An analysis of CMAQ gas-phase dry deposition over North America through grid-scale and land-use-specific diagnostics in the context of AQMEII4
Authors: Hogrefe Christian , Bash Jesse O. , Pleim Jonathan E. , Schwede Donna B. , Gilliam Robert C. , Foley Kristen M. , Appel K. Wyat , Mathur Rohit 
Published: July 2023

Abstract. The fourth phase of the Air Quality Model Evaluation
International Initiative (AQMEII4) is conducting a diagnostic
intercomparison and evaluation of deposition simulated by regional-scale air
quality models over North America and Europe. In this study, we analyze
annual AQMEII4 simulations performed with the Community Multiscale Air
Quality Model (CMAQ) version 5.3.1 over North America. These simulations
were configured with both the M3Dry and Surface Tiled Aerosol and Gas
Exchange (STAGE) dry deposition schemes available in CMAQ. A comparison of
observed and modeled concentrations and wet deposition fluxes shows that the
AQMEII4 CMAQ simulations perform similarly to other contemporary
regional-scale modeling studies. During summer, M3Dry has higher ozone
(O3) deposition velocities (Vd) and lower mixing ratios than STAGE
for much of the eastern US, while the reverse is the case over eastern
Canada and along the US West Coast. In contrast, during winter STAGE has higher
O3 Vd and lower mixing ratios than M3Dry over most of the
southern half of the modeling domain, while the reverse is the case for much
of the northern US and southern Canada. Analysis of the diagnostic
variables defined for the AQMEII4 project, i.e., grid-scale and land-use-specific effective conductances and deposition fluxes for the major dry deposition pathways, reveals generally higher summertime stomatal and
wintertime cuticular grid-scale effective conductances for M3Dry and
generally higher soil grid-scale effective conductances (for both vegetated
and bare soil) for STAGE in both summer and winter. On a domain-wide basis,
the stomatal grid-scale effective conductances account for about half of the
total O3 Vd during daytime hours in summer for both schemes.
Employing land-use-specific diagnostics, results show that daytime Vd varies
by a factor of 2 between land use (LU) categories. Furthermore, M3Dry vs. STAGE
differences are most pronounced for the stomatal and vegetated soil pathway
for the forest LU categories, with M3Dry estimating larger effective
conductances for the stomatal pathway and STAGE estimating larger effective
conductances for the vegetated soil pathway for these LU categories. Annual
domain total O3 deposition fluxes differ only slightly between M3Dry
(74.4 Tg yr−1) and STAGE (76.2 Tg yr−1), but pathway-specific fluxes to
individual LU types can vary more substantially on both annual and seasonal
scales, which would affect estimates of O3 damage to sensitive
vegetation. A comparison of two simulations differing only in their LU
classification scheme shows that the differences in LU cause seasonal mean
O3 mixing ratio differences on the order of 1 ppb across large portions
of the domain, with the differences generally being largest during summer and in
areas characterized by the largest differences in the fractional coverages
of the forest, planted and cultivated, and grassland LU categories. These
differences are generally smaller than the M3Dry vs. STAGE differences
outside the summer season but have a similar magnitude during summer.
Results indicate that the deposition impacts of LU differences are caused
by differences in the fractional coverages and spatial distributions of
different LU categories and the characterization of these categories
through variables like surface roughness and vegetation fraction in lookup
tables used in the land surface model and deposition schemes. Overall, the
analyses and results presented in this study illustrate how the diagnostic
grid-scale and LU-specific dry deposition variables adopted for AQMEII4 can
provide insights into similarities and differences between the CMAQ M3Dry
and STAGE dry deposition schemes that affect simulated pollutant budgets and
ecosystem impacts from atmospheric pollution.