Comprehensive assessment of vertical variations in urban atmospheric CO2 concentrations by using tall tower measurement and an atmospheric transport model.

In this study, we performed a comprehensive assessment of the vertical CO 2 concentration in the urban atmosphere using measurements at two different heights (113 m and 420 m) in Seoul, South Korea. The difference in CO 2 concentration between the two altitudes (△CO 2 = CO 2 at 113 m minus CO 2 at 420 m) showed a significant diurnal variation, with the highest at 07:00 (19.9 ppm) and the lowest at 16:00 (3.9 ppm). When the planetary boundary layer (PBL) rose above the two sites (daytime), the CO 2 concentrations at the two altitudes were highly correlated (r = 0.87) with low △CO 2. In contrast, when the PBL was located between the two sites (night time), the correlation coefficient of the CO 2 concentration between the two altitudes decreased by 0.55 with a high △CO 2. To explain the cause of this variation in △CO 2 according to PBL, we performed Weather Research and Forecasting-stochastic time-inverted Lagrangian transport (WRF-STILT) simulations. Simulations showed that CO 2 measurements at two different heights were influenced by the same nearby urban areas during the daytime. However, the site above the PBL only measured the CO 2 of air transported from the outside downtown area during the night time. Consequently, the observed night time △CO 2 is explained by the difference in air mass between the two measurements owing to PBL variations. The night time △CO 2 further implicates the local attribution of observed CO 2 below the PBL by removing the effect from the remote area. Because of this unique night time characteristic of △CO 2 , we evaluated the changes in CO 2 concentration in Seoul during the COVID-19 period. Compared to the pre-COVID-19 period, △CO 2 clearly decreased from 26.5 ppm to 6.2 ppm with the implementation of social distancing, thus confirming the decreasing local influence of CO 2 concentrations. Our findings highlight the potential of atmospheric CO 2 monitoring at high altitudes as an observation-based method to assess the effectiveness of local carbon management. • Emerging importance of tall tower CO 2 observation in carbon monitoring. • Observational evidence that PBL constrains the vertical variations of CO 2. • Urban atmospheric vertical structure can be identified via vertical CO 2 monitoring. • Variation of urban CO 2 emissions can be detected via vertical CO 2 monitoring. [ABSTRACT FROM AUTHOR]