A Four Carbon Organonitrate as a Significant Product of Secondary Isoprene Chemistry.

Oxidation of isoprene by nitrate radicals (NO3) or by hydroxyl radicals (OH) under high NOx conditions forms a substantial amount of organonitrates (ONs). ONs impact NOx concentrations and consequently ozone formation while also contributing to secondary organic aerosol. Here we show that the ONs with the chemical formula C4H7NO5 are a significant fraction of isoprene-derived ONs, based on chamber experiments and ambient measurements from different sites around the globe. From chamber experiments we found that C4H7NO5 isomers contribute 5%-17% of all measured ONs formed during nighttime and constitute more than 40% of the measured ONs after further daytime oxidation. In ambient measurements C4H7NO5 isomers usually dominate both nighttime and daytime, implying a long residence time compared to C5 ONs which are removed more rapidly. We propose potential nighttime sources and secondary formation pathways, and test them using a box model with an updated isoprene oxidation scheme.

A Method for the Measurement of the Water Solubility Distribution of Atmospheric Organic Aerosols.

A method for the measurement of the water solubility distribution of atmospheric organic aerosols is presented. This method is based on the extraction of organic aerosols collected on filters, using different amounts of water and measurement of the corresponding water-soluble organic carbon concentration. The solubility distribution is then estimated using the solubility basis set. The method was applied on both ambient and source-specific aerosols. Approximately 60% of the atmospheric urban organic aerosol analyzed had water solubility higher than 0.6 g L-1. Around 10% of the fresh cooking organic aerosol had water solubility higher than 10 g L-1, while 80% of the total fresh cooking organic aerosol had solubility lower than 0.1 g L-1. The ambient measurements suggested that the solubility distributions are roughly consistent with the positive matrix factorization analysis results determined during the analysis of the high-resolution time-of-flight aerosol mass spectrometry data. Most of the oxidized organic aerosol appears to have water solubility above 0.6 g L-1, while the hydrocarbon-like organic aerosol and cooking organic aerosol have water solubility less than 0.002 and 0.1 g L-1, respectively. The biomass burning organic aerosol seems to have mostly intermediate solubility in water, between 0.04 and 0.6 g L-1. The proposed approach can quantify the solubility distribution in the 0.002-15 g L-1 range. Future extension of the method to higher solubility ranges would be useful for capturing the complete solubility range for atmospheric cloud condensation studies (0.1-100 g L-1).

Gas-Particle Partitioning and SOA Yields of Organonitrate Products from NO3-Initiated Oxidation of Isoprene under Varied Chemical Regimes.

Alkyl nitrate (AN) and secondary organic aerosol (SOA) from the reaction of nitrate radicals (NO3) with isoprene were observed in the Simulation of Atmospheric PHotochemistry In a large Reaction (SAPHIR) chamber during the NO3Isop campaign in August 2018. Based on 15 day-long experiments under various reaction conditions, we conclude that the reaction has a nominally unity molar AN yield (observed range 90 ± 40%) and an SOA mass yield of OA + organic nitrate aerosol of 13-15% (with ∼50 µg m-3 inorganic seed aerosol and 2-5 µg m-3 total organic aerosol). Isoprene (5-25 ppb) and oxidant (typically ∼100 ppb O3 and 5-25 ppb NO2) concentrations and aerosol composition (inorganic and organic coating) were varied while remaining close to ambient conditions, producing similar AN and SOA yields under all regimes. We observe the formation of dinitrates upon oxidation of the second double bond only once the isoprene precursor is fully consumed. We determine the bulk partitioning coefficient for ANs (K p ∼ 10-3 m3 µg-1), indicating an average volatility corresponding to a C5 hydroxy hydroperoxy nitrate.