Processes regulating the sources and sinks of ammonia in the Canadian Arctic.

As part of the NETCARE project, measurements of gas phase ammonia (NH₃) were made onboard the Canadian Coast Guard Ship Amundsen operating in the Canadian Arctic Archipelago and at the Global Atmospheric Watch station at Alert, NU in the summer of 2016. Comparing with our previous measurements from the Amundsen for a similar summer period in 2014 (median mixing ratio of 220 pptv), we found similar levels of NH₃ in the region in 2016 (140 pptv from the ship and 230 pptv at Alert). We also characterized the NH₃ emission potential of the tundra soil, finding that there is sufficient NH₄⁺ and high enough pH in the soil that it may act as a source, especially under elevated soil temperatures. Using the NH₃ emission potential of three tundra soil samples collected near Alert, and the bidirectional flux framework, we found that the average net tundra-air exchange during the study period ranged between -2.7 mg N m^-2 h^-1 (deposition) to +3.1 mg N m^-2 h^-1 (emission). This implies that warming Arctic soils may act as sources of NH₃ to the local atmosphere. Analysis of submicron particles collected onboard the Amundsen and at Alert show that the NH₃ is predominantly in the gas phase (gas fraction is 64-99% on the Amundsen and 85-98% at Alert). The ammonium content of rain and snow samples indicates that wet deposition is an important sink of atmospheric NH₃ in the summer, especially relative to winter deposition measurements at Alert. Frequent drizzle limits the lifetime of NH₃ against wet deposition to a timescale of ∼1 day.