Evaluating high-resolution forecasts of atmospheric CO and CO ₂ from a global prediction system during KORUS-AQ field campaign

page11008Accurate and consistent monitoring of anthropogenic combustion is imperative because of its significant health and environmental impacts, especially at city-to-regional scale. Here, we assess the performance of the Copernicus Atmosphere Monitoring Service (CAMS) global prediction system using measurements from aircraft, ground sites, and ships during the Korea-United States Air Quality (KORUS-AQ) field study in May to June 2016. Our evaluation focuses on CAMS CO and inline-formulaCO₂ analyses as well as two higher-resolution forecasts (16 and 9 km horizontal resolution) to assess their capability in predicting combustion signatures over east Asia. Our results show a slight overestimation of CAMS inline-formulaCO₂ with a mean bias against airborne inline-formulaCO₂ measurements of 2.2, 0.7, and 0.3 ppmv for 16 and 9 km inline-formulaCO₂ forecasts, and analyses, respectively. The positive inline-formulaCO₂ mean bias in the 16 km forecast appears to be consistent across the vertical profile of the measurements. In contrast, we find a moderate underestimation of CAMS CO with an overall bias against airborne CO measurements of inline-formula−19.2 (16 km), inline-formula−16.7 (9 km), and inline-formula−20.7 ppbv (analysis). This negative CO mean bias is mostly seen below 750 hPa for all three forecast/analysis configurations. Despite these biases, CAMS shows a remarkable agreement with observed enhancement ratios of CO with inline-formulaCO₂ over the Seoul metropolitan area and over the West (Yellow) Sea, where east Asian outflows were sampled during the study period. More efficient combustion is observed over Seoul (inline-formula $M12inlinescrollmathml\mathrm{normal\; d}chem\mathrm{normal\; CO}/\mathrm{normal\; d}chem{\mathrm{normal\; CO}}_{\mathrm{normal\; 2}}=\mathrm{normal\; 9}$ 77pt14ptsvg-formulamathimge15dbed157728345ac15fd0589ea867d acp-18-11007-2018-ie00001.svg77pt14ptacp-18-11007-2018-ie00001.png  ppbv ppmvinline-formula⁻¹) compared to the West Sea (inline-formula $M14inlinescrollmathml\mathrm{normal\; d}chem\mathrm{normal\; CO}/\mathrm{normal\; d}chem{\mathrm{normal\; CO}}_{\mathrm{normal\; 2}}=\mathrm{normal\; 28}$ 83pt14ptsvg-formulamathimg3be8931fd245903ba9a1c1d59edabca9 acp-18-11007-2018-ie00002.svg83pt14ptacp-18-11007-2018-ie00002.png  ppbv ppmvinline-formula⁻¹). This “combustion signature contrast” is consistent with previous studies in these two regions. CAMS captured this difference in enhancement ratios (Seoul: 8–12 ppbv ppmvinline-formula⁻¹, the West Sea: inline-formula∼30 ppbv ppmvinline-formula⁻¹) regardless of forecast/analysis configurations. The correlation of CAMS CO bias with inline-formulaCO₂ bias is relatively high over these two regions (Seoul: 0.64–0.90, the West Sea: inline-formula∼0.80) suggesting that the contrast captured by CAMS may be dominated by anthropogenic emission ratios used in CAMS. However, CAMS shows poorer performance in terms of capturing local-to-urban CO and inline-formulaCO₂ variability. Along with measurements at ground sites over the Korean Peninsula, CAMS produces too high CO and inline-formulaCO₂ concentrations at the surface with steeper vertical gradients (inline-formula∼0.4 ppmv hPainline-formula⁻¹ for inline-formulaCO₂ and 3.5 ppbv hPainline-formula⁻¹ for CO) in the morning samples than observed (inline-formula∼0.25 ppmv hPainline-formula⁻¹ for inline-formulaCO₂ and 1.7 ppbv hPainline-formula⁻¹ for CO), suggesting weaker boundary layer mixing in the model. Lastly, we find that the combination of CO analyses (i.e., improved initial condition) and use of finer resolution (9 km vs. 16 km) generally produces better forecasts.