26 research outputs found
Investigation of Isoprene Dynamics During the DayâtoâNight 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 wellâmixed conditions. However, an endâofâday peak (around 21:00 LT) occurs on 23% of the campaign days, followed by a rapid removal (from 21:00â22:00 LT) at rate of 0.57 hrâ1 during the dayâtoânight transition period. During the endâofâday peak, inâcanopy isoprene concentrations increase by 77% (from 3.5 to 6.2âppbv) on average. Stratification and weak winds (<3.4âmâsâ1 at 46âm) significantly suppress turbulent exchanges between inâ and aboveâcanopy, leading to accumulation of isoprene emitted at dusk. A critical standard deviation of the vertical velocity (Ïw) of 0.14, 0.2, and 0.29âmâsâ1 is identified to detect the endâofâday peak for the height of 13, 21, and 34âm, respectively. In 85% of the endâofâday cases, the wind speed increases above 2.5âmâsâ1 after the peak along with a shift in wind direction, and turbulence is reestablished. Therefore, the wind speed of 2.5âmâsâ1 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 endâofâday peak is reduced by vertical turbulent mixing, and we hypothesize that horizontal advection may play a role.Key PointsLow turbulent mixing during clear and calm nights leads to accumulation of isoprene within the canopyTurbulent mixing accounts for 80% of the observed nighttime isoprene loss ratesIsoprene flux measurements did not capture the majority of the removal of the accumulated isoprenePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163406/2/jgrd56554.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163406/1/jgrd56554_am.pd