Using Short‐Term CO/CO₂ Ratios to Assess Air Mass Differences Over the Korean Peninsula During KORUS‐AQ

One‐second in situ measurements of CO and CO 2 mole fractions were made aboard the National Aeronautics and Space Administration DC‐8 aircraft during the 2016 KORUS‐AQ joint air quality and atmospheric chemistry field campaign in South Korea. The ratio of CO to CO 2 enhancement is used to characterize regional combustion source signatures. Calculations of the ∆CO/∆CO 2 ratio were made with a short duration rolling window (60 s), filtered by the coefficient of determination ( R 2 ), and plotted as distributions to characterize air masses measured from the aircraft during the campaign. The KORUS‐AQ sampling domain was divided into analysis regions to facilitate the analysis. Over Seoul, the boundary layer shows a low‐ratio signature in the ∆CO/∆CO 2 ratios, with more than 50% of the correlated slopes in the boundary layer falling below 1% ∆CO/∆CO 2 , and 80% of the slopes between 0% and 2% ∆CO/∆CO 2 . However, this behavior changes to a larger ratio distribution at higher altitudes. The West Sea receptor region was divided into three analysis sectors, by meteorological regime, and used in conjunction with measurements collected over China during the KORUS‐AQ campaign time period to characterize the Chinese ∆CO/∆CO 2 ratio signature. Chinese‐type emissions have a slope distribution that is shifted to higher ratios and broadened compared to measurements over Seoul, with the bulk of the measurements between 2% and 4% ∆CO/∆CO 2 , with few negative slopes. The measured ratio trends over South Korea are consistent with inventoried CO and CO 2 emissions.