Airborne Observations of the Human Impact on Remote Regions of Earth’s Atmosphere

Abstract

Two of the most important aspects of our atmosphere for human life are insulating greenhouse gases in the atmosphere and a protective layer of ozone approximately 25 kilometers above the Earth’s surface. Since the Industrial Revolution, these have been significantly altered by human activity. We explore two distinct aspects of better understanding the atmosphere and its changes, through airborne in situ measurements (1) in the stratosphere over the United States, and (2) in the troposphere over the North Slope of Alaska, the only region of the U.S. with permafrost. In the stratosphere, HCl observations are an important diagnostic for the evaluation of catalytic processes that impact the ozone layer. We report in situ balloon-borne observations of HCl employing off-axis integrated cavity output spectroscopy (ICOS) engaging a new instrument fitted with a re-injection mirror. We deployed the HCl instrument alongside an ozone instrument in August 2018 on a balloon-borne descent over New Mexico between 20- 80 hPa (29-18 km altitude). The observations agreed with nearby satellite measurements made by the Earth Observing System Microwave Limb Sounder within 10 % on average. This is the first time that stratospheric measurements of HCl have been made with ICOS and the first time any cavity enhanced HCl instrument has been tested in-flight. Through this work, we have established a viable airborne HCl instrument as part of a portfolio of in situ measurements for future stratospheric campaigns. The chemical partitioning of HCl, ozone, and other species in the future stratosphere depends on the future extent of climate change. The dominant uncertainty in projecting this is poor understanding of climate feedbacks, including the potential feedback from thawing permafrost. The microbial by-product nitrous oxide (N2O), a potent greenhouse gas and ozone depleting substance, has conventionally been assumed to have minimal emissions in permafrost regions. This assumption has been questioned by recent in situ studies. However, every known in situ study focused on permafrost N2O fluxes, have used chambers to examine small areas ( m2). In late August 2013, we used the airborne eddy-covariance technique to make in situ N2O flux measurements over the North Slope of Alaska from a low-flying aircraft spanning a much larger area: around 310 km2. We observed large variability of N2O fluxes with many areas exhibiting negligible emissions. The daily mean averaged over our flight campaign was 3.8 (2.2–4.7) mg N2O m−2 d−1 with the 90 % confidence interval shown in parentheses. Assuming these measurements are representative of the whole month, then the permafrost areas we observed emitted a total of around 0.04–0.09 g m−2 for August, which is comparable to what is typically assumed to be the upper limit of yearly emissions for these regions. If permafrost N2O emissions are already not negligible as we have observed, their predicted increase with warming permafrost soil temperatures could result in a non-carbon climate feedback of a currently unanticipated magnitude.