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

Abstract

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-tonight 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.

Large Eddy Simulation for Investigating Coupled Forest Canopy and Turbulence Influences on Atmospheric Chemistry

“…1). Horizontal mean isoprene is maximum 9.7 ppbv, which is on the higher side of daytime hourly isoprene mixing ratios observed at temperate deciduous forests (Goldstein et al, 1998;Helmig et al, 1998;McKinney et al, 2011;Wei et al, 2020). Both NO and NO 2 peak at the ground, but NO 3 and N 2 O 5 peak just above the ground in the lower canopy.…”

“…Isoprene's maximum happens at 0.8$$h$$, where isoprene emissions maximize (not shown), which is slightly above maximum LAD at 0.6$$h$$ but where the sunlight fraction of leaves exceeds the shaded fraction of leaves (Figure 1). Horizontal mean isoprene is maximum 9.7 ppbv, which is on the higher side of daytime hourly isoprene mixing ratios observed at temperate deciduous forests (Goldstein et al., 1998; Helmig et al., 1998; McKinney et al., 2011; Wei et al., 2020). Both NO and NO 2 peak at the ground, but NO 3 and N 2 O 5 peak just above the ground in the lower canopy.…”

Advancing Cross‐Disciplinary Understanding of Land‐Atmosphere Interactions

“…Steiner pointed to aircraft, radiosonde, and satellite observations as a means by which complementary atmospheric measurements might be obtained. Steiner discussed research opportunities that would emerge from considering both local and non‐local land‐atmosphere feedbacks, using precipitation recycling (J. Wei & Dirmeyer, 2019), soil moisture‐temperature‐drought interactions (Schumacher et al., 2019), and diurnal patterns of isoprene concentrations (Wei et al., 2020) as key examples. Steiner proposed that improved understanding of local versus non‐local feedbacks could lead to better predictive models.…”