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 (σ_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.

中文翻译：

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昼夜过渡期异戊二烯动力学研究

在2016年AMOS野外活动期间，在密歇根大学生物站，异戊二烯的浓度通常在混合条件下的午后（当地时间LT 15:00）达到峰值。但是，在活动的23％时出现了一天结束时的高峰（约21:00 LT），然后在运动期间以0.57 hr ^-1的速率快速移除（从21：00-22：00 LT）。昼夜过渡期。在高峰结束时，冠层中的异戊二烯浓度平均增加77％（从3.5到6.2 ppbv）。分层和弱风（在46 m处< 3.4 m s ^-1）显着抑制了冠层内和冠层之间的湍流交换，导致黄昏时散发的异戊二烯积聚。垂直速度的临界标准偏差（σ_w）分别确定为0.14、0.2和0.29 m s ^-1，以分别检测出13 m，21 m和34 m高度的日末峰值。在85％的日终情况下，风速在达到峰值后随风向的变化而增加到2.5 m s ^-1以上，并重新建立了湍流。因此，风速为2.5 m s ^-1被认为是湍流从不依赖风速切换到依赖风速的阈值点。恢复的湍流占随后异戊二烯去除的80％，其余20％可以通过与羟基，臭氧和硝酸根的化学反应来解释。观察到的异戊二烯通量不支持这样的论点，即垂直湍流混合会降低日终峰值，我们假设水平对流可能会起作用。