Inter-annual variations of atmospheric isoprene in a major metropolis, Taipei, were reported based on a two-year continuous measurements of non-methane hydrocarbons (NMHCs) with an hourly resolution. It is by far the largest dataset ever collected in the urban subtropical environment with thriving vegetation and traffic flows. The dataset revealed the detailed interplay between the two major sources of biogenic and vehicular throughout the year. To separate the vehicular contribution from the biogenic one for the ambient isoprene, ethylene was used as an indicator of traffic emissions. While dramatic surge of isoprene was observed at noontime in hot months with the highest average peak mixing ratio of 1.6 ppbv in August, its abundance decreased to 0.2 ppbv on average in cold months. The vehicular contribution to ambient isoprene was largely masked over by the noontime surge of isoprene in hot seasons, but was still able to be vaguely observed on the slopes of the isoprene peaks mimicking the rush-hour features of ethylene. In winter, the diurnal variations of isoprene were very similar to those of ethylene, which suggests that ambient isoprene in cold months was almost of traffic origin.
This study based on the continuous dataset could enhance the key findings in previous flask studies in the same metropolis. While canister sampling had a major advantage of area coverage, the highly time-resolved fixed-site monitoring could better reveal the evolution process from a biogenically overwhelmed condition in hot months to the condition where the biogenic source weakened to reveal the traffic source in cold months.