Analysis of High-Ozone Episodes Using a Chemical Metric Based on the Reactivities of Organic Precursors

Abstract

This study examines high-ozone episodes in an urban area of northern Taiwan influenced by local photochemical processes and transboundary pollution from the Asian continent. Long-term data sets from Photochemical Assessment Monitoring Stations and Air Quality Monitoring Stations were used to develop a photochemical metric based on volatile organic compounds ratios. We analyzed the ratio of ln(n-butane/ethane) versus ln(n-pentane/ethane) based on which we derived the δ value to assess the degree of regional air mass aging from a chemical reactivity perspective. In summer, extreme ozone events with low δ values are primarily driven by local photochemical activity. However, in fall and spring, the slightly higher δ values in some episodes signify the involvement of long-range transport (LRT) bringing in continental ozone and cooler air masses while local photochemistry remains active. The interaction between transported and locally produced ozone is pronounced during milder ozone episodes (100–120 ppbv). We further examined the m,p-xylene/ethylbenzene ratio as a complementary chemical indicator for characterizing ozone events with mild LRT. Slightly lower temperatures (below 28.8°C) are often accompanied by elevated nocturnal ozone levels (40–80 ppbv) and a less defined m,p-xylene/ethylbenzene ratio. Overall, high-ozone episodes (≥100 ppbv) are primarily driven by local photochemical processes. However, LRT can elevate the ozone baseline, particularly during spring and fall, when aged air masses bring in residual ozone leading to increased daytime ozone levels. These findings highlight how local photochemistry and regional transport shape high-ozone episodes emphasizing the value of chemical indicators for understanding air mass aging and ozone formation.