Ozone uptake by an evergreen forest canopy: temporal variation and possible mechanisms

Area of Science:

• Atmospheric Chemistry
• Forest Ecology
• Biogeochemical Cycles

Background:

• Ozone (O3) is a significant air pollutant impacting forest health and ecosystem function.
• Understanding ozone deposition dynamics in forest canopies is crucial for assessing environmental impacts.
• Evergreen forests play a key role in atmospheric gas exchange and pollutant cycling.

Purpose of the Study:

• To investigate ozone concentration, deposition velocity, and flux patterns in an evergreen forest.
• To compare different measurement methods for ozone flux.
• To identify key drivers of ozone flux and deposition in relation to tree physiology and environmental factors.

Main Methods:

• Comparison of gradient and eddy correlation methods for measuring ozone deposition velocity (vd).
• Analysis of ozone concentration ([O3]), vd, and ozone flux (Fc) over an evergreen forest canopy.
• Correlation analysis of Fc with CO2 and water vapor fluxes, stomatal conductance, and environmental variables.

Main Results:

• Both gradient and eddy correlation methods showed similar diurnal patterns in vd.
• Ozone flux (Fc) was correlated with CO2 and water vapor fluxes during the day.
• Fc was primarily driven by stomatal conductance and surface/gas reactions, not ambient [O3] (10-70 ppb).
• Diurnal patterns showed highest [O3] in the afternoon and higher vd in the morning, leading to consistent Fc.
• Nighttime Fc was substantial, exceeding half of the daytime flux.

Conclusions:

• Stomatal conductance is the primary determinant of ozone flux in this evergreen forest.
• Ozone deposition occurs significantly throughout the day and night.
• Emissions of nitric oxide and terpenes act as important ozone removal factors in Norway spruce dominated forests.