Long-term (2005-2015) trend analysis of PM_2.5 precursor gas NO₂ and SO₂ concentrations in Taiwan

Ground air monitoring stations have been installed in Taiwan since 1993 to ensure whether the criteria air pollutants meet the ambient air quality standards. In the present study, the data from the monitoring stations were used to evaluate long-term (2005-2015) trend of NO₂ and SO₂ in three metropolitan cities (northern Taipei, central Taichung, and southern Kaohsiung), two eastern coastal cities (Hualien and Taitung), and one agricultural city in west-central plain (Douliu); those cities essentially covered the entire region of Taiwan. The results indicate that SO₂ and NO₂ concentrations of all studied six cities meet the annual average standards of 30 and 50 ppb, respectively. After deseasonalizing the original data and using 7-month moving average, the trend analysis reveals a decreasing trend ranging from 0.15 to 0.57 ppb/year (R² from 0.33 to 0.85) for NO₂ and 0.06 to 0.45 ppb/year (R² from 0.32 to 0.92) for SO₂; the corresponding reductions over the 10-year span are 4 to 42% for NO₂ and 22 to 52% for SO₂. The reduction trend, despite the growth in GDP, vehicle numbers and energy consumption, industrial output, etc., is similar to those of developed countries. Clearly, there are seasonal/monthly variation patterns for these two precursor gases with minimum levels in summer (July) and maximum in winter (December). The concentration reductions, however, were lagging behind the respective emission reductions. There are significant correlations among six cites for NO₂ (r = 0.58-0.93) and, to some extent, SO₂ (0.32-0.66). The correlation between SO₂ and NO₂ (r = 0.46-0.74) indicates same or similar emission sources. Furthermore, the correlation between observed pollutant concentrations and their emission is excellent for SO₂ in two cities (0.79-0.96). The SO₂/NO₂ ratios vary with city and time and the value is site specific. For example, in 2005, the SO₂/NO₂ ratio was 0.38 in Kaohsiung and 0.18 in both Taipei and Taichung, the latter reflecting significant contribution from mobile sources. However, they all converged to 0.18-0.28 in 2015 in the six cities evaluated. All in all, the policies/measures made by the central and local government are effective in reducing ambient SO₂ and NO₂ levels.