Vertical distribution of Arctic methane in 2009–2018 using ground-based remote sensing

Abstract:

We have created a 10-year time series of atmospheric methane vertical profiles. The vertical profile information is inferred from the line shapes of methane absorption features in high resolution short wave infrared spectra measured by a ground-based Fourier Transform Spectrometer at Sodankylä, Finland. In the retrieval we used dimension reduction method, where the variability of the profile around the prior is described by four singular vectors calculated form the prior covariance matrix. This reduces the retrieved parameters from full profile of 100 layers to 4 coefficients and also constrains the retrieval to smooth posterior profiles. Here we have used ground-based FTS data but the method could be applied to other remote sensing instruments as well.

We compared the retrieved profiles to the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) satellite measurements and the AirCore balloon-borne profile measurements. The lowest retrieved layer was also compared to in-situ measurements from a 50-meter mast. In general, the ground-based FTS and ACE-FTS profiles agreed within 10% below 20 km and within 40% in the highly variable stratosphere between 20 km and 40 km. Total columns calculated from the profiles were within 6 ppb of the official TCCON GGG2014 columns.

In addition to one-to-one comparison we also used dynamic linear model (DLM) to calculate smoothed growth rates over the time series from the retrieval and the instruments used as the reference. Our method produced similar trend characteristics as the references. Our trend estimates suggest that the most recent tropospheric CH4 growth rate has been lower than the global average. Meanwhile the stratospheric growth rate has become stronger, which may point to an enhanced circulation from the tropics or to a decrease in the stratospheric methane sink.