Biogenic isoprene in subtropical urban settings and implications for air quality

Abstract Isoprene has potentially a large impact on secondary oxidant formation, particularly in the polluted urban atmospheres. The environmental conditions in tropical and subtropical cities with high temperatures and light flux are conducive to the production of large amounts of biogenic isoprene. Measurements of speciated volatile organic compounds (VOCs) were conducted in Taipei, a subtropical metropolis, to investigate the characteristics of biogenic and anthropogenic isoprene during the hot seasons (summer and autumn) and to assess their significance in secondary pollutant formation. The daily and daytime average concentrations of isoprene at the subtropical urban site in summer were 0.72 and 1.26 ppbv, respectively, which were considerably higher than the concentrations of isoprene in most temperate cities. Furthermore, summertime isoprene ranks highest in OH reactivity and second highest in terms of ozone formation potential (OFP) among 66 measured VOCs. The ratios of isoprene to 1,3-butadiene, an exhaust tracer, were used to estimate the fractions of biogenic and anthropogenic isoprene in the urban area. The results reveal that the biogenic contribution apparently overwhelmed the anthropogenic contribution in summertime, although traffic in the city is heavy. Furthermore, the residual isoprene (mostly biogenic) after daytime photochemical loss persisted into the nighttime and contributed a large fraction to nighttime isoprene. In autumn, daytime isoprene was also predominantly from biogenic sources because the hot and sunny conditions persist into the autumn months. The high biogenic isoprene levels in subtropical urban settings and its coherence with OH diurnal cycles accentuate the significance of biogenic isoprene and its potentially great impact on atmospheric oxidant capacity, urban air quality, and even regional climate.