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Gas-Phase Water-Soluble Organic Carbon: CMAQ Model Evaluation in Baltimore County
Ellie N Smith1, Kirk R Baker2, Marwa M H El-Sayed3
1Department of Chemistry, University of California, Irvine, California 92617, United States.
ACS Earth & Space Chemistry
|June 25, 2025
Summary
The Community Multiscale Air Quality (CMAQ) model struggles to predict gas-phase water-soluble organic carbon (WSOCg), a key precursor for secondary organic aerosol. Model performance for WSOCg differs significantly from real-world measurements.
Area of Science:
- Atmospheric Chemistry
- Air Quality Modeling
- Aerosol Science
Background:
- Gas-phase water-soluble organic carbon (WSOCg) is a crucial precursor for secondary organic aerosol (SOA) formed in atmospheric waters (aqSOA).
- Accurate modeling of WSOCg is essential for understanding aqSOA formation, yet its prediction has not been adequately evaluated in atmospheric models.
- The Community Multiscale Air Quality (CMAQ) model is a widely used tool for simulating air quality, but its performance for WSOCg requires investigation.
Purpose of the Study:
- To evaluate the predictive capability of the CMAQ model for gas-phase water-soluble organic carbon (WSOCg).
- To compare CMAQ-simulated WSOCg concentrations and diurnal patterns with continuous mist chamber measurements.
- To assess the model's ability to capture the chemical transformations leading to WSOCg formation.
Main Methods:
- Paired CMAQ model predictions with mist chamber measurements of WSOCg in Baltimore County, MD.
- Simulated mist chamber collection of WSOCg using compound-specific collection efficiencies based on Henry's law.
- Evaluated model skill for WSOCg, nitrogen dioxide (NO2), and ozone (O3) using statistical metrics.
Main Results:
- CMAQ predicted higher WSOCg concentrations in August, contrary to measurements showing peaks in February-March.
- The model failed to replicate the observed diurnal patterns of WSOCg across all months.
- CMAQ showed excellent skill in predicting NO2 (R² = 0.5) and O3 (R² = 0.6), indicating sufficient representation of relevant precursor gases and chemistry for these pollutants.
Conclusions:
- The CMAQ model's simulation of organic gases and their chemistry is adequate for predicting criteria pollutants like NO2 and O3.
- However, the model's current representation may not fully capture the complex chemical transformations that produce WSOCg, impacting aqSOA precursor predictions.
- Further improvements in modeling WSOCg formation pathways are needed for accurate aqSOA predictions.
