Measurement-based investigation of ozone deposition to vegetation under the effects of coastal and photochemical air pollution in the Eastern Mediterranean

Abstract

Dry deposition of ozone (O3) to vegetation is an important pathway for its removal from the troposphere, and it can lead to adverse effects in plants and changes in climate. However, our mechanistic understanding of O3 dry deposition is insufficient to adequately account for it in global and regional models, primarily because this process is highly complicated by feedback mechanisms and sensitivity to specific characteristics of vegetative environment and atmospheric dynamics and composition. We hypothesized that measuring dry deposition of O3 to vegetation near the Eastern Mediterranean (EM) coast, where large variations in meteorological conditions and photochemical air pollution frequently occur, would enable identifying the mechanisms controlling O3 deposition to vegetation. Moreover, we have only limited knowledge of O3 deposition to vegetation occurring near a coastline, under air pollution, or in the EM. This study investigated O3 deposition to mixed Mediterranean vegetation between the summers of 2015 and 2017, 3.6 km away from the EM coast, using the eddy covariance technique to quantify vertical flux of O3 and its partitioning to stomatal and non-stomatal flux, concurrent with nitrogen oxide (NOx), sulfur dioxide and carbon monoxide. Surprisingly, nighttime O3-deposition velocity (Vd) was smaller than daytime Vd by only ~20–37% on average for all measurement periods, primarily related to moderate nighttime atmospheric stability due to proximity to the seashore. We provide evidence for the role of sea-salt aerosols in enhancing O3 deposition via surface-wetness buildup at low relative humidity near the coast, and for daytime enhancement of O3 deposition by the combined effects of biogenic volatile organic compound emission and surface-wetness buildup. We further show that NOx emitted from elevated emission sources can reduce O3 deposition, and even lead to a positive O3 flux, demonstrating the importance of adequately taking into account the impact of air pollution on O3 deposition to vegetation.