Description:
<jats:p>The regional Weather Research and Forecasting Model with Chemistry (WRF‐Chem) version 3.2 is used to analyze the carbon monoxide (CO) budget and spatiotemporal variability over the United States in summer 2008. CO tracers for different emission sources are used to separate the modeled CO fields into the contributions from individual sources (pollution inflow to the model domain, chemical production within the model domain, and local emissions by type). The implementation of tagged CO tracers into WRF‐Chem constitutes an innovative aspect of this work. We evaluate WRF‐Chem CO concentrations using aircraft, satellite, and surface observations. The model reproduces fairly well the observed CO concentrations for the entire altitude range but tends to underestimate fire emissions and overestimate anthropogenic sources and CO from pollution inflow. Evaluation results also show that the model gives a good representation of background CO mixing ratios with mean biases better than ∼15 ppbv in the free troposphere (FT) and less than 20 ppbv toward the surface. The analysis of the CO budget over the contiguous United States shows that at the surface, CO from inflow is the dominant source, with a mean relative contribution of 63 ± 19%. Anthropogenic and photochemically produced CO contribute to surface CO to a lesser extent (18 ± 14% and 14 ± 8%, respectively). The average contribution from fire emissions to surface CO during the period examined is small (2 ± 5%) but can have a large impact in certain regions and times. Similar trends are found in the planetary boundary layer (PBL). In the FT, the average CO relative contributions are estimated as 84 ± 12% for CO from inflow, 5 ± 4% for anthropogenic CO, 9 ± 7% for photochemically produced CO, and 1 ± 5% for CO from fires. Using WRF‐Chem simulations, we also examine the representation of surface and PBL CO concentration variability that would be captured by current near infrared (NIR) and thermal infrared (TIR) satellite observations. We find that CO total columns are impacted by variability in the lowermost troposphere (LMT) at the ∼10% level, indicating limited sensitivity for air quality applications. The same is generally true for the FT CO column obtained from TIR measurements, although this does provide a good measure for capturing the pollution inflow variability and is therefore valuable in providing initial and boundary conditions to constrain regional models. We further analyze the situations under which the LMT concentrations obtained from recently demonstrated multispectral (NIR + TIR) observations capture the surface CO variability.</jats:p>