Long-term trace gas and black carbon measurements at the high-altitude station Mount Kenya: tropical atmospheric variability and the influence of African emissions

Abstract

Long-term measurements of atmospheric composition are essential for understanding regional and global climate impacts. Although the Global Atmosphere Watch (GAW) programme provides a network of worldwide measurements, continuous atmospheric measurements across Africa remain scarce. This study presents multi-year in-situ measurements of trace gases and black carbon from the Mount Kenya GAW station (MKN) from 2020 to 2024, offering a unique dataset from equatorial Africa. Its location exposes MKN to contrasting air masses from both hemispheres, enabling detection of emissions and providing insights into tropical variability such as seasonal and diurnal cycles. We present carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), ozone (O3), and black carbon (BC) measurements and describe seasonal and diurnal variability. Atmospheric transport modelling combined with emissions estimates for CO and methane were used to distinguish African and non-African contributions to greenhouse gases and air pollution. CO and BC were mainly linked to household fuel use and industrial energy, with biomass burning contributing during dry seasons. Methane variability was driven by agriculture and seasonal wetlands, but large uncertainties remain in all emission estimates. We further compare the measurements from 2020–2024 with trace-gas data from 2002–2012. More positive trends were observed for CO2 and CH4 in agreement with global patterns, whereas O3 exhibited non-significant positive trends in the recent period, consistent with findings from previous studies. CO trends were less conclusive due to the influence of sporadic biomass burning events, which complicate long-term trend detection. Comparison of the observations with Copernicus Atmospheric Monitoring Service (CAMS) model products shows CAMS fails to capture O3 and BC dynamics during rainy seasons. Overall, our results demonstrate the value of MKN observations for evaluating atmospheric models and emission inventories, and underscore the urgent need to expand measurement infrastructure across Africa to improve understanding of atmospheric processes and climate impacts.