Investigation of Isoprene Dynamics During the Day-to-Night Transition Period

At the University of Michigan Biological Station during the 2016 AMOS field campaign, isoprene concentrations typically peak in the early afternoon (around 15:00 local time, LT) under well-mixed conditions. However, an end-of-day peak (around 21:00 LT) occurs on 23% of the campaign days, followed by a rapid removal (from 21:00-22:00 LT) at rate of 0.57 hr(-1) during the day-to-night transition period. During the end-of-day peak, in-canopy isoprene concentrations increase by 77% (from 3.5 to 6.2 ppbv) on average. Stratification and weak winds (3.4 m s(-1) at 46 m) significantly suppress turbulent exchanges between in- and above-canopy, leading to accumulation of isoprene emitted at dusk. A critical standard deviation of the vertical velocity (sigma(w)) of 0.14, 0.2, and 0.29 m s(-1) is identified to detect the end-of-day peak for the height of 13, 21, and 34 m, respectively. In 85% of the end-of-day cases, the wind speed increases above 2.5 m s(-1) after the peak along with a shift in wind direction, and turbulence is reestablished. Therefore, the wind speed of 2.5 m s(-1) is considered as the threshold point where turbulence switches from being independent of wind speed to dependent on wind speed. The reinstated turbulence accounts for 80% of the subsequent isoprene removal with the remaining 20% explained by chemical reactions with hydroxyl radicals, ozone, and nitrate radicals. Observed isoprene fluxes do not support the argument that the end-of-day peak is reduced by vertical turbulent mixing, and we hypothesize that horizontal advection may play a role.