CMAQv5.0 Sulfur Chemistry
Golam Sarwar, Sergey Napelenok, Shawn Roselle, Kathleen Fahey
A number of updates have been implemented in CMAQv5.0 that directly impact the predictions of ambient sulfate. In CMAQv5.0, ISORROPIA 2.1 (Fountoukis and Nenes, 2007) was implemented, along with an updated treatment and tracking of crustal species (e.g., Ca2+, K+, Mg2+) and trace metals (e.g., Fe, Mn). These species impact aqueous-phase oxidation of S(IV) to S(VI) by altering the pH and ionic strength of the droplets. The explicit treatment of Fe and Mn now also modulate the Fe(III)/Mn(II) metal-catalyzed aqueous-phase sulfur oxidation.
CMAQ contains five aqueous-phase chemical pathways for SO2 oxidation involving (1) H2O2, (2) O3, (3) methylhydroperoxide (MHP), (4) peroxyacetic acid (PAA), and (5) metal catalysis. Rate constants for all pathways (except metal catalysis) have been updated following Jacobson (1997). The rate constant for SO2 oxidation via metal catalysis has been updated following Martin and Goodman (1991).
In previous versions of the CMAQ model, sulfate production via the above reaction was calculated using the prescribed background concentrations of 0.01 μg/m3 for Fe(III) and 0.005 μg/m3 for Mn(II). As CMAQv5.0 contains predictions of Fe and Mn, their modeled concentrations are now used to estimate Fe(III) and Mn(II) values for the metal catalyzed oxidation pathway. To estimate aqueous-phase Fe(III) and Mn(II) concentrations from total (activated) aerosol iron and manganese, the solubility and oxidation state of these species need to be estimated. Iron solubility and oxidation state is highly variable and dependent on a number of factors including origin of the aerosol and time of day, with more soluble iron aerosol found in anthropogenic source regions compared to those areas with high levels of natural dust emissions (Alexander et al., 2009; Seifert et al., 1998). Manganese is typically more soluble than iron and exists mainly as Mn(II) in cloud/fog droplets. Iron cycles diurnally, and exists mainly as Fe(II) during the day and Fe(III) at night (Alexander et al., 2009). In CMAQv5.0, the solubility of iron and manganese is kept constant at 10% and 50%, respectively (Alexander et al., 2009). All dissolved manganese is assumed to be Mn(II), and Fe(III) is assumed to be 90% of the dissolved Fe at night and 10% during the day. Note that while only Fe(III) and Mn(II) impact the S(IV) oxidation rate, all Fe and Mn in the activated droplets is subjected to scavenging/deposition.
The CB05TUCL mechanism in CMAQ contains one gas-phase chemical reaction for SO2 oxidation. The rate constant for the reaction in CMAQv5.0 has been updated using the 2006 NASA/JPL recommendation. The gas-phase rate constant for the SO2 oxidation is now consistent between the CB05 and SAPRC07 mechanisms.
Impact of the changes
In summer, the resulting changes increase sulfate by up to 0.5 μg/m3 and decrease SO2. Their impact on aerosol sulfate in winter is mixed. They increase aerosol sulfate in some areas while decrease in other areas. The impact on ozone is small. Please see Sarwar et al. (2011) for more details.
References:
Alexander, B., R.J. Park, D.J. Jacob, S. Gong, 2009. Transition metal-catalyzed oxidation of atmospheric sulfur: global implications for the sulfur budget, GRL, 114, D02309.
Fountoukis C. and A. Nenes, 2007. ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+-Ca2+-Mg2+-NH4+ -Na+-SO42--NO3- -Cl- -H2O aerosols. Atmospheric Chemistry and Physics, 7, 4639-4659.
Jacobson, M., 1997. Development and application of a new air pollution modeling system II. Aerosol module structure and design, Atmospheric Environment, 31, 131-144.
Martin, R.L. and T.W. Good, 1991. Catalyzed oxidation of sulfur dioxide in solution: the iron-manganese synergism, Atmospheric Environment, 25A, 2395-2399.
Sarwar, G., K. Fahey, S. Napelenok, S. Roselle, R. Mathur, 2011. Examining the impact of CMAQ model updates on aerosol sulfate predictions, the 10th Annual CMAS Models-3 User's Conference, October, Chapel Hill, NC.
Seifert, R.L., A. M. Johansen, M. R. Hoffman, and S.O. Pehkonen, 1998. Measurements of trace metal (Fe, Cu, Mn, Cr) oxidation states in fog and stratus clouds, J. Air and Waste Manage. Assoc., 48, 128-143.
Affected files:
AQ_DATA.F, aqchem.F, convcld_acm.F, rescld.F, aq_map.F, AE_cb05cl_ae6_aq_recon.csv, AE_cb05cl_ae6_aq_recon.nml, mech.def, RXCM.EXT, RXDT.EXT, hrdriver.F, hrcalcks.F, hrdata_mod.F, hrg1.F, hrg2.F, hrg3.F, hrg4.F, hrinit.F, hrprodloss.F, hrrates.F, hrsolver.F
