Reply on RC1

Abstract

Dry deposition is a key process for surface ozone (O3) removal. Stomatal resistance is a major component of O3 dry deposition, which is parameterized differently in current land surface models and chemical transport models. We developed and used a standalone terrestrial biosphere model, driven by a unified set of prescribed meteorology, to evaluate two widely used dry deposition modeling frameworks, Wesely (1989) and Zhang et al. (2003), with different configurations of stomatal resistance: 1) the default multiplicative method in each deposition scheme; 2) the traditional photosynthesis-based Farquhar-Ball-Berry (FBB) stomatal algorithm; 3) the Medlyn stomatal algorithm based on an optimization theory. We found that using the FBB stomatal approach that captures ecophysiological responses to environmental factors, especially to water stress, can generally improve the simulated dry deposition velocities compared with multiplicative schemes. The Medlyn stomatal approach produces higher stomatal conductance (reverse of stomatal resistance) than FBB and is likely to overestimate dry deposition velocities for major vegetation types, but its performance is greatly improved when spatially varying slope parameters based on annual mean precipitation are used. Large discrepancies were also found in simulated stomatal responses to rising CO2 levels, and that multiplicative stomatal method with an empirical CO2 response function produces reduction (−35 %) in global stomatal conductance, which is much larger than that with photosynthesis-based stomatal method (−14–19 %) when atmospheric CO2 level increases from 390 ppm to 550 ppm. Our results show the potential biases in O3 sink caused by errors in model structure especially in the Wesely dry deposition scheme, and the importance of using photosynthesis-based representation of stomatal resistance in dry deposition schemes under a changing climate and rising CO2 concentration.