Date Published:

2019

Abstract:

Dry deposition of ozone (O3) to vegetation is an important removal pathway for tropospheric O3, while O3 uptake through plant stomata negatively affects vegetation and leads to climate change. Both processes are controlled by vegetation characteristics and ambient conditions via complex mechanisms. Recent studies have revealed that these processes can be fundamentally impacted by coastal effects, and by dry and warm conditions in ways that have not been fully characterized, largely due to lack of measurements under such conditions. Hence, we hypothesized that measuring dry deposition of O3 to vegetation along a sharp spatial climate gradient, and at different distances from the coast, can offer new insights into the characterization of these effects on O3 deposition to vegetation and stomatal uptake, providing important information for afforestation management and for climate and air-quality model improvement. To address these hypotheses, several measurement campaigns were performed at different sites, including pine, oak, and mixed Mediterranean forests, at distances of 20–59 km from the Eastern Mediterranean coast, under semiarid, Mediterranean and humid Mediterranean climate conditions. The eddy covariance technique was used to quantify vertical O3 flux (Ftot) and its partitioning to stomatal flux (Fst) and non-stomatal flux (Fns). Whereas Fst tended to peak around noon under humid Mediterranean and Mediterranean conditions in summer, it was strongly limited by drought under semiarid conditions from spring to early winter, with minimum average Fst/Ftot of 8–11% during the summer. Fns in the area was predominantly controlled by relative humidity (RH), whereas increasing Fns with RH for RH < 70% indicated enhancement of Fns by aerosols, via surface wetness stimulation. At night, efficient turbulence due to sea and land breezes, together with increased RH, resulted in strong enhancement of Ftot. Extreme dry surface events, some induced by dry intrusion from the upper troposphere, resulted in positive Fns events.

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