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

Long-term trends in total oxidants (O-x, defined as NO2 + O-3) and ozone (O-3) 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 O-3 (+0.51 ppbv/yr) and relatively constant levels of O-x at approximately 46 ppbv. However, when divided into two time periods, the O-3 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 O-3 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 O-3 and O-x 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 O-3 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 O-3 levels in recent years, resulting in altered O-3 and O-x levels after 2007. The oceanic baseline O-3 averages revealed by AQS on the satellite islands around Taiwan also indicated consistent results in recent years, with O-3 values notably lower (38-40 ppbv) than the peak values of 40-45 ppbv in the early 2000s. (C) 2014 Elsevier Ltd. All rights reserved.