Water-soluble inorganics (WSIs) (Cl⁻, NO₃⁻, SO₄²⁻, NH₄⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺) in both PM_2.5 and PM₁₀ in urban Hong Kong were studied using hourly measurement data collected by an online instrument MARGA 2S spanning 4 years from July 2013 to June 2017. Major secondary inorganic aerosols (SIAs) including NO₃⁻, SO₄²⁻ and NH₄⁺ were the dominating species of WSIs in PM_2.5, accounting for over 85%. In PM_Coarse (PM_2.5-10), NO₃⁻ was the largest contributor (40%) to the WSIs while other constituents had comparable contributions. The semi-volatile species NH₄NO₃ was found to preferentially reside in particle phase under low acidity condition and to notably dissociate back into its precursors when temperature increased. The subsequent uptake of HNO₃ by coarse particles occurred in the afte...[ Read more ]

Water-soluble inorganics (WSIs) (Cl⁻, NO₃⁻, SO₄²⁻, NH₄⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺) in both PM_2.5 and PM₁₀ in urban Hong Kong were studied using hourly measurement data collected by an online instrument MARGA 2S spanning 4 years from July 2013 to June 2017. Major secondary inorganic aerosols (SIAs) including NO₃⁻, SO₄²⁻ and NH₄⁺ were the dominating species of WSIs in PM_2.5, accounting for over 85%. In PM_Coarse (PM_2.5-10), NO₃⁻ was the largest contributor (40%) to the WSIs while other constituents had comparable contributions. The semi-volatile species NH₄NO₃ was found to preferentially reside in particle phase under low acidity condition and to notably dissociate back into its precursors when temperature increased. The subsequent uptake of HNO₃ by coarse particles occurred in the afternoon through considerable processing of sea spray and crustal dust aerosol species, particularly salts containing Na⁺, Mg²⁺ and Ca²⁺. Coarse nitrate budget was assessed for the first time in Hong Kong. Greater than 40% of NO₃⁻ in PM_Coarse was associated with crustal dust processing, which was more prominent during cold seasons. Chloride depletion contributed over 20% of the nitrate formation in coarse particle and was more favourable during warm seasons. Episodic pollution days and their relation to aerosol NO₃⁻ were also assessed to provide policy implications. During PM₁₀ episodic days, NO₃⁻, a significant component in fine particle (19.7 μg/m³, 12.8% of PM₁₀ mass concentration), also had comparably abundant presence in the coarse particle (17.6 μg/m³, 11.4% of PM₁₀ mass concentration). It was further found that as relative contribution of NO₃⁻ increased in PM₁₀ episodic days, larger proportions were taken up by coarse fraction in the form of NH₄NO₃. Additionally, as sulfate reduction over the years would favour the transition from NH₄⁺ poor to NH₄⁺ rich regimes, the formation of aerosol NO₃^- would become more favourable. Understanding of nitrate formation with less sulfate becomes important for the formulation of pollution control strategies.