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Secondary inorganic soluble aerosol in Hong Kong : continuous measurement, formation mechanism discussion and improvement of an observation-based model to study control strategies

Authors Xue, Jian
Issue Date 2012
Summary Secondary inorganic aerosols (SIA), comprising of sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+), are major constituents of fine particle (PM2.5, particle matter with aerodynamic diameter smaller than 2.5 μm). They play important roles in PM-related adverse impacts on human health, acid deposition, visibility degradation and climate system. Past studies on SIA in Hong Kong (HK) or the Pearl River Delta (PRD) mostly rely on filter-based measurements which typically need 12 h or longer to collect enough material for off-line analysis. Work in this thesis focuses on half-hourly or hourly measurements of PM2.5 SIA in two locations in HK using a continuous system, PILS (Particle-into-Liquid System) coupled to two ion chromatographs. The high-resolution data sets allow the examination of SIA temporal dynamics in the scale of hours that the filter-based approach is incapable of providing. In addition, the PILS measurements of semi-volatile SIA species, i.e., NO3- and NH4+, are more reliable as the filter-based sampling is subjected to various sampling artifacts due to gas-particle interactions and/or evaporation of semi-volatile species. The high-time resolution PM2.5 ionic chemical composition data have been explored and the main findings are summarized below: (1) Impacts of local emissions, regional transports and their interactions on chemical composition and concentrations of PM2.5 SIA and other ionic species were investigated at the Hong Kong University of Science and Technology (HKUST), a receptor site, under three synoptic conditions. It is found air masses that passed the heavy urban areas located on the HK Island most likely attributed a night time NO3- episode in fall. Low temperature and high relative humidity (RH) that was associated with an approaching cold front efficiently shifted gaseous phase HNO3 to aerosol phase. Regional transport from north PRD was likely responsible for elevated NH4+ and SO42- levels in a SO42- episode in fall. During a cold front event, the variation of surface PM2.5 and its SIA components was closely related to wind regime. That is, at the beginning of the cold front passage, the strong wind efficiently diluted the boundary layer pollutants, while as the wind weakened (in ~2 days), elevated pollutants levels were observed. (2) Chemical compositions and size characteristics of ionic species were investigated at Tung Chung, a new town area located in the Southwest part of HK. The sampling period was from 17 to 26 December 2009, covering both normal conditions and an aerosol episode. The three major secondary inorganic ions, SO42, NH4+ and NO3-, accounted for 47 ± 6% of PM2.5 mass. Nitrate in PM2.5 had an average concentration of 4.5 μg m-3, ranging from 0.8 to 40.5 μg m-3. During the episode hours (defined to be periods during which the air pollution index exceeding 100), nitrate explained 37% of PM2.5 mass and its concentration exceeded that of SO42- to become the most abundant PM2.5 species. Variable size distributions of NO3- were observed, with three sets of samples showing a dominant droplet mode, two sets showing a dominant coarse mode, and the rest four sets bimodally distributed in both droplet and coarse modes. During the episode, 84% of NO3- was distributed in fine mode particles, significantly contributing to the elevated PM2.5 level. Further examination of size characteristics of NO3- shows that fine mode NO3- is more likely to occur in environments when the fine particles are less acidic and the sea-salt aerosol contributions are low. (3) The ionic chemical composition of PM2.5 and meteorological parameters (e.g., temperature, RH) obtained at the HKUST site under all three different synoptic conditions are input into Aerosol Inorganic Model (AIM-III) for estimation of in situ pH through calculation of H+ amount and aerosol liquid water content (LWC). The particle pHIS ranged from -1.87 to 3.12, with an average at -0.03, indicating the PM2.5 particles in Hong Kong are highly acidic. Unlike particle strong acidity, which was dominated by SO42- concentration, the amount of aerosol liquid water content could significantly influence in situ particle acidity. Principal factor analysis has identified the equivalent concentration ratio between cations and anions (i.e., R+/-) and RH to be the two most important factors influencing the particle pHIS. pHIS under different synoptic conditions in this study could be well approximated by a single linear regression equation (slope: 0.95, R2: 0.93), i.e., pHIS = 4.94 R+/- + 3.11 RH - 5.70. Such an empirical equation provides a convenient mean in estimating particle in-situ acidity for assessing the role of acid-catalyzed aerosol reactions. The second part of this thesis work is to improve an observation-based model (OBAMAP) for SIA, which was first developed by Dr. Zibing Yuan (2006) to evaluate the sensitivity of formation of nitrate ad sulfate to changes in the emissions of their precursors (i.e., NOx, SO2, and VOCs). The improvement work includes incorporating updated chemical mechanisms, thermodynamic equilibrium for gas-aerosol phase apportionment and size distribution of SIA. While NOx and SO2 are recognized precursor gases to nitrate and sulfate, levels of volatile organic compounds (VOCs) also have impact on the production of SIA through mediating the level of oxidants. In the model, a sequence of present time-frame observations of precursors and particle compositions and meteorological data are used to drive the simulation and to determine responses to perturbed emission rates. The use of observational data ensures that the calculations are carried out for the proper combinations of NOx and VOCs conditions. The new OBM for SIA is applied to hourly gaseous and particulate composition data measured during a wintertime pollution episode encountered in Tung Chung for probing effectiveness of different precursor control strategies. The OBM analysis suggests that local formation of PM2.5 SIA was strongly enhanced and contributed 78% of NO3- increase and 14% of SO42- increase in the episode hours. The NO3- level is found to be sensitive to the change in NOx and anthropogenic VOCs (AVOCs) emissions. Each percent reduction (near 10% reduction rate) in NOx and AVOCs results in a reduction of 0.53% and 0.66% in NO3- mass formation, respectively. However, the significance of NOx and AVOC control would be expected to decrease as more precursors are reduced. Reduction of SO42- formation, on the other hand, is most sensitive to reduction of SO2. At the measured mbient level, each percent reducing of SO2 results in a reduction of 1.20% SO42- mass formation and this value increases to 1.70% if the reduction of SO2 increases to 60%, suggesting controlling of SO2 would become more effective as more SO2 is reduced. The OBM is demonstrated to provide a relatively simple and cost-effective tool for analyzing the increasing database of high time resolution measurements of VOCs and major aerosol ionic species. The PILS measurements and the modeling framework are also used to investigate different sulfate formation regimes in respective of relevant precursor abundance. Past studies have shown formation of SO42- could be severely inhibited under high NOx conditions because oxidative potential of the atmosphere is greatly diminished through the reaction of NO2+OH. Such a theory fails to explain high SO42- loadings under low solar actinic fluxes conditions observed in mega cities in China. In this thesis, we propose a new production regime of SO42- in which oxidation of S(IV) is dominated by NO2 and O3 in the aqueous phase. Simulated with a simplified version of OBAMAP, it is shown elevation of NOx favors productions of SO42- in this regime, especially under high-SO2 conditions. We then study the importance of NO2-derived and O3-derived SO42- during haze episodes in PRD and during winter at urban/suburban locations in PRD. Our findings reveal these two pathways account for >70% of SO42- productions. Since production of NO2-derived SO42- is independent on solar actinic fluxes while production by other pathways is, NO2-derived SO42- plays a more important role under low solar actinic fluxes conditions, even during the night time. In addition, it is noted that high levels of NO2-derived SO42- can only be expected under high-SO2 conditions (like in PRD) because level of atmospheric SO2 is the limiting parameter. Results in this thesis contribute to improving our understanding of the complexity of chemical processes that SIA and their precursors are involved in atmospheric environments typical of rapid-developing regions. In addition, the OBM analysis provides valuable information for formulation of effective control strategies of SIA precursors.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2012
Language English
Format Thesis
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