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Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids. Here we show that isoprene, carbon monoxide and methane can each suppress the instantaneous mass and the overall mass yield derived from monoterpenes in mixtures of atmospheric vapours. We find that isoprene ‘scavenges’ hydroxyl radicals, preventing their reaction with monoterpenes, and the resulting isoprene peroxy radicals scavenge highly oxygenated monoterpene products. These effects reduce the yield of low-volatility products that would otherwise form secondary organic aerosol. Global model calculations indicate that oxidant and product scavenging can operate effectively in the real atmosphere. Thus highly reactive compounds (such as isoprene) that produce a modest amount of aerosol are not necessarily net producers of secondary organic particle mass and their oxidation in mixtures of atmospheric vapours can suppress both particle number and mass of secondary organic aerosol. We suggest that formation mechanisms of secondary organic aerosol in the atmosphere need to be considered more realistically, accounting for mechanistic interactions between the products of oxidizing precursor molecules (as is recognized to be necessary when modelling ozone production). Adding reactive gases such as isoprene to mixtures lowers the production of secondary organic aerosol in the atmosphere, thus reducing the atmospheric particulate burden, with implications for human health and climate.

This work presents the results from flow reactor studies on the formation of carboxylic acids from limonene oxidation under various conditions. A High Resolution Time Of Flight acetate Chemical Ionisation Mass Spectrometer (HR – TOF – CIMS) was used in combination with the Filter Inlet for Gases and AEROsols (FIGAERO) to measure the carboxylic acid profile in the gas and particle phases. The results revealed that limonene oxidation produced large amounts of carboxylic acids which are important contributors to secondary organic aerosol (SOA) formation. The highest 10 acids contributed 56–91 % to the total gas-phase signal and the dominant gas-phase species in most experiments were C8H12O4, C9H14O4, C7H10O4 and C10H16O3. The particle-phase composition was generally more complex than the gas-phase composition and the highest 10 acids contributed 47–92 % to the total signal. The dominant species in the particle phase were C8H12O5, C9H14O5, C9H12O5 and C10H16O4. The measured concentrations of dimers in the particle phase were very low, indicating that acidic dimers play a minor role in SOA formation via ozone/OH oxidation of limonene. Spearman correlation analysis of the produced carboxylic acid species and experimental parameters were helpful in interpreting the results. Based on the various experimental conditions, the acidic composition for all experiments were modelled using the Master Chemical Mechanisms (MCM). Significant concentrations of 11 acids, from a total of 16 acids, included in MCM were measured with the CIMS. However, the model predictions were, in some cases, inconsistent with the measurement results, especially in the case of the OH dependence. Reaction mechanisms are suggested to fill-in the knowledge gaps. Based on the mechanisms proposed in this work, nearly 75 % of the qualitative gas-phase signal of the low concentration (ppb converted), humid, mixed oxidant experiment can be explained.

The gas phase nitrate radical (NO3•) initiated oxidation of limonene can produce organic nitrate species with varying physical properties. Low-volatility products can contribute to secondary organic aerosol (SOA) formation and organic nitrates may serve as a NOx reservoir, which could be especially important in regions with high biogenic emissions. This work presents the measurement results from flow reactor studies on the reaction of NO3• and limonene using a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS) combined with a Filter Inlet for Gases and AEROsols (FIGAERO). Major condensed-phase species were identified, and the identity and volatility properties of the most prevalent organic nitrates in the produced SOA were determined. Analysis of multiple experiments resulted in the identification of several dominant species (including C10H15NO6, C10H17NO6, C8H11NO6, C10H17NO7, and C9H13NO7) that occurred in the SOA under all conditions considered. The observed and expected (listed) products (associated with the Master Chemical Mechanism (MCM) limonene mechanism) were compared, and many non-listed species were identified. Additionally, the formation of dimers was consistently observed and these species resided almost completely in the particle phase. The identities of these species are discussed, and formation mechanisms are proposed. Cluster analysis of the desorption temperatures corresponding to the analyzed particle-phase species yielded at least five distinct groupings based on a combination of molecular weight and desorption profile. Overall, the results indicate that the oxidation of limonene by NO3• produces a complex mixture of highly oxygenated monomer and dimer products that contribute to SOA formation.