Long-term land-atmosphere carbon monoxide exchange from a tall tower in a rural Central European region

Abstract. Carbon monoxide (CO) is a crucial atmospheric trace gas, significantly influencing the atmosphere's oxidative capacity and indirectly affecting climate. However, the net land-atmosphere exchange of CO remains highly uncertain due to the scarcity of long-term monitoring. This study presents and analyses a 10-year-long CO flux data series from a tall-tower eddy covariance (EC) system operated in a rural Central European region. The measured fluxes were evaluated separately for cases when the flux footprint of the measurement covered almost exclusively quasi-natural areas (arable land, forests), and for cases when the footprint of the measurement covered populated areas (villages, roads). The vegetation-dominated sector (agricultural fields and forests) acted as a weak net CO source from April to September with an average emission of 0.58 nmol m^‑2 s^‑1 (monthly range 0.31–0.91 nmol m^‑2 s^‑1), with the highest value occurring in July. The diurnal peaks of the median hourly emissions can be observed around noon, and they range from 1.59 to 2.66 nmol m^‑2 s^‑1. The nighttime (20–04 h LST) hourly median values range from ‑1.01 to +1.17 nmol m^‑2 s^‑1. Their deviations from zero are not statistically significant at a p<0.05 probability level. Solar radiation was identified as the primary driver of CO exchange. The measured emissions from the populated areas significantly exceed the activity-based estimations, presumably due to the underestimation of the emissions from residential heating. This study also presents the advantages and challenges of tall-tower EC flux measurements.