Development of a multiphase chemical model and its application in evaluation of gas-aerosol partitioning of glyoxal
by Xi Chen
M.Phil. Atmospheric, Marine and Coastal Environment
viii, 67 leaves : ill. ; 30 cm
This MPhil research work consists of two separate projects and therefore is separately summarized below....[ Read more ]
This MPhil research work consists of two separate projects and therefore is separately summarized below.
Project 1: Organic materials make up a significant fraction of ambient particulate mass. It is important to quantify their contributions to the total aerosol mass for the identification of aerosol sources and subsequently formulating effective control measures. The organic carbon (OC) mass can be determined by an aerosol carbon analyzer; however, there is no direct method for the determination of the mass of organic compounds, which also contain N, H, and O atoms in addition to C. The often-adopted approach is to estimate the organic mass (OM) from organic carbon (OC) multiplying by a factor. However, this OC-to-OM multiplier was rarely measured for a lack of appropriate methods for OM. We report here a top-down approach that couples thermal gravimetric and chemical analyses for the determination of OM. OM is taken to be the mass difference of a filter before and after heating at 550°C in air for 4 hrs minus mass losses due to elemental carbon (EC), volatile inorganic compounds (e.g., NH4NO3), and aerosol-associated water that arise from the heating treatment. The losses of EC and inorganic compounds are determined through chemical analysis of the filter before and after the heating treatment. We analyzed 37 ambient aerosol samples collected in Hong Kong during the winter of 2003, spring of 2004 and summer of 2005. A value of 2.1 ± 0.3 was found to be the appropriate factor for the year round to convert OC to OM in these Hong Kong aerosol samples. If the dominant air mass is classified into two categories, then 2.2 is applicable for continental originated air mass, 1.9 for marine air mass.
Project 2: A multiphase box model is developed to describe and investigate the pathways leading to the formation of C2 secondary organic aerosol (SOA) components through heterogeneous reactions involving precursors including glyoxal, glycoaldehyde, glycolic acid, and glyoxylic acid. Among the above C2 oxygenated compounds, glyoxal has the most abundant gas-phase concentrations. Its partitioning behaviors between the gas phase and the condensed phase (i.e., cloud or wet aerosol phase) are key to accurately modeling the potential SOA mass that can be produced from the C2 precursors. Recent smog chamber studies suggest that glyoxal could polymerize in wet aerosols. As the first application of the multiphase chemical model, this thesis work investigates the extent of the gas-condensed phase partitioning of glyoxal under two extreme scenarios. In one scenario, glyoxal is assumed to irreversibly undergo polymerization upon entering cloud water. In the second scenario, it is assumed that the partitioning is entirely governed by Henry's Law.
The two-scenarios are simulated for urban conditions adopting CAPRAM 2.4 [Ervens et al, 2003] for aqueous phase mechanism, RACM [Stockwell et al, 1997] mechanism for gas phase chemical mechanism, and the resistance model of Schwartz [I989] to treat phase transfer of 38 species. The mechanisms involves 209 species and 635 chemical reactions. In the simulation the cloud droplets are assumed to be monodispersed with a diameter of 10μm and a Liquid Water Content of 3 x 107 (vol/vol) and the duration is assumed to be 4 hrs every day. The simulation predicts the fraction of glyoxal residing in the gas-phase to be 0.863 when averaged over cloud and cloud-free periods in the scenario governed by the Henry's Law. In the polymerization scenario, this fraction is predicted to be less than 0.1. Only one study by Mutsunaga et al  measured gas-particle distribution of glyoxal and reported an average value of 0.54 for the fraction of glyoxal residing in the gas-phase. The overprediction in the Henry's Law scenario and the under-prediction of the polymerization scenario suggest that the Henry's law can not be the only mechanism governing the gas-aerosol phase partitioning of glyoxal and irreversible reactions (e.g., polymerization) forming nonvolatile species plays an important role.