Atmosphere/Snow Interactions: Shepson Atmospheric Chemistry Group: Purdue University

Atmosphere/Snow Interactions


  1. To understand air/snow surface reactions involving halogenated and carbonyl compounds.
  2. To understand snowpack chemistry and the photochemical production/destruction of various reactive species.
  3. To determine the effect of increased aerosols due to open water in leads on halogens and photochemistry.

Overview of Arctic Halogen Chemistry:

It has been known for several decades that surface level ozone (and mercury) undergoes rapid depletion, to near zero concentrations, in springtime in polar environments on or near sea ice. This chemistry occurs via a photochemical heterogeneous catalytic chain reaction between ozone and molecular bromine known as a “bromine explosion”.  During a bromine explosion Br2 is first generated by photochemical oxidation in the snow pack.  Once Br2 is released into the atmosphere, it is photolysed to form a radical bromine atom.  This then begins a catalytic destruction cycle with ozone.

CIMS_BarrowLab     I2_PhotochemProd

While Br2 is the most potent destroyer of ozone, other halogens (Cl2 and I2) are also produced photochemically in the snowpack, and significantly affect atmospheric chemistry in the Arctic.  In the Shepson group all three molecular halogens, as well as various other halogen species are measured using chemical ionization mass spectrometry (see picture above on left).  This method has been used to demonstrate photochemical production of halogens in the snowpack (see picture above on right).

Interested in Halogen Chemistry? Click here for more details!

We have demonstrated that the snowpack produces halogens, there are many details about this chemistry that are still poorly understood. The rate of Xproduction in the snowpack, and how they mix into the atmosphere above is not well understood. The picture below shows the set-up for eddy covariance fluxes measured on the arctic tundra in spring 2016.   In addition to fluxes, vertical profiles of halogens were measured up to an altitude of 7 meters. These are the first measurements of halogens greater than 3 m high and lower than 300 m.


CHemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA):

CHACHA is a 2 month research campaign in Utqiagvik (formerly Barrow) utilizing 2 research aircraft, the Airborne Laboratory for Atmospheric Research (ALAR) and the University of Wyoming King Air, to address the below hypotheses:

  1. Open sea ice leads are a source of sea spray aerosol, containing particulate halides, to the Arctic boundary layer, affecting cloud composition and the vertical and horizontal propagation of halogen chemistry.
  2. Oil and gas extraction emissions significantly modify regional scale natural halogen chemistry, impacting multiphase halogen recycling and activation, as well as the fate of NOx and O3.
  3. Atmospheric oxidative processing of sulfur, nitrogen, and other chemical species differs between clear-sky conditions, and those impacted by in-cloud chemical processing.

See the project website here:

Keep track of our progress with the live Field catalog here:

And Dr. Shepson's blog here:

This is a large collaborative project with several Co PI's including the Pratt lab at the University of Michigan, the Simpson lab at the University of Alaska-Fairbanks, the Fuentes lab at Pennsylvania State University, and the Lance lab at the University of Albany.

See the campaign webpage and Dr. Shepson's blog to keep up to date with the project!

Aircraft in Barrow

Previous Field Campaigns and Projects