Linking gas, particulate, and toxic endpoints to air emissions in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM)
Authors: Pye Havala O. T. , Place Bryan K. , Murphy Benjamin N. , Seltzer Karl M. , D'Ambro Emma L. , Allen Christine , Piletic Ivan R. , Farrell Sara , Schwantes Rebecca H. , Coggon Matthew M. , Saunders Emily , Xu Lu , Sarwar Golam , Hutzell William T. , Foley Kristen M. , Pouliot George , Bash Jesse , Stockwell William R. 
Published: May 2023

Abstract. Chemical mechanisms describe the atmospheric
transformations of organic and inorganic species and connect air emissions
to secondary species such as ozone, fine particles, and hazardous air
pollutants (HAPs) like formaldehyde. Recent advances in our understanding of
several chemical systems and shifts in the drivers of atmospheric chemistry
warrant updates to mechanisms used in chemical transport models such as the
Community Multiscale Air Quality (CMAQ) modeling system. This work builds on
the Regional Atmospheric Chemistry Mechanism version 2 (RACM2) and develops
the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM)
version 1.0, which demonstrates a fully coupled representation of chemistry
leading to ozone and secondary organic aerosol (SOA) with consideration of
HAPs. CRACMMv1.0 includes 178 gas-phase species, 51 particulate species,
and 508 reactions spanning gas-phase and heterogeneous pathways. To support
estimation of health risks associated with HAPs, nine species in CRACMM
cover 50 % of the total cancer and 60 % of the total non-cancer
emission-weighted toxicity estimated for primary HAPs from anthropogenic and
biomass burning sources in the US, with the coverage of toxicity higher
(> 80 %) when secondary formaldehyde and acrolein are
considered. In addition, new mechanism species were added based on the
importance of their emissions for the ozone, organic aerosol, or atmospheric
burden of total reactive organic carbon (ROC): sesquiterpenes, furans,
propylene glycol, alkane-like low- to intermediate-volatility organic
compounds (9 species), low- to intermediate-volatility oxygenated species (16 species), intermediate-volatility aromatic hydrocarbons (2 species), and
slowly reacting organic carbon. Intermediate- and lower-volatility organic
compounds were estimated to increase the coverage of anthropogenic and
biomass burning ROC emissions by 40 % compared to current operational
mechanisms. Autoxidation, a gas-phase reaction particularly effective in
producing SOA, was added for C10 and larger alkanes, aromatic
hydrocarbons, sesquiterpenes, and monoterpene systems including second-generation aldehydes. Integrating the radical and SOA chemistry put
additional constraints on both systems and enabled the implementation of
previously unconsidered SOA pathways from phenolic and furanone compounds,
which were predicted to account for ∼ 30 % of total aromatic
hydrocarbon SOA under typical atmospheric conditions. CRACMM organic aerosol
species were found to span the atmospherically relevant range of species carbon
number, number of oxygens per carbon, and oxidation state with a slight high
bias in the number of hydrogens per carbon. In total, 11 new emitted species
were implemented as precursors to SOA compared to current CMAQv5.3.3
representations, resulting in a bottom-up prediction of SOA, which is
required for accurate source attribution and the design of control strategies.
CRACMMv1.0 is available in CMAQv5.4.