Recent improvement in air quality as evidenced by the island-wide monitoring network in Taiwan

Long-term trends in total oxidants (Ox, 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 to 2012 revealed an increasing trend for O3 (+0.51 ppbv/yr) and relatively constant levels of Ox 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 Ox level drastically decreased (−0.66 ppbv/yr) during this same period. These recent changes in the O3 and Ox trends island-wide were consistent with the trends observed in the largest metropolis, Taipei, suggesting a ubiquitous phenomenon. The changes in the O3 and Ox trends around 2007 can be attributed to two causes from the observation point of view. As for the sink perspective, NOx 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 Ox 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 Ox 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.