Abstract

In this study, the canopy effects on the vertical ozone exchange within and above Californian orchard are investigated. We examined the comprehensive dataset obtained from the Canopy Horizontal Array Turbulence Study (CHATS). CHATS typifies a rural central Californian site, with O₃ mixing ratios of less than 60 ppb and moderate NO_x mixing ratios. The CHATS campaign covered a complete irrigation cycle, with our analysis including periods before and after irrigation. Lower O₃ mixing ratios were found following irrigation, together with increased wind speeds, decreased air temperatures and increased specific humidity. Friction velocity, sensible heat and gas fluxes above the canopy were estimated using variations on the flux-gradient method, including a method which accounts for the roughness sublayer (RSL). These methods were compared to fluxes derived from observed eddy diffusivities of heat and friction velocity. We found that the use of the RSL parameterization, which accounts for the canopy-induced turbulent mixing above the canopy, resulted in a stronger momentum, heat, and ozone exchange fluxes above this orchard, compared to the method which omits the RSL. This was quantified by the increased friction velocity, heat flux and ozone deposition flux of up to 12, 29, and 35% at 2.5 m above the canopy, respectively. Within the canopy, vertical fluxes, as derived from local gradients and eddy diffusivity of heat, were compared to fluxes calculated using the Lagrangian inverse theory. Both methods showed a presence of vertical flux divergence of friction velocity, heat and ozone, suggesting that turbulent mixing was inefficient in homogenizing the effects driven by local sources and sinks on vertical exchange of those quantities. This weak mixing within the canopy was also corroborated in the eddy diffusivities of friction velocity and heat, which were calculated directly from the observations. Finally, the influence of water stress on the O₃ budget was examined by comparing the results prior and after the irrigation. Although the analysis is limited to the local conditions, our in situ measurements indicated differences in the O₃ mixing ratio prior and after irrigation during CHATS. We attribute these O₃ mixing ratio changes to enhanced biological emission of volatile organic compounds (VOCs), driven by water stress.

摘要

在这项研究中，研究了冠层对加利福尼亚果园内及以上垂直臭氧交换的影响。我们检查了从冠层水平阵列湍流研究（CHATS）获得的综合数据集。CHATS代表了加利福尼亚州中部的一个农村地区，O ₃混合比小于60 ppb，NO _x混合比中等。CHATS活动涵盖了完整的灌溉周期，我们的分析包括灌溉前后的时期。下O ₃灌溉后发现混合比，以及增加的风速，降低的空气温度和增加的比湿度。使用通量梯度方法（包括考虑粗糙度子层（RSL）的方法）的变化来估算顶盖上方的摩擦速度，显热和气体通量。将这些方法与从观察到的热和摩擦速度的涡流扩散率得出的通量进行了比较。我们发现，与忽略RSL的方法相比，使用RSL参数化可解决该果园上方的冠层引起的冠层引起的湍流混合，从而产生了更大的动量，热量和臭氧交换通量。这可以通过在树冠上方2.5 m处分别增加高达12％，29％和35％的摩擦速度，热通量和臭氧沉积通量来量化。在顶篷内，将从局部梯度和热涡流导出的垂直通量与使用拉格朗日逆理论计算的通量进行比较。两种方法均显示出摩擦速度，热量和臭氧在垂直方向上的通量发散，这表明湍流混合无法有效地均化本地源和汇在垂直方向交换这些量所带来的影响。冠层内部的这种弱混合也可以通过直接从观测值计算得到的摩擦速度和热量的涡流扩散得到证实。最后，水分胁迫对O的影响 两种方法均显示出摩擦速度，热量和臭氧在垂直方向上的通量发散，这表明湍流混合无法有效地均化本地源和汇在垂直方向交换这些量所带来的影响。冠层内部的这种弱混合也可以通过直接从观测值计算得到的摩擦速度和热量的涡流扩散得到证实。最后，水分胁迫对O的影响 两种方法均显示出摩擦速度，热量和臭氧在垂直方向上的通量发散，这表明湍流混合无法有效地均化本地源和汇在垂直方向交换这些量所带来的影响。冠层内部的这种弱混合也可以通过直接从观测值计算得到的摩擦速度和热量的涡流扩散得到证实。最后，水分胁迫对O的影响通过比较灌溉前后的结果来检查₃预算。尽管分析仅限于当地条件，但我们的现场测量结果表明，在CHATS灌溉前后，O ₃混合比存在差异。我们将这些O ₃混合比的变化归因于水分胁迫导致挥发性有机化合物（VOC）的生物排放增加。