Abstract
An observation-based model for secondary inorganic aerosol (OBM-SIA) is developed to determine the sensitivity of formation of sulfate (SO42–) and nitrate (NO3–) to changes in their precursors. The model incorporates CB05 chemical mechanism with inclusion of two recently discovered OH enhancement pathways and essential aqueous phase chemical reactions, thermodynamic equilibriums for gas-aerosol phase apportionment and size distribution of SO42– and NO3–. A sequence of present time-frame observations of precursors and particle compositions are used to drive the simulation and to determine responses to perturbed emission rates of precursors. OBM-SIA obviates the need for uncertain emission inventories and boundary layer dynamic conditions, and makes use of data that are increasingly available due to recent advances in online instruments for various gaseous and aerosol components, thereby offering a cost-effective tool for the analysis of SIA-precursor relationships in the atmosphere. OBM-SIA is applied to hourly gaseous and particulate composition data during a wintertime pollution episode in Hong Kong. The major pathway responsible for the production of NO3– is the reaction of OH and NO2 in the gas phase, while the gas phase oxidation of SO2 by OH and aqueous phase oxidation of S(IV) by O3 contribute most significantly on SO42– production. NO3– production is more sensitive to the reduction of NOx and anthropogenic VOCs. Reduction of SO42– formation is however most sensitive to SO2 reduction and it becomes more effective as more SO2 is reduced. The work illustrates the utility of OBM-SIA in suggesting evidence-based control strategies for effective reduction of targeted SIAs.
Highlights
Ambient particle formation and accumulation in the atmosphere involve a series of physical and chemical processes in the lower troposphere
observation-based model for secondary inorganic aerosol (OBM-secondary inorganic aerosol (SIA)) uses a sequence of present time-frame observations of precursors and particle compositions and meteorological data to drive the simulation, ensuring that the calculations are carried out for the proper combinations of nitrogen oxides (NOx) and VOCs conditions
Utility of the model is demonstrated by carrying out an OBM-SIA analysis of observation data of gaseous precursors, SIA components, and relevant meteorological parameters collected in a residential/commercial neighborhood site in Hong Kong during a wintertime period (17–26 December, 2009)
Summary
Ambient particle formation and accumulation in the atmosphere involve a series of physical and chemical processes in the lower troposphere. OBM-SIA uses the hourly concentration of the observed atmospheric species as well as the meteorological condition data to calculate the formation potential of secondary SO42– and NO3–. This is reasonable, as the majority of NO3– is contributed by local production, while SO42– is mostly contributed by transport function. A series of simplifying assumptions, including (1) transport is assumed negligible when resolving concentrations of the unspecified species by Eq (2); (2) transport function is assumed unvaried when the control strategies is conducted; and (3) thermodynamic equilibrium is assumed constantly achieved in the sensitivity studies; (4) since OBM-SIA is a 0-D model, it uses surface observations to the entire PBL. OBM-SIA cannot replace the EBM in prognostic modeling to calculate the pollutants level or to estimate precursor reduction needed to bring an area into attainment (Cardelino and Chameides, 1995)
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