Kinetics and Product Formation during the Photooxidation of Butanol on Atmospheric Mineral Dust

Mineral dust particles have photochemical properties that can promote heterogeneous reactions on their surfaces and therefore alter atmospheric composition. Even though dust photocatalytic nature has received significant attention recently, most studies have focused on inorganic trace gases. Here, we investigated how light changes the chemical interactions between butanol and Arizona test dust, a proxy for mineral dust, under atmospheric conditions. Butanol uptake kinetics were measured, exploring the effects of UV light irradiation intensity (0–1.4 mW/cm²), relative humidity (0–10%), temperature (283–298 K), and butanol initial concentration (20–55 ppb). The composition of the gas phase was monitored by a high-resolution proton-transfer-reaction mass spectrometer (PTR-ToF-MS) operating in H₃O⁺ mode. Water was observed to play a significant role, initially reducing heterogeneous processing of butanol but enhancing reaction rates once it evaporated. Gas phase products were identified, showing that surface reactions of adsorbed butanol led to the emission of a variety of carbonyl containing compounds. Under actinic light these compounds will photolyze and produce hydroxyl radicals, changing dust processing from a sink of VOC into a source of reactive compounds

Nitrogen-Containing Compounds Enhance Light Absorption of Aromatic-Derived Brown Carbon

The formation of secondary brown carbon (BrC) is chemically complex, leading to an unclear relationship between its molecular composition and optical properties. Here, we present an in-depth investigation of molecular-specific optical properties and aging of secondary BrC produced from the photooxidation of ethylbenzene at varied NOx levels for the first time. Due to the pronounced formation of unsaturated products, the mass absorption coefficient (MAC) of ethylbenzene secondary organic aerosols (ESOA) at 365 nm was higher than that of biogenic SOA by a factor of 10. A high NOx level ([ethylbenzene]0/[NOx]0 –1) was found to significantly increase the average MAC300–700nm of ESOA by 0.29 m2 g–1. The data from two complementary high-resolution mass spectrometers and quantum chemical calculations suggested that nitrogen-containing compounds were largely responsible for the enhanced light absorption of high-NOx ESOA, and multifunctional nitroaromatic compounds (such as C8H9NO3 and C8H9NO4) were identified as important BrC chromophores. High-NOx ESOA underwent photobleaching upon direct exposure to ultraviolet light. Photolysis did not lead to the significant decomposition of C8H9NO3 and C8H9NO4, indicating that nitroaromatic compounds may serve as relatively stable nitrogen reservoirs and would effectively absorb solar radiation during the daytime

The High Pressure Inside Aerosol Particles Enhances Photochemical Reactions of Biomass Burning Compounds

Ultrafine aerosols (d < 100 nm) are the most abundant particles in the atmosphere with strong implications for climate and air quality. Their formation and evolution remain a subject of significant uncertainty. Recently, the implication of a fundamental and hitherto unconsidered characteristic of ultrafine aerosols has been highlighted: the Young–Laplace pressure. Here, the photochemical reaction of vanillin, a proxy for biomass burning compounds, under various high pressures was investigated. Using high-resolution mass spectrometry and UV–visible spectroscopy, we demonstrated that vanillin photodegradation was faster by ∼40% under high pressures typical of atmospheric nanoparticles. Chemical characterization shows that dimer formation, ring-opening, and cleavage processes were greatly favored (i.e., up to ∼250%) at high pressures. While the formation of light-absorbing compounds appears to be nonaffected, their decomposition through photooxidative processes was shown to be 50% faster at high pressures. This study establishes that the high pressure inside nanometric-sized aerosols has to be considered as a key property that can significantly impact photochemical processes involved in aerosol growth and aging

Quantification and Mechanistic Investigation of the Spontaneous H₂O₂ Generation at the Interfaces of Salt-Containing Aqueous Droplets

There is now much evidence that OH radicals and H2O2 are spontaneously generated at the air–water interface of atmospheric aerosols. Here, we investigated the effect of halide anions (Cl–, Br–, I–), which are abundant in marine aerosols, on this H2O2 production. Droplets were generated via nebulization of water solutions containing Na2SO4, NaCl, NaBr, and NaI containing solutions, and H2O2 was monitored as a function of the salt concentration under atmospheric relevant conditions. The interfacial OH radical formation was also investigated by adding terephthalic acid (TA) to our salt solutions, and the product of its reaction with OH, hydroxy terephthalic acid (TAOH), was monitored. Finally, a mechanistic investigation was performed to examine the reactions participating in H2O2 production, and their respective contributions were quantified. Our results showed that only Br– contributes to the interfacial H2O2 formation, promoting the production by acting as an electron donor, while Na2SO4 and NaCl stabilized the droplets by only reducing their evaporation. TAOH was observed in the collected droplets and, for the first time, directly in the particle phase by means of online fluorescence spectroscopy, confirming the interfacial OH production. A mechanistic study suggests that H2O2 is formed by both OH and HO2 self-recombination, as well as HO2 reaction with H atoms. This work is expected to enhance our understanding of interfacial processes and assess their impact on climate, air quality, and health

Photochemistry of Atmospheric Dust: Ozone Decomposition on Illuminated Titanium Dioxide

The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 × 10−6 to 3 × 10−5. The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume

NH₃ Weakens the Enhancing Effect of SO₂ on Biogenic Secondary Organic Aerosol Formation

Anthropogenic air pollutants can be involved in biogenic secondary organic aerosol (SOA) formation. However, such interactions are currently one of the least understood aspects of atmospheric chemistry. Herein, SOA formation via chemical interactions between anthropogenic SO2, NH3, and O3 and biogenic β-caryophyllene was investigated. It is shown that although SO2 considerably enhanced SOA formation, this enhancing effect was weakened by NH3 when SO2 and NH3 coexisted. NH3-induced neutralization of particle acidity generated by SO2 oxidation may be the primary driving factor of this weakening effect. Molecular-level characterization using high-resolution quadrupole time-of-flight mass spectrometry revealed additional connections between NH3-induced changes in SOA composition and aerosol acidity. Specifically, the lower relative abundances of several main products generated in the presence of SO2 and NH3 than those formed in the presence of only SO2 were consistent with their suppressed formation by lower seed acidity. The suppression of oligomer formation by NH3 provided more evidence for the weakening of acid-catalyzed processes caused by acidity neutralization. Accordingly, the current study demonstrates that NH3 as a less effectively regulated alkaline gas resulting from an unbalanced reduction of different pollutants must be considered with caution when evaluating effects of SO2 on SOA formation via anthropogenic–biogenic interactions

Reactive Uptake of Ozone by Chlorophyll at Aqueous Surfaces

We report the results of two complementary studies of the heterogeneous reaction between gas-phase ozone and aqueous chlorophyll. In the first experiment, the chlorophyll is present at the air–water interface and its concentration is measured as a function of time, using laser-induced fluorescence, to obtain the surface kinetics. Under most experimental conditions, these are well described using a Langmuir–Hinshelwood formalism. The second experiment was carried out in a wetted-wall flowtube apparatus and measured the uptake coefficient of ozone by the chlorophyll solution. The uptake coefficient decreases with increasing ozone concentration, consistent with the surface mechanism found in the fluorescence experiment. The two experiments agree that the uptake coefficient for ozone by such chlorophyll samples is ∼2–5 × 10−6 with unpolluted boundary layer ozone concentrations. At low wind speed, the reaction between ozone and chlorophyll at the sea surface may represent the driving force for ozone deposition at the ocean surface, significantly increasing its deposition velocity there

Photoenhanced Uptake of NO₂ by Pyrene Solid Films

We report uptake kinetics measurements of the heterogeneous reaction of gas phase NO2 with solid films of pyrene. By using a coated flow tube equipped with several near-ultraviolet (UV) emitting lamps (range 300−420 nm), we examined the effect of actinic radiation on the heterogeneous loss kinetics of nitrogen dioxide. With atmospherically relevant concentrations of NO2, (20−119 ppbv), the uptake ranged from below 10−7 in the dark to 3.5 × 10−6 under near-UV irradiation. Under illuminated conditions, the uptake coefficient decreased markedly with increasing gas-phase concentration, suggestive of a Langmuir−Hinshelwood-type surface reaction mechanism. The NO2 reactivity was not a function of deposited Pyrene mass or of the relative humidity (in the range 10−89%) and depended linearly on the intensity of illumination. Gas-phase product analysis indicated that approximately 50% of the NO2 loss could be accounted for by HONO and NO release. These experimental results are discussed along with a possible nitration mechanism

Differences in Photosensitized Release of VOCs from Illuminated Seawater versus Freshwater Surfaces

Recent studies have shown that photochemical reactions occurring at the air–water interface are a source of volatile organic compounds (VOCs) to the atmosphere. We report here the photosensitized formation of VOCs from illuminated freshwater and seawater mimics containing nonanoic acid (NA) and/or Suwannee River natural organic matter (SRNOM). Under an atmosphere of air, the total blank-corrected steady-state concentration of VOCs formed from illuminated seawater coated with nonanoic acid is somewhat smaller than that formed from freshwater, suggesting some differences in photochemical pathways for the two substrates. The total blank-corrected steady-state concentration of VOCs more than doubles from both freshwater and seawater NA-coated surfaces under nitrogen compared to air. The addition of SRNOM as a photosensitizer induces some photochemistry from the seawater sample under air, but no chemistry is seen with freshwater or under nitrogen for either substrate. Adding SRNOM to the nonanoic acid-containing solutions roughly doubles the total steady-state concentration of VOCs emitted from both freshwater and seawater surfaces under air. The small differences in product formation for the two substrates imply some difference in the photochemical mechanisms operating in freshwater versus seawater, which may be due to the presence of halides and metals as well as pH differences between the two aqueous systems