A modified micrometeorological gradient method for estimating O<sub>3</sub> dry depositions over a forest canopy

Abstract

Abstract. Small pollutant concentration gradients between levels above a plant canopy result in large uncertainties in estimated air-surface exchange fluxes when using existing micrometeorological gradient methods, including the aerodynamic gradient method (AGM) and the modified Bowen ratio method (MBR). A modified micrometeorological gradient method (MGM) is proposed in this study for estimating O 3 dry deposition fluxes over a forest canopy using concentration gradients between a level above and a level below the canopy top, taking advantage of relatively large gradients between these levels due to significant pollutant uptake in the top layers of the canopy. The new method is compared with the AGM and MBR methods and is also evaluated using eddy-covariance (EC) flux measurements collected at the Harvard Forest Environmental Measurement Site, Massachusetts, during 1993-2000. All three gradient methods (AGM, MBR, and MGM) produced similar diurnal cycles of O 3 dry deposition velocity (V d (O 3 )) to the EC measurements, with the MGM method being the closest in magnitude to the EC measurements. The multi-year average V d (O 3 ) differed significantly between these methods, with the AGM, MBR, and MGM method being 2.28, 1.45, and 1.18 times that of the EC, respectively. Sensitivity experiments identified several input parameters for the MGM method as first-order parameters that affect the estimated V d (O 3 ). A 10 % uncertainty in the wind speed attenuation coefficient or canopy displacement height can cause about 10 % uncertainty in the estimated V d (O 3 ). An unrealistic leaf area density vertical profile can cause an uncertainty of a factor of 2.0 in the estimated V d (O 3 ). Other input parameters or formulas for stability functions only caused an uncertainly of a few percent. The new method provides an alternative approach to monitoring/estimating long-term deposition fluxes of similar pollutants over tall canopies.

Sensitivity of Ozone Dry Deposition to Ecosystem‐Atmosphere Interactions: A Critical Appraisal of Observations and Simulations

“…For the growing season, both models simulate lower V d,O3 than observed at Ulborg. This discrepancy may reflect uncertainties in V d,O3 measurements derived from the gradient method, which in general are greater than those obtained with the eddy correlation method (Wu et al, ).…”

Dry Deposition of Ozone Over Land: Processes, Measurement, and Modeling

“…While commonly used ozone UV absorbance instruments are likely adequate for this technique, this method requires detection of likely small gradients in ozone and the reference quantity. Previous work suggests substantial biases (50–100%) in ozone fluxes estimated with the modified Bowen Ratio technique with carbon dioxide fluxes relative to ozone EC at Harvard Forest (Z. Y. Wu et al, ).…”

Evaluation and Intercomparison of Five North American Dry Deposition Algorithms at a Mixed Forest Site

“…A database of hourly V d values for O 3 and SO 2 covering 1 May 2008 to 30 April 2013 was previously developed at Borden Forest (Wu et al, ). In the process of developing this database, dry deposition fluxes ( F ) of O 3 and SO 2 were calculated using concentration gradients between a level above and a level below the canopy top based on a modified gradient method (MGM) described in Wu et al (). Briefly, the MGM is similar to the classic aerodynamic gradient method (AGM) in that fluxes are calculated by combining measurements of concentration gradients and micrometeorology (e.g., u* , L ) with empirical models of wind and thermodynamic profiles.…”

“…However, uncertainties of the order of approximately 20% in the estimated V d values likely exist due to the assigned canopy characteristics, limitation of the algorithm, and measurement uncertainties in concentrations. Uncertainties are expected to be higher in stable conditions (e.g., nighttime and winter) when accuracy of the stability correction functions decreases (Wu et al, ) and are higher for SO 2 than O 3 as concentrations of SO 2 were relatively low through the measuring period (Wu et al, ). These uncertainties are considered when using the V d database to evaluate the five dry deposition models below.…”