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

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

Emission of Volatile Organic Compounds to the Atmosphere from Photochemistry in Thermokarst Ponds in Subarctic Canada

Climate warming is accelerating the thawing of permafrost, which contains almost twice as much carbon as the atmosphere, to a point where a large quantity of dissolved organic matter (DOM) is being mobilized toward surface waters, including thermokarst ponds. DOM can be partially photodegraded into volatile organic compounds (VOCs), which are little studied in Arctic environments. The main objective of this work is to identify and quantify the VOCs emitted to the gas phase by photochemistry from thermokarst water sampled in four ponds from two study sites in northern Quebec. VOC emissions were characterized by proton-transfer reaction mass spectrometry. Results show rapid photoproduction of between 35 and 59 VOCs when DOM water samples are exposed to radiation. Our results also show that the quality of DOM is a more important factor to control VOC photoproduction than the quantity of DOM. Depending on the assumptions used in upscaling our laboratory results to the field sites, calculations yield net carbon fluxes between 1.93 and 174 μmol C m–2 d–1. While these values are small compared to literature values of CO2 and CH4 fluxes from thermokarst ponds, this process represents an important flux of reactive molecules that could affect Arctic atmospheric chemistry

New Directions: Fundamentals of atmospheric chemistry: Keeping a three-legged stool balanced

New Directions: Fundamentals of atmospheric chemistry: Keeping a three-legged stool balance

Spontaneous Iodide Activation at the Air–Water Interface of Aqueous Droplets

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air–water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3– formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir–Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols

Particle-Phase Photosensitized Radical Production and Aerosol Aging

Atmospheric aerosol particles may contain light absorbing (brown carbon, BrC), triplet forming organic compounds that can sustain catalytic radical reactions and thus contribute to oxidative aerosol aging. We quantify UVA induced radical production initiated by imidazole-2-carboxaldehyde (IC), benzophenone (BPh). and 4-benzoylbenzoic acid (BBA) in the presence of the nonabsorbing organics citric acid (CA), shikimic acid (SA), and syringol (Syr) at varying mixing ratios. We observed a maximum HO₂ release of 10¹³ molecules min^–1 cm^–2 at a mole ratio <i>X</i>_BPh < 0.02 for BPh in CA. Mixtures of either IC or BBA with CA resulted in 10¹¹–10¹² molecules min^–1 cm^–2 of HO₂ at mole ratios (<i>X</i>_IC and <i>X</i>_BBA) between 0.01 and 0.15. HO₂ release was affected by relative humidity (<i>RH</i>) and film thickness suggesting coupled photochemical reaction and diffusion processes. Quantum yields of HO₂ formed per absorbed photon for IC, BBA and BPh were between 10^–7 and 5 × 10^–5. The nonphotoactive organics, Syr and SA, increased HO₂ production due to the reaction with the triplet excited species ensuing ketyl radical production. Rate coefficients of the triplet of IC with Syr and SA measured by laser flash photolysis experiments were <i>k</i>_Syr = (9.4 ± 0.3) × 10⁸ M^–1 s^–1 and <i>k</i>_SA = (2.7 ± 0.5) × 10⁷ M^–1 s^–1. A simple kinetic model was used to assess total HO₂ and organic radical production in the condensed phase and to upscale to ambient aerosol, indicating that BrC induced radical production may amount to an upper limit of 20 and 200 M day^–1 of HO₂ and organic radical respectively, which is greater or in the same order of magnitude as the internal radical production from other processes, previously estimated to be around 15 M per day

Leakage Rates of Refrigerants CFC-12, HCFC-22, and HFC-134a from Operating Mobile Air Conditioning Systems in Guangzhou, China: Tests inside a Busy Urban Tunnel under Hot and Humid Weather Conditions

Determining the leakage rates of halogenated refrigerants from operating mobile air conditioning systems (MACs) is a challenging task. Here, we take advantage of a heavily trafficked tunnel with a traffic flow of over 40,000 motor vehicles per day in south China. We carried out measurements in 2014 on hot and humid days, and therefore, it is reasonable to assume that essentially all of the MAC units would be turned on to ensure the thermo-comfort of the occupants. Thus, we obtained the leakage rates of the three most important refrigerants from the operating MACs aboard the on-road vehicles. The emission factors (EFs) of HFC-134a, HCFC-22, and CFC-12 from the on-road operating MACs are 1.27 ± 0.11, 0.47 ± 0.04, and 0.17 ± 0.04 mg km^–1 veh^–1, respectively. Normalized by the percentages of vehicles using different refrigerants in their MACs, the emission rates of HFC-134a, HCFC-22, and CFC-12 are 52.2, 329, and 59.5 mg h^–1 veh^–1, respectively. This emission rate of HFC-134a is approximately 10 times higher than those previously reported in Europe for stationary conditions and a whole-lifetime average of fugitive losses. The unusually high leakage rates suggest that improving the leak tightness of MACs in China would help to greatly lower their emissions. The global warming potentials associated with refrigerant leakage is equal to 1.4% of the CO₂ directly emitted due to fuel consumptions

SO₂ Uptake on Oleic Acid: A New Formation Pathway of Organosulfur Compounds in the Atmosphere

Organosulfates are tracers for secondary organic aerosol (SOA) formation. We propose a new mechanism of organosulfur product formation in the atmosphere, in which sulfur dioxide (SO₂) reacts directly with alkenes. The experiments were conducted at the gas–liquid interface with a coated-wall flow tube reactor. It was shown, for the first time, that SO₂ reacts efficiently with the unsaturated bond in oleic acid under atmospheric conditions (without ozone), leading to the formation of C₉ and C₁₈ organosulfur products. The associated uptake coefficients were in excess of 10^–6, decreasing with initial SO₂ concentration and increasing with humidity. These results might explain a fraction of organosulfur products detected in atmospheric particles. This work tends to elucidate the role of organosulfates’ interfacial chemistry as a potentially unrecognized pathway for their contribution to SOA formation; however, it remains to be determined how significant this pathway is to the overall organosulfate abundances measured in ambient aerosol

Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface

We report on experiments that probe photosensitized chemistry at the air/water interface, a region that does not just connect the two phases but displays its own specific chemistry. Here, we follow reactions of octanol, a proxy for environmentally relevant soluble surfactants, initiated by an attack by triplet-state carbonyl compounds, which are themselves concentrated at the interface by the presence of this surfactant. Gas-phase products are determined using PTR-ToF-MS, and those remaining in the organic layer are determined by ATR-FTIR spectroscopy and HPLC-HRMS. We observe the photosensitized production of carboxylic acids as well as unsaturated and branched-chain oxygenated products, compounds that act as organic aerosol precursors and had been thought to be produced solely by biological activity. A mechanism that is consistent with the observations is detailed here, and the energetics of several key reactions are calculated using quantum chemical methods. The results suggest that the concentrating nature of the interface leads to its being a favorable venue for radical reactions yielding complex and functionalized products that themselves could initiate further secondary chemistry and new particle formation in the atmospheric environment

Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth’s climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments