Importance of dry deposition parameterization choice in global simulations of surface ozone
Authors: Wong Anthony Y. H. , Geddes Jeffrey A. , Tai Amos P. K. , Silva Sam J. 
Published: November 2019

Abstract. Dry deposition is a major sink of tropospheric ozone.
Increasing evidence has shown that ozone dry deposition actively links
meteorology and hydrology with ozone air quality. However, there is little
systematic investigation on the performance of different ozone dry
deposition parameterizations at the global scale and how parameterization
choice can impact surface ozone simulations. Here, we present the results of
the first global, multidecadal modelling and evaluation of ozone dry
deposition velocity (vd) using multiple ozone dry deposition
parameterizations. We model ozone dry deposition velocities over 1982–2011
using four ozone dry deposition parameterizations that are representative of
current approaches in global ozone dry deposition modelling. We use
consistent assimilated meteorology, land cover, and satellite-derived leaf
area index (LAI) across all four, such that the differences in simulated
vd are entirely due to differences in deposition model structures or
assumptions about how land types are treated in each. In addition, we use
the surface ozone sensitivity to vd predicted by a chemical transport
model to estimate the impact of mean and variability of ozone dry deposition
velocity on surface ozone. Our estimated vd values from four different
parameterizations are evaluated against field observations, and while
performance varies considerably by land cover types, our results suggest
that none of the parameterizations are universally better than the others.
Discrepancy in simulated mean vd among the parameterizations is
estimated to cause 2 to 5 ppbv of discrepancy in surface ozone in the
Northern Hemisphere (NH) and up to 8 ppbv in tropical rainforests in July,
and up to 8 ppbv in tropical rainforests and seasonally dry tropical forests
in Indochina in December. Parameterization-specific biases based on
individual land cover type and hydroclimate are found to be the two main
drivers of such discrepancies. We find statistically significant trends in
the multiannual time series of simulated July daytime vd in all
parameterizations, driven by warming and drying (southern Amazonia, southern
African savannah, and Mongolia) or greening (high latitudes). The trend in
July daytime vd is estimated to be 1 % yr−1 and leads
to up to 3 ppbv of surface ozone changes over 1982–2011. The interannual coefficient of
variation (CV) of July daytime mean vd in NH is found to be
5 %–15 %, with spatial distribution that varies with the dry deposition
parameterization. Our sensitivity simulations suggest this can contribute
between 0.5 to 2 ppbv to interannual variability (IAV) in surface ozone, but
all models tend to underestimate interannual CV when compared to long-term
ozone flux observations. We also find that IAV in some dry deposition
parameterizations is more sensitive to LAI, while in others it is more sensitive
to climate. Comparisons with other published estimates of the IAV of
background ozone confirm that ozone dry deposition can be an important part
of natural surface ozone variability. Our results demonstrate the importance
of ozone dry deposition parameterization choice on surface ozone modelling
and the impact of IAV of vd on surface ozone, thus making a strong case
for further measurement, evaluation, and model–data integration of ozone dry
deposition on different spatiotemporal scales.