Nitrite-induced oxidation of organic coatings on models for airborne particles

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components

Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, , demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of ∼1 × 1018 molecule m-2. Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.

Atmospheric chemistry of bioaerosols: heterogeneous and multiphase reactions with atmospheric oxidants and other trace gases

Once airborne, biologically-derived aerosol particles are prone to reaction with various atmospheric oxidants such as OH, NO₃, and O₃.

Advances in analytical techniques and instrumentation have now established methods for detecting, quantifying, and identifying the chemical and microbial constituents of particulate matter in the atmosphere. For example, recent cryo-TEM studies of sea spray have identified whole bacteria and viruses ejected from ocean seawater into air. A focal point of this perspective is directed towards the reactivity of aerosol particles of biological origin with oxidants (OH, NO₃, and O₃) present in the atmosphere. Complementary information on the reactivity of aerosol particles is obtained from field investigations and laboratory studies. Laboratory studies of different types of biologically-derived particles offer important information related to their impacts on the local and global environment. These studies can also unravel a range of different chemistries and reactivity afforded by the complexity and diversity of the chemical make-up of these particles. Laboratory experiments as the ones reviewed herein can elucidate the chemistry of biological aerosols.

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

Optical trapping of micron-sized droplet morphology and heterogeneous kinetics with gas-phase ozone with nitrite in a wall-less apparatus.

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.

Hydroxyl radical oxidation of phospholipid-coated NaCl particles

Biological organic compounds mixed with NaCl and other inorganic compounds in sea-salt aerosol particles react in air with oxidants such as ozone and hydroxyl (OH) radicals. Laboratory studies of model systems can provide insight into these reactions. We report here studies of the kinetics and mechanism of oxidation of unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on NaCl by gas phase OH in air at room temperature and 1 atm pressure using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) and matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF-MS) to identify possible structures of surface-bound reaction products. For comparison, some studies were also carried out on the saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) on NaCl. The calculated concentration of hydroxyl radicals, generated by photolysis of isopropyl nitrite, was (1.6-6.4) × 10(8) radicals cm(-3). Surface-bound aldehydes, ketones, organic nitrates and nitrate ions were identified as products of these reactions and structures of potential products were proposed based on mechanistic considerations combined with the MALDI-TOF-MS and DRIFTS spectra. The loss rate of vinyl hydrogen, =C-H, at 3008 cm(-1) was used to obtain a lower limit for the rate constant (k1) for addition of OH to the double bond, k1 > (3 ± 1) × 10(-13) cm(3) molecule(-1) s(-1) (1s), corresponding to a reaction probability of γ(add) > (4 ± 1) × 10(-3) (1s). Assuming that abstraction from -CH2- groups in POPC is the same as for DPPC, the percentage of the reaction that occurs by addition is ~80%. This is similar to the percent addition predicted using structure-reactivity relationships for gas-phase reactions. Decreasing the amount of POPC relative to NaCl resulted in more nitrate ion formation and less relative loss of POPC, and increasing the OH concentration resulted in a more rapid loss of POPC and faster product formation. These studies suggest that under atmospheric conditions with an OH concentration of 5 × 10(6) radicals cm(-3), the lifetime of POPC with respect to OH is <6 days.

Heterogeneous oxidation of a phosphocholine on synthetic sea salt by ozone at room temperature

The ozonolysis of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) adsorbed on salt mixtures as models for sea-salt particles was studied in real time using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) at room temperature with and without added water vapor. The salt substrates were a mixture of MgCl(2)·6H(2)O with NaCl or a commercially available synthetic sea salt. Ozone concentrations ranged from (0.25 to 3.9) × 10(13) molecules cm(-3) (0.1-1.6 ppm). The major products identified by FTIR and confirmed using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry were the secondary ozonide (SOZ) and a phospholipid aldehyde and carboxylic acid formed by scission of the double bond. The reaction probabilities for the two substrates were similar, γ = (6-7) × 10(-7), with an estimated overall uncertainty of a factor of two. The presence of water vapor decreased the yield of SOZ relative to the products formed by C[double bond, length as m-dash]C scission, but also increased the availability of the double bond for reaction, particularly on the less hygroscopic commercial sea-salt substrate. Thus, water not only affects the mechanisms and products, but also the structure of the phospholipid on the salt in a manner that affects its reactivity. The results of these studies suggest that the reactivity and products of oxidation of unsaturated phospholipids on sea-salt particles in air will be very sensitive to the nature and phase of the substrate, the amount of water present, and whether there is phase separation between the organics and the inorganic salt mixture.