By Dr Rebecca Fisher, Royal Holloway University of London
I’m writing this half way through a 24 hour air sampling campaign in the Stordalen wetland in Abisko, Sweden. Abisko (68⁰21’N, 19⁰03’E) is about 100 km NW of Kiruna where the aircraft team is based. Methane emissions from the wetland here have been studied for more than 30 years. There has been an increase in CH4 emissions over this period as discontinuous permafrost has thawed.
Why am I staying here all night? I’m collecting samples of air from just above the surface of the wetland (30 cm and 3 m above ground height), every two hours for 24 hours. Unfortunately (for a sleep deprived postdoc) the highest concentrations of methane in air tend to be in the early hours of the morning, just before sunrise. Overnight emissions of methane from the wetland soil can get trapped near the ground.
How much the methane concentration will increase overnight depends on the meteorology which influences the mixing height – highest concentrations are usually measured in still conditions, particularly if a temperature inversion occurs. The wind is behaving tonight - there was a slight westerly breeze in the afternoon but the wind completely dropped as soon as the sun went below the horizon, which means local emissions should build up. When the sun rises the atmosphere gets mixed up and the concentration of measure down near the ground will decrease. Because of this mixing of the atmosphere during the day the air close to the ground is cleanest in the early afternoon.
When the air samples get back to our laboratory at Royal Holloway, we will measure the concentration of methane in the air, and then use a mass spectrometer to measure the isotopic composition (i.e. the ratio of carbon-13 to carbon-12) in the methane. The isotopic signature of methane that is released to the atmosphere from wetlands can vary depending on factors such as the production mechanism, temperature, water level and vegetation type. We’re carrying out these experiments at a number of wetland sites to see how variable the isotopic signature of Arctic wetland methane is. We will then compare the isotopic measurements from samples on the aircraft with the ground based measurements.
So what’s the wetland like at 2am? It certainly doesn’t feel like the middle of the night as it’s not very dark. Although the sun set at around 21.30 and won’t rise until after 4 the sky has been very light towards the northern horizon and there’s just enough light to walk along the boardwalks without falling in (so far). The temperature has dropped from 15⁰C during the day to 7⁰C but the soil temperature has been fairly constant (around 10⁰C at 10 cm depth), so we expect the flux of methane from the soil to be fairly constant between day and night. It’s very quiet until I turn my air sampling pump on and the noise disturbs some birds. I have a few mosquitoes for company but thankfully there aren’t as many this week as we have experienced in the past. I can hear the distant hum of a greenhouse gas analyser and whirr of automatic chambers opening and closing. No wolves or bears have come to investigate what I’m doing yet. Maybe they’re used to the unusual activity of scientists around Abisko.
Previously in this series: