Characteristics of major secondary ions in typical polluted atmospheric aerosols during autumn in central Taiwan

In autumn of 2008, the chemical characteristics of major secondary ionic aerosols at a suburban site in central Taiwan were measured during an annually occurring season of high pollution. The semicontinuous measurement system measured major soluble inorganic species, including NH₄ ⁺, NO₃ ⁻, and SO₄ ²⁻, in PM₁₀ with a 15 min resolution time. The atmospheric conditions, except for the influences of typhoons, were dominated by the local sea-land breeze with clear diurnal variations of meteorological parameters and air pollutant concentrations. To evaluate secondary aerosol formation at different ozone levels, daily ozone maximum concentration (O₃,dₐᵢₗy ₘₐₓ) was used as an index of photochemical activity for dividing between the heavily polluted period (O₃,dₐᵢₗy ₘₐₓ ≧80 ppb) and the lightly polluted period (O₃,dₐᵢₗy ₘₐₓ<80 ppb). The concentrations of PM₁₀, NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺ and total major ions during the heavily polluted period were 1.6, 1.9, 2.4, 2.7 and 2.3 times the concentrations during the lightly polluted period, respectively. Results showed that the daily maximum concentrations of PM₁₀ occurred around midnight and the daily maximum ozone concentration occurred during daytime. The average concentration of SO₂ was higher during daytime, which could be explained by the transportation of coastal industry emissions to the sampling site. In contrast, the high concentration of NO₂ at night was due to the land breeze flow that transport inland urban air masses toward this site. The simulations of breeze circulations and transitions were reflected in transports and distributions of these pollutants. During heavily polluted periods, NO₃ ⁻ and NH₄ ⁺ showed a clear diurnal variations with lower concentrations after midday, possibly due to the thermal volatilization of NH₄NO₃ during daytime and transport of inland urban plume at night. The diurnal variation of PM₁₀ showed the similar pattern to that of NO₃ ⁻ and NH₄ ⁺ aerosols. This indicated that the formatted secondary aerosols in the inland urban area could be transported to the coastal area by the weak land breeze and deteriorated the air quality in the coastal area at night.