Recent Improvement in Air Quality as Evidenced by the Island-wide Monitoring Network in Taiwan

Abstract:

Long-term trends in total oxidants (O_x, defined as NO₂ + O₃) and ozone (O₃) were analyzed based on the annual averages calculated from 56 air-quality stations (AQS) across Taiwan from 1994 to 2012. A simple regression of the annual averages for the period from 1994-2012 revealed an increasing trend for O₃ (+0.51 ppbv/yr) and relatively constant levels of O_x at approximately 46 ppbv. However, when divided into two time periods, the O₃ trend leveled off and remained constant at approximately 29 ppbv from 2007 to 2012, whereas the O_x level drastically decreased (-0.66 ppbv/yr) during this same period. These recent changes in the O₃ and O_x trends island-wide were consistent with the trends observed in the largest metropolis, Taipei, suggesting a ubiquitous phenomenon. The changes in the O₃ and O_x trends around 2007 can be attributed to two causes from the observation point of view. As for the sink perspective, NO_x levels have continued to decrease rapidly since 1994 (-0.82 ppbv/yr or -44% over 19 years), diminishing the NO-titration and, thus, increasing O₃ levels, resulting in constant O_x levels for the period prior to 2007. From the source perspective, continuous decreases in the levels of reducing agents (Non-methane hydrocarbons, NMHC: -0.012 ppmC/yr or -53% over 19 years, CO: -0.022 ppm/yr or -49% over 19 years) have lowered the photochemical O₃ levels in recent years, resulting in altered O₃ and O_x levels after 2007. The oceanic baseline O₃ averages revealed by AQS on the satellite islands around Taiwan also indicated consistent results in recent years, with O₃ values notably lower (38-40 ppbv) than the peak values of 40-45 ppbv in the early 2000s.