis of nitric acid on the surface has been found recently to be greatly enhanced from that in the gas phase. Yet, photolysis of particulate nitrate (pNO₃) associated with atmospheric aerosols is still relatively unknown. Here, aerosol filter samples were collected both near the ground surface and throughout the troposphere on board the NSF/NACR C-130 aircraft. The photolysis rate constants of pNO₃ were determined from these samples by directly monitoring the production rates of nitrous acid (HONO) and nitrogen dioxide (NO₂) under UV light (>290 nm) irradiation. Scaled to the tropical noontime condition on the ground level (solar zenith angle = 0°), the normalized photolysis rate constants (j_pNO3^N) are in the range from 6.2 × 10^-6 s^-1 to 5.0 × 10^-4 s^-1 with a median of 8.3 × 10^-5 s^-1 and a mean (±1 SD) of (1.3 ± 1.2) × 10^-4 s^-1. Chemical compositions, specifically nitrate loading and organic matter, affect the rate of photolysis. Extrapolated to ambient pNO₃ loading conditions, e.g. ≤ 10 nmol m^-3, the mean j_pNO3^N value is over 1.8 × 10^-4 s^-1 in the suburban, rural, and remote environments. Photolysis of particulate nitrate is thus a source of HONO and NO₂ in the troposphere.

Heterogeneous Photochemical Conversion of NO2 to HONO on the Humic Acid Surface under Simulated Sunlight

The poor understanding of HONO sources in the daytime highlights the importance of the heterogeneous photochemical reaction of NO2 with aerosol or soil surfaces. The conversion of NO2 to HONO on humic acid (HA) under simulated sunlight was investigated using a flow tube reactor at ambient pressure. The uptake coefficient (γ) of NO2 linearly increased with irradiation intensity and HA mass in the range of 0-2.0 μg/cm(2), while it decreased with the NO2 concentration. The HONO yield was found to be independent of irradiation intensity, HA mass, and NO2 concentration. The temperature (278-308 K) had little influence on both γ and HONO yield. Additionally, γ increased continuously with relative humidity (RH, 7-70%), and a maximum HONO yield was observed at 40% RH. The heterogeneous photochemical reaction of NO2 with HA was explained by the Langmuir-Hinshelwood mechanism.

Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

HONO formation by photolysis of HNO₃ on clean surfaces is no significant source of HONO and NO_x in the atmosphere.

Review of the bulk and surface chemistry of iron in atmospherically relevant systems containing humic-like substances

The current state of knowledge and future research directions of the bulk and surface chemistry of iron relevant to atmospheric surfaces are reviewed.

Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint

Gaseous nitrogen dioxide (NO2) represents an oxidant that is present in relatively high concentrations in various indoor settings. Remarkably increased NO2 levels up to 1.5 ppm are associated with homes using gas stoves. The heterogeneous reactions of NO2 with adsorbed water on surfaces lead to the generation of nitrous acid (HONO). Here, we present a HONO source induced by heterogeneous reactions of NO2 with selected indoor paint surfaces in the presence of light (300 nm<λ<400 nm). We demonstrate that the formation of HONO is much more pronounced at elevated relative humidity. In the presence of light (5.5 W m(-2)), an increase of HONO production rate of up to 8.6·10(9) molecules cm(-2) s(-1) was observed at [NO2]=60 ppb and 50% relative humidity (RH). At higher light intensity of 10.6 (W m(-2)), the HONO production rate increased to 2.1·10(10) molecules cm(-2) s(-1). A high NO2 to HONO conversion yield of up to 84% was observed. This result strongly suggests that a light-driven process of indoor HONO production is operational. This work highlights the potential of paint surfaces to generate HONO within indoor environments by light-induced NO2 heterogeneous reactions.

Oxidation and nitration of tyrosine by ozone and nitrogen dioxide: reaction mechanisms and biological and atmospheric implications

The nitration of tyrosine by atmospheric oxidants, O3 and NO2, is an important cause for the spread of allergenic diseases. In the present study, the mechanism and pathways for the reaction of tyrosine with the atmospheric oxidants O3 and NO2 are studied using DFT-M06-2X, B3LYP, and B3LYP-D methods with the 6-311+G(d,p) basis set. The energy barrier for the initial oxidation reactions is also calculated at the CCSD(T)/6-31+G(d,p) level of theory. The reaction is studied in gas, aqueous, and lipid media. The initial oxidation of tyrosine by O3 proceeds by H atom abstraction and addition reactions and leads to the formation of six different intermediates. The subsequent nitration reaction is studied for all the intermediates, and the results show that the nitration affects both the side chain and the aromatic ring of tyrosine. The rate constant of the favorable oxidation and nitration reaction is calculated using variational transition state theory over the temperature range of 278-350 K. The spectral properties of the oxidation and nitration products are calculated at the TD-M06-2X/6-311+G(d,p) level of theory. The fate of the tyrosine radical intermediate is studied by its reaction with glutathione antioxidant. This study provides an enhanced understanding of the oxidation and nitration of tyrosine by O3 and NO2 in the context of improving the air quality and reducing the allergic diseases.

Surface water enhances the uptake and photoreactivity of gaseous catechol on solid iron(III) chloride

Uptake and photoreactivity of catechol-Fe complexes are investigated at the gas/solid interface under humid and dry conditions, along with the nature of the hydrogen-bonding network of adsorbed water. Catechol was chosen as a simple model for organics in aerosols. Iron chloride was used to distinguish ionic mobility from binding to coordinated iron(III) in hematite. Studies were conducted using diffuse reflectance infrared Fourier transform spectroscopy as a function of irradiation time. Results show that adsorbed water at 30% relative humidity (RH), not light, increases the concentration of adsorbed catechol by a factor of 3 over 60 min relative to dry conditions. Also, our data show that, at 30% RH and under light and dark conditions, growth factors describing the concentration of adsorbed catechol are very similar suggesting that light does not significantly enhance the uptake of catechol vapor on FeCl3. Surface water also enhances the initial photodecay kinetics of catechol-Fe complexes at 30% RH by a factor of 10 relative to control experiments (RH < 1%, or no FeCl3 under humid conditions). Absorptions assigned to carbonyl groups were not observed with irradiation time, which was explained by the dominance of FeCl(2+) species relative to FeOH(2+) in the highly acidic "quasi-liquid" phase at 30% RH. Clear differences in the hydrogen-bonding network upon gaseous catechol uptake are observed in the dark and light and during the photodecay of adsorbed catechol. The implications of these results on our understanding of interfacial processes in aged iron-containing surfaces are discussed.

Multiphase chemical kinetics of the nitration of aerosolized protein by ozone and nitrogen dioxide

Proteins contained in pollen and other biological particles are nitrated by ozone and nitrogen dioxide in polluted air. The nitration can enhance the allergenic potential of proteins, which may contribute to the increasing prevalence of allergic diseases. The reactive uptake of NO(2) by aerosolized protein (bovine serum albumin) was investigated in an aerosol flow tube using the short-lived radioactive tracer (13)N. In the absence of O(3), the NO(2) uptake coefficient was below detection limit (γ(NO2) < 10(-6)), but with 20-160 ppb O(3) γ(NO2) increased from ~10(-6) to ~10(-4). Using the kinetic multilayer model of surface and bulk chemistry (KM-SUB), the observed time and concentration dependence can be well reproduced by a multiphase chemical mechanism involving ozone-generated reactive oxygen intermediates (ROIs), but not by NO(3) radicals formed in the gas phase. Product studies show the formation of protein dimers, suggesting that the ROIs are phenoxy radical derivatives of the amino acid tyrosine (tyrosyl radicals) which are also involved in physiological protein nitration processes. Our results imply that proteins on the surface of aerosol particles undergo rapid nitration in polluted air, while the rate of nitration in bulk material may be low depending on phase state and surface-to-volume ratio.

Atmospheric photosensitized heterogeneous and multiphase reactions: from outdoors to indoors

This proposal involves direct photolysis processes occurring in the troposphere incorporating photochemical excitation and intermolecular energy transfer. The study of such processes could provide a better understanding of ·OH radical formation pathways in the atmosphere and in consequence, of a more accurate prediction of the oxidative capacity of the atmosphere. Compounds that readily absorb in the tropospheric actinic window (ionic organic complexes, PAHs, aromatic carbonyl compounds) acting as potential photosensitizers of atmospheric relevant processes are explored. The impact of hotosensitation on relevant systems which could act as powerful atmospheric reactors,that is, interface ocean-atmosphere, urban and forest surfaces and indoor air environments is also discussed.