Observations of Atmospheric Methane and Carbon Dioxide Mixing Ratios: Tall-Tower or Mountain-Top Stations?

Abstract

Mountain-top observations of greenhouse gas mixing ratios may be an alternative to tall-tower measurements for regional scale source and sink estimation. To investigate the equivalence or limitations of a mountain-top site as compared to a tall-tower site, we used the unique opportunity of comparing in situ measurements of methane ( $$\hbox {CH}_{4}$$ ) and carbon dioxide ( $$\hbox {CO}_{2}$$ ) mixing ratios at a mountain top (986 m above sea level, a.s.l.) with measurements from a nearby (distance 28.4 km) tall tower, sampled at almost the same elevation (1009 m a.s.l.). Special attention was given to, (i) how local wind statistics and greenhouse gas sources and sinks at the mountain top influence the observations, and (ii) whether mountain-top observations can be used as for those from a tall tower for constraining regional greenhouse gas emissions. Wind statistics at the mountain-top site are clearly more influenced by local flow systems than those at the tall-tower site. Differences in temporal patterns of the greenhouse gas mixing ratios observed at the two sites are mostly related to the influence of local sources and sinks at the mountain-top site. Major influences of local sources can be removed by applying a statistical filter ( $$5{\mathrm{th}}$$ percentile) or a filter that removes periods with unfavourable flow conditions. In the best case, the bias in mixing ratios between the mountain-top and the tall-tower sites after the application of the wind filter was $${-}0.0005\pm 0.0010$$ ppm for methane (September, 0000–0400 UTC) and $$0.11\pm 0.18$$ ppm for $$\hbox {CO}_{2}$$ (February, 1200–1600 UTC). Temporal fluctuations of atmospheric $$\hbox {CH}_{4}$$ and $$\hbox {CO}_{2}$$ mixing ratios at both stations also showed good agreement (apart from $$\hbox {CO}_{2}$$ during summertime) as determined by moving bi-weekly Pearson correlation coefficients (up to 0.96 for $$\hbox {CO}_{2}$$ and 0.97 for $$\hbox {CH}_{4}$$ ). When only comparing mixing ratios minimally influenced by local sources (low bias and high correlation coefficients), our measurements indicate that mountain-top observations are comparable to tall-tower observations.