An Experimental Study of the Gas-Phase Reactions of NO₃ Radicals with a Series of Unsaturated Aldehydes: <i>trans</i>-2-Hexenal, <i>trans</i>-2-Heptenal, and <i>trans</i>-2-Octenal

Rate constants for the gas-phase reactions of the NO₃ radical with a series of unsaturated aldehydes, <i>trans</i>-2-hexenal, <i>trans</i>-2-heptenal, and <i>trans</i>-2-octenal, have been measured using absolute rate method at 294 ± 3 K and atmospheric pressure. This work was performed to clarify discrepancies found in the literature and thus led to a clearer view of the effect of the increasing carbon chain length on the reactivity of <i>trans</i>-2-alkenals. The rate constants were determined to be (4.7 ± 1.5) × 10^–15, (5.3 ± 1.6) × 10^–15, and (5.6 ± 2.3) × 10^–15 cm³ molecule^–1 s^–1 for <i>trans</i>-2-hexenal, <i>trans</i>-2-heptenal, and <i>trans</i>-2-octenal, respectively. These results clearly indicate that the carbon chain lengthening of the <i>trans</i>-2-alkenals does not significantly affect the rate constant. In addition, the mechanism for the reaction of NO₃ with these unsaturated aldehydes was also investigated. Unsaturated peroxynitrate-type compounds that are exclusively formed through the abstraction channel were observed as the main products

High-NO_<i>x</i> Photooxidation of <i>n</i>‑Dodecane: Temperature Dependence of SOA Formation

The temperature and concentration dependence of secondary organic aerosol (SOA) yields has been investigated for the first time for the photooxidation of <i>n</i>-dodecane (C₁₂H₂₆) in the presence of NO_<i>x</i> in the CESAM chamber (French acronym for “Chamber for Atmospheric Multiphase Experimental Simulation”). Experiments were performed with and without seed aerosol between 283 and 304.5 K. In order to quantify the SOA yields, a new parametrization is proposed to account for organic vapor loss to the chamber walls. Deposition processes were found to impact the aerosol yields by a factor from 1.3 to 1.8 between the lowest and the highest value. As with other photooxidation systems, experiments performed without seed and at low concentration of oxidant showed a lower SOA yield than other seeded experiments. Temperature did not significantly influence SOA formation in this study. This unforeseen behavior indicates that the SOA is dominated by sufficiently low volatility products for which a change in their partitioning due to temperature would not significantly affect the condensed quantities

Atmospheric Simulation Chamber Studies of the Gas-Phase Photolysis of Pyruvic Acid

Pyruvic acid is an atmospherically abundant α-keto-acid that degrades efficiently from the troposphere via gas-phase photolysis. To explore conditions relevant to the environment, 2–12 ppm pyruvic acid is irradiated by a solar simulator in the environmental simulation chamber, CESAM. The combination of the long path length available in the chamber and its low surface area to volume ratio allows us to quantitatively examine the quantum yield and photochemical products of pyruvic acid. Such details are new to the literature for the low initial concentrations of pyruvic acid employed here. We determined photolysis quantum yields of ϕ_obs^N₂ = 0.84 ± 0.1 in nitrogen and ϕ_obs^Air = 3.2 ± 0.5 in air, which are higher than those reported by previous studies that used higher partial pressures of pyruvic acid. The quantum yield greater than unity in air is due to secondary chemistry, driven by O₂, that emerges under the conditions in these experiments. The low concentration of pyruvic acid and the resulting oxygen effect also alter the product distribution such that acetic acid, rather than acetaldehyde, is the primary product in air. These results indicate that tropospheric pyruvic acid may degrade in part via photoinduced mechanisms that are different than previously expected

Methylglyoxal Uptake Coefficients on Aqueous Aerosol Surfaces

In order to predict the amount of secondary organic aerosol formed by heterogeneous processing of methylglyoxal, uptake coefficients (γ) and estimates of uptake reversibility are needed. Here, uptake coefficients are extracted from chamber studies involving ammonium sulfate and glycine seed aerosol at high relative humidity (RH ≥ 72%). Methylglyoxal uptake coefficients on prereacted glycine aerosol particles had a strong dependence on RH, increasing from γ = 0.4 × 10^–3 to 5.7 × 10^–3 between 72 and 99% RH. Continuous methylglyoxal losses were also observed in the presence of aqueous ammonium sulfate at 95% RH (γ_AS,wet = 3.7 ± 0.8 × 10^–3). Methylglyoxal uptake coefficients measured at ≥95% RH are larger than those reported for glyoxal on nonacidified, aqueous aerosol surfaces at 90% RH. Slight curvature in first-order uptake plots suggests that methylglyoxal uptake onto aqueous aerosol surfaces is not entirely irreversible after 20 min. Methylglyoxal uptake by cloud droplets was rapid and largely reversible, approaching equilibrium within the 1 min mixing time of the chamber. PTR-MS measurements showed that each cloud event extracted 3 to 8% of aerosol-phase methylglyoxal and returned it to the gas phase, likely by an oligomer hydrolysis mechanism

Nitrogen-Containing, Light-Absorbing Oligomers Produced in Aerosol Particles Exposed to Methylglyoxal, Photolysis, and Cloud Cycling

Aqueous methylglyoxal chemistry has often been implicated as an important source of oligomers in atmospheric aerosol. Here we report on chemical analysis of brown carbon aerosol particles collected from cloud cycling/photolysis chamber experiments, where gaseous methylglyoxal and methylamine interacted with glycine, ammonium, or methylammonium sulfate seed particles. Eighteen N-containing oligomers were identified in the particulate phase by liquid chromatography/diode array detection/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry. Chemical formulas were determined and, for 6 major oligomer products, MS² fragmentation spectra were used to propose tentative structures and mechanisms. Electronic absorption spectra were calculated for six tentative product structures by an ab initio second order algebraic-diagrammatic-construction/density functional theory approach. For five structures, matching calculated and measured absorption spectra suggest that they are dominant light-absorbing species at their chromatographic retention times. Detected oligomers incorporated methylglyoxal and amines, as expected, but also pyruvic acid, hydroxyacetone, and significant quantities of acetaldehyde. The finding that ∼80% (by mass) of detected oligomers contained acetaldehyde, a methylglyoxal photolysis product, suggests that daytime methylglyoxal oligomer formation is dominated by radical addition mechanisms involving CH₃CO*. These mechanisms are evidently responsible for enhanced browning observed during photolytic cloud events

Brown Carbon Production in Ammonium- or Amine-Containing Aerosol Particles by Reactive Uptake of Methylglyoxal and Photolytic Cloud Cycling

The effects of methylglyoxal uptake on the physical and optical properties of aerosol containing amines or ammonium sulfate were determined before and after cloud processing in a temperature- and RH-controlled chamber. The formation of brown carbon was observed upon methylglyoxal addition, detected as an increase in water-soluble organic carbon mass absorption coefficients below 370 nm and as a drop in single-scattering albedo at 450 nm. The imaginary refractive index component <i>k</i>₄₅₀ reached a maximum value of 0.03 ± 0.009 with aqueous glycine aerosol particles. Browning of solid particles occurred at rates limited by chamber mixing (<1 min), and in liquid particles occurred more gradually, but in all cases occurred much more rapidly than in bulk aqueous studies. Further browning in AS and methylammonium sulfate seeds was triggered by cloud events with chamber lights on, suggesting photosensitized brown carbon formation. Despite these changes in optical aerosol characteristics, increases in dried aerosol mass were rarely observed (<1 μg/m³ in all cases), consistent with previous experiments on methylglyoxal. Under dry, particle-free conditions, methylglyoxal reacted (presumably on chamber walls) with methylamine with a rate constant <i>k</i> = (9 ± 2) × 10^–17 cm³ molecule^–1 s^–1 at 294 K and activation energy <i>E</i>_a = 64 ± 37 kJ/mol