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<p>The black carbon (BC) and carbon monoxide (CO) emission ratios were estimated and compiled from long-term, harmonized observations of the <span class="inline-formula">ΔBC∕ΔCO</span> ratios under conditions unaffected by wet deposition at four sites in East Asia, including two sites in South Korea (Baengnyeong and Gosan) and two sites in Japan (Noto and Fukuoka). Extended spatio-temporal coverage enabled estimation of the full seasonality and elucidation of the emission ratio in North Korea for the first time. The estimated ratios were used to validate the Regional Emission inventory in ASia (REAS) version 2.1 based on six study domains (“East China”, “North China”, “Northeast China”, South Korea, North Korea, and Japan). We found that the <span class="inline-formula">ΔBC∕ΔCO</span> ratios from four sites converged into a narrow range (6.2–7.9&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>&thinsp;ppb<span class="inline-formula">⁻¹)</span>, suggesting consistency in the results from independent observations and similarity in source profiles over the regions. The <span class="inline-formula">BC∕CO</span> ratios from the REAS emission inventory (7.7&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>&thinsp;ppb<span class="inline-formula">⁻¹</span> for East China – 23.2&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>&thinsp;ppb<span class="inline-formula">⁻¹</span> for South Korea) were overestimated by factors of 1.1 for East China to 3.0 for South Korea, whereas the ratio for North Korea (3.7&thinsp;ng&thinsp;m<span class="inline-formula">⁻³</span>&thinsp;ppb<span class="inline-formula">⁻¹</span> from REAS) was underestimated by a factor of 2.0, most likely due to inaccurate emissions from the road transportation sector. Seasonal variation in the <span class="inline-formula">BC∕CO</span> ratio from REAS was found to be the highest in winter (China and North Korea) or summer (South Korea and Japan), whereas the measured <span class="inline-formula">ΔBC∕ΔCO</span> ratio was the highest in spring in all source regions, indicating the need for further characterization of the seasonality when creating a bottom-up emission inventory. At levels of administrative districts, overestimation in Seoul, the southwestern regions of South Korea, and Northeast China was noticeable, and underestimation was mainly observed in the western regions in North Korea, including Pyongyang. These diagnoses are useful for identifying regions where revisions in the inventory are necessary, providing guidance for the refinement of BC and CO emission rate estimates over East Asia.</p>