The Community Multiscale Air Quality (CMAQ) model versions 5.3 and 5.3.1: system updates and evaluation
Authors: Appel K. Wyat , Bash Jesse O. , Fahey Kathleen M. , Foley Kristen M. , Gilliam Robert C. , Hogrefe Christian , Hutzell William T. , Kang Daiwen , Mathur Rohit , Murphy Benjamin N. , Napelenok Sergey L. , Nolte Christopher G. , Pleim Jonathan E. , Pouliot George A. , Pye Havala O. T. , Ran Limei , Roselle Shawn J. , Sarwar Golam , Schwede Donna B. , Sidi Fahim I. , Spero Tanya L. , Wong David C. 
Published: May 2021

Abstract. The Community Multiscale Air Quality (CMAQ) model version 5.3 (CMAQ53), released to the public in August 2019 and followed by version 5.3.1 (CMAQ531) in December 2019, contains numerous science updates,
enhanced functionality, and improved computation efficiency relative to the
previous version of the model, 5.2.1 (CMAQ521). Major science advances in
the new model include a new aerosol module (AERO7) with significant updates
to secondary organic aerosol (SOA) chemistry, updated chlorine chemistry,
updated detailed bromine and iodine chemistry, updated simple halogen chemistry, the
addition of dimethyl sulfide (DMS) chemistry in the CB6r3 chemical
mechanism, updated M3Dry bidirectional deposition model, and the new
Surface Tiled Aerosol and Gaseous Exchange (STAGE) bidirectional deposition
model. In addition, support for the Weather Research and Forecasting (WRF)
model's hybrid vertical coordinate (HVC) was added to CMAQ53 and the
Meteorology-Chemistry Interface Processor (MCIP) version 5.0 (MCIP50).
Enhanced functionality in CMAQ53 includes the new Detailed Emissions
Scaling, Isolation and Diagnostic (DESID) system for scaling incoming
emissions to CMAQ and reading multiple gridded input emission files. Evaluation of CMAQ531 was performed by comparing monthly and seasonal mean
daily 8 h average (MDA8) O3 and daily PM2.5 values from several
CMAQ531 simulations to a similarly configured CMAQ521 simulation
encompassing 2016. For MDA8 O3, CMAQ531 has higher O3 in the
winter versus CMAQ521, due primarily to reduced dry deposition to snow,
which strongly reduces wintertime O3 bias (2–4 ppbv monthly
average). MDA8 O3 is lower with CMAQ531 throughout the rest of the
year, particularly in spring, due in part to reduced O3 from the
lateral boundary conditions (BCs), which generally increases MDA8 O3
bias in spring and fall (∼0.5 µg m−3). For daily
24 h average PM2.5, CMAQ531 has lower concentrations on average in
spring and fall, higher concentrations in summer, and similar concentrations
in winter to CMAQ521, which slightly increases bias in spring and fall and
reduces bias in summer. Comparisons were also performed to isolate updates
to several specific aspects of the modeling system, namely the lateral BCs,
meteorology model version, and the deposition model used. Transitioning from
a hemispheric CMAQ (HCMAQ) version 5.2.1 simulation to a HCMAQ version 5.3
simulation to provide lateral BCs contributes to higher O3 mixing
ratios in the regional CMAQ simulation in higher latitudes during winter
(due to the decreased O3 dry deposition to snow in CMAQ53) and lower
O3 mixing ratios in middle and lower latitudes year-round (due to reduced
O3 over the ocean with CMAQ53). Transitioning from WRF version 3.8 to
WRF version 4.1.1 with the HVC resulted in consistently higher (1.0–1.5 ppbv) MDA8 O3 mixing ratios and higher PM2.5 concentrations (0.1–0.25 µg m−3) throughout the year. Finally, comparisons of the
M3Dry and STAGE deposition models showed that MDA8 O3 is generally
higher with M3Dry outside of summer, while PM2.5 is consistently higher
with STAGE due to differences in the assumptions of particle deposition
velocities to non-vegetated surfaces and land use with short vegetation
(e.g., grasslands) between the two models. For ambient NH3, STAGE has
slightly higher concentrations and smaller bias in the winter, spring, and
fall, while M3Dry has higher concentrations and smaller bias but larger
error and lower correlation in the summer.