Abstract
Abstract. The atmospheric oxidation of aromatic compounds is an important source of secondary organic aerosol (SOA) in urban areas. The oxidation of aromatics depends strongly on the levels of nitrogen oxides (NOx). However, details of the mechanisms by which oxidation occurs have only recently been elucidated. Xu et al. (2015) developed an updated version of the gas-phase Caltech Atmospheric Chemistry Mechanism (CACM) designed to simulate toluene and m-xylene oxidation in chamber experiments over a range of NOx conditions. The output from such a mechanism can be used in thermodynamic predictions of gas–particle partitioning leading to SOA. The current work reports the development of a model for SOA formation that combines the gas-phase mechanism of Xu et al. (2015) with an updated lumped SOA-partitioning scheme (Model to Predict the Multi-phase Partitioning of Organics, MPMPO) that allows partitioning to multiple aerosol phases and that is designed for use in larger-scale three-dimensional models. The resulting model is termed aroCACM/MPMPO 1.0. The model is integrated into the University of California, Irvine – California Institute of Technology (UCI-CIT) Airshed Model, which simulates the South Coast Air Basin (SoCAB) of California. Simulations using 2012 emissions indicate that “low-NOx” pathways to SOA formation from aromatic oxidation play an important role, even in regions that typically exhibit high-NOx concentrations.
Highlights
Atmospheric aerosol particles negatively affect human health, contribute to reduced visibility and impact Earth’s climate through their ability to scatter and absorb radiation and affect cloud properties (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006)
The formation of isoprene-derived organic nitrates, along with their subsequent reactions with OH, were added. These modifications had little effect on modeled NO and NO2 concentrations or Secondary organic aerosol (SOA) concentration. This would be expected as biogenic volatile organic compounds (VOCs) play a much greater role in SOA formation in a forested region, than in the relatively high-nitrogen oxides (NOx), arid South Coast Air Basin (SoCAB) region
This work reports the development of aroCACM/MPMPO, a gas-phase and SOA model designed for use in largescale chemical transport models that includes important routes to SOA formation from the oxidation of aromatic species. aroCACM/MPMPO makes use of updated schemes for vapor pressure estimation and grouping SOA species and includes both aqueous- and organic-phase partitioning for all SOA species using an equilibrium-partitioning module
Summary
Atmospheric aerosol particles negatively affect human health, contribute to reduced visibility and impact Earth’s climate through their ability to scatter and absorb radiation and affect cloud properties (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). In order to simulate the formation of SOA and other secondary pollutants such as ozone, air quality models require chemical mechanisms that predict gas-phase oxidation chemistry over a wide range of NOx concentrations in a computationally efficient manner. Such mechanisms can be highly reduced (lumped) or highly specific depending on computational demands and application. As an intermediate approach to the non-specific and comprehensive methods mentioned above, Xu et al (2015) updated the Caltech Atmospheric Chemical Mechanism (CACM) to include SOA formation from the gas-phase oxidation of toluene and m-xylene and evaluated the model through comparison to chamber experiments. The updated CACM of Xu et al (2015) was incorporated along with appropriate lumped SOA species and a treatment of aqueous-phase partitioning of aromatic-derived SOA
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