Tropospheric methane in northern Finland: seasonal variations, transport patterns and correlations with other trace gases

Methane mixing ratios have been continuously observed at Pallas, Finland since winter 2004. The seasonal variation in monthly means was ca. 40 ppb, showing largest mixing ratios in winter and also high values during late summer. Examination of back-trajectories showed that the air masses with elevated methane mixing ratios arrived from continental Eastern and Central Europe while lowmethane mixing ratios were connected with Atlantic and Arctic air masses. During summer, air masses with highest mixing ratios arrived from Northwestern Russia indicating wetland sources, while the influence of southern emissions became more significant in winter. Methane was positively correlated with carbon dioxide and negatively correlated with ozone in winter. The average slope of the selected wintertime background hourly mean mixing ratios was 7.0 ± 1.2 ppb(CH₄)/ppm(CO₂). Nocturnal summertime low-altitude measurements above a local wetland source indicated slopes of about 10 ± 1 ppb(CH₄)/ppm(CO₂). The different slopes reflect the differences in emission parameters.