The reversible partitioning of glyoxal was studied in simulation chamber experiments for the first time by time-resolved measurements of gas-phase and particle-phase concentrations in sulfate-containing aerosols. Two complementary methods for the measurement of glyoxal particle-phase concentrations are compared: (1) an offline method utilizing filter sampling of chamber aerosols followed by HPLC-MS/MS analysis and (2) positive matrix factorization (PMF) analysis of aerosol mass spectrometer (AMS) data. Ammonium sulfate (AS) and internally mixed ammonium sulfate/fulvic acid (AS/FA) seed aerosols both show an exponential increase of effective Henry's law coefficients (KH,eff) with AS concentration (cAS, in mol kg(-1) aerosol liquid water, m = molality) and sulfate ionic strength, I(SO4(2-)) (m). A modified Setschenow plot confirmed that "salting-in" of glyoxal is responsible for the increased partitioning. The salting constant for glyoxal in AS is K(S)CHOCHO = (-0.24 ± 0.02) m(-1), and found to be independent of the presence of FA. The reversible glyoxal uptake can be described by two distinct reservoirs for monomers and higher molecular weight species filling up at characteristic time constants. These time constants are τ1 ≈ 10(2) s and τ2 ≈ 10(4) s at cAS < 12 m, and about 1-2 orders of magnitude slower at higher cAS, suggesting that glyoxal uptake is kinetically limited at high salt concentrations.

Abstract.A new off-line instrument to quantify peroxides in aerosol particles using iodometry in long path absorption spectroscopy has been developed and is called peroxide long path absorption photometer (Peroxide-LOPAP). The new analytical setup features important technical innovations compared to hitherto published iodometric peroxide measurements. Firstly, the extraction, chemical conversion and measurement of the aerosol samples are performed in a closed oxygen-free (∼ 1 ppb) environment. Secondly, a 50-cm optical detection cell is used for an increased photometric sensitivity. The limit of detection was 0.1 µM peroxide in solution or 0.25 nmol m −3 with respect to an aerosol sample volume of 1 m 3 . The test reaction was done at a constant elevated temperature of 40 • C and the reaction time was 60 min.Calibration experiments showed that the test reaction with all reactive peroxides, i.e. hydrogen peroxide (H 2 O 2 ), peracids and peroxides with vicinal carbonyl groups (e.g. lauroyl peroxide) goes to completion and their sensitivity (slope of calibration curve) varies by only ±5 %. However, very inert peroxides have a lower sensitivity. For example, tert-butyl hydroperoxide shows only 37 % sensitivity compared to H 2 O 2 after 1 h. A kinetic study revealed that even after 5 h only 85 % of this inert compound had reacted.The time trends of the peroxide content in secondary organic aerosol (SOA) from the ozonolysis and photooxidation of α-pinene in smog chamber experiments were measured. The highest mass fraction of peroxides with 34 % (assuming a molecular weight of 300 g mol −1 ) was found in freshly generated SOA from α-pinene ozonolysis. Mass fractions decreased with increasing NO levels in the photooxidation experiments. A decrease of the peroxide content was also observed with aging of the aerosol, indicating a decomposition of peroxides in the particles.

Fossil fuel-powered vehicles emit significant particulate matter, for example, black carbon and primary organic aerosol, and produce secondary organic aerosol. Here we quantify secondary organic aerosol production from two-stroke scooters. Cars and trucks, particularly diesel vehicles, are thought to be the main vehicular pollution sources. This needs re-thinking, as we show that elevated particulate matter levels can be a consequence of 'asymmetric pollution' from two-stroke scooters, vehicles that constitute a small fraction of the fleet, but can dominate urban vehicular pollution through organic aerosol and aromatic emission factors up to thousands of times higher than from other vehicle classes. Further, we demonstrate that oxidation processes producing secondary organic aerosol from vehicle exhaust also form potentially toxic 'reactive oxygen species'.

A series of smog chamber (SC) experiments was conducted to identify factors responsible for the discrepancy between ambient and SC aerosol degree of oxygenation. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer is used to compare mass spectra from α-pinene photooxidation with ambient aerosol. Composition is compared in terms of the fraction of particulate CO₂⁺, a surrogate for carboxylic acids, vs. the fraction of C₂H₃O⁺, a surrogate for aldehydes, alcohols and ketones, as well as in the Van Krevelen space, where the evolution of the atomic hydrogen-to-carbon ratio (H : C) vs. the atomic oxygen-to-carbon ratio (O : C) is investigated. Low (near-ambient) organic mass concentrations were found to be necessary to obtain oxygenation levels similar to those of low-volatility oxygenated organic aerosol (LV-OOA) commonly identified in ambient measurements. The effects of organic mass loading and OH (hydroxyl radical) exposure were decoupled by inter-experiment comparisons at the same integrated OH concentration. An OH exposure between 3 and 25 × 10⁷ cm⁻³ h is needed to increase O : C by 0.05 during aerosol aging. For the first time, LV-OOA-like aerosol from the abundant biogenic precursor α-pinene was produced in a smog chamber by oxidation at typical atmospheric OH concentrations. Significant correlation between measured secondary organic aerosol (SOA) and reference LV-OOA mass spectra is shown by Pearson's R² values larger than 0.90 for experiments with low organic mass concentrations between 1.2 and 18 μg m⁻³ at an OH exposure of 4 × 10⁷ cm⁻³ h, corresponding to about two days of oxidation time in the atmosphere, based on a global mean OH concentration of ~ 1 × 10⁶ cm⁻³. α-Pinene SOA is more oxygenated at low organic mass loadings. Because the degree of oxygenation influences the chemical, volatility and hygroscopic properties of ambient aerosol, smog chamber studies must be performed at near-ambient concentrations to accurately simulate ambient aerosol properties

Abstract. This study investigates the evolution of shipemitted aerosol particles using the stochastic particleresolved model PartMC-MOSAIC (Particle Monte Carlo model-Model for Simulating Aerosol Interactions and Chemistry). Comparisons of our results with observations from the QUANTIFY (Quantifying the Climate Impact of Global and European Transport Systems) study in 2007 in the English Channel and the Gulf of Biscay showed that the model was able to reproduce the observed evolution of total number concentration and the vanishing of the nucleation mode consisting of sulfate particles. Further process analysis revealed that during the first hour after emission, dilution reduced the total number concentration by four orders of magnitude, while coagulation reduced it by an additional order of magnitude. Neglecting coagulation resulted in an overprediction of more than one order of magnitude in the number concentration of particles smaller than 40 nm at a plume age of 100 s. Coagulation also significantly altered the mixing state of the particles, leading to a continuum of internal mixtures of sulfate and black carbon. The impact on cloud condensation nuclei (CCN) concentrations depended on the supersaturation threshold S at which CCN activity was evaluated. For the base case conditions, characterized by a low formation rate of secondary aerosol species, neglecting coagulation, but simulating condensation, led to an underestimation of CCN concentrations of about 37 % for S = 0.3 % at the end of the 14-h simulation. In contrast, for supersaturations higher than 0.7 %, neglecting coagulation resulted in an overestimation of CCN concentration, about 75 % for S = 1 %. For S lower than 0.2 % the differences between simulations including coagulation and neglecting coagulation were negligible. Neglecting condensation, but simulating coagulation did not impact the CCN concentrations below 0.2 % and resulted in an underestimation of CCN concentrations for larger supersaturations, e.g., 18 % for S = 0.6 %. We also explored the role of nucleation for the CCN concentrations in the ship plume. For the base case the impact of nucleation on CCN concentrations was limited, but for a sensitivity case with higher formation rates of secondary aerosol over several hours, the CCN concentrations increased by an order of magnitude for supersaturation thresholds above 0.3 %.