Why Does NOAA ESRL Consider Summit a Key Arctic Site?

Summit Station, Greenland, operated by the National Science Foundation (NSF) and located at the top of the Greenlandic ice cap, is a unique Arctic observation site owing to its altitude (3,238m); all other Arctic atmospheric observatories are near sea level, where regional change influences measurements. At the elevation of Summit, surface measurements of atmospheric gases and aerosols are often representative of free troposphere concentrations. We have found that abundances and seasonal variations observed for a number of trace gases in the free troposphere from Summit are substantially different than observed at Arctic sites situated within the marine boundary layer. Seasonally-dependent vertical gradients of gases such as carbon dioxide (CO2), carbonyl sulfide (COS), methyl chloride (CH3Cl) and methyl bromide (CH3Br) are readily apparent by comparing results from Summit vs. measurements at Alert, Canada (200 m asl) or Barrow, US (11 m asl). NOAA ESRL is able to characterize vertical gradients of trace gases and their seasonal variations in other regions of the northern hemisphere by comparing results from Mauna Loa, HI (3,397 m asl) with Cape Kumukahi, HI (3 m asl) (both at 19°N) and Trinidad Head, CA (107 m asl) with Niwot Ridge, CO (3,523 m asl) at 40°N. These vertical gradients can provide unique insights into the nature and scale of processes influencing trace gases that are otherwise difficult to identify from surface measurements within the marine boundary layer alone. For example, spring-time ozone depletion events that occur around Alert and Barrow within the marine boundary layer influence trace gas abundances in unique ways that are not generally observed at Summit. Sampling in the free troposphere in the Arctic also allows us to better characterize the global atmospheric burden of trace gases. Balloonborne ozone monitoring at Summit is driven by the need to better understand stratospheric ozone depletion in the Arctic. Long range transport of anthropogenic pollutants and forest fire smoke from Asia and Canada are observed at Summit in the ozone and black carbon measurements. NOAA plans on expanding measurements in the coming years to the point that Summit will be a full Atmospheric Baseline Observatory along the lines of the Barrow Observatory.