First look at the isotope data

Lab Note #21
May 14, 2014
Maria has finished running each sample twice.  She plans to do at least one more round, but yesterday we discussed the data she has collected so far at our group meeting.  Note that all of the interpretations below are very preliminary and may change as additional data becomes available.  My aim is to give you some insight into our progress as it develops.

First, I will tell you what we expected to see.  The image below shows nitrate isotope measurements (x-axis) as a function of depth (y-axis) in a snow pit that Maria dug on the surface of sea ice near Antarctica during the austral spring of 2012.  It shows a clear sign of active recycling of reactive nitrogen in the snow, with "light" nitrogen isotopes near the snow surface, rapidly increasing with depth over the first 2 cm, overlying "heavy" nitrogen isotopes at depth in the snow.  The star on the map of Antarctica shows where the samples were collected.  



The profile above is characteristic of active recycling of reactive nitrogen because the sun preferentially recycles the "light" isotopes, leaving the "heavy" isotopes behind at depth in the snow pack.  These "light" isotopes get re-deposited to the surface layer of the snow, creating the steep gradient in the upper 2 cm.

Many of Maria's snowpits from Utah did not look like this.  The image below shows the results from our first snow pit in the campaign.  The isotopes are in black, and the nitrate concentration is in red.  The brown shaded region represents the approximate location of the dusty layer.  It had not snowed for over a month when we arrived.  So dust had been depositing to and accumulating on the surface of the snow during this time.  


The dust will reduce the recycling of nitrogen in two ways.  1. It absorbs UV radiation, which is required for the chemistry to occur in the snow.  2. Dust is alkaline.  This means that it will reduce the acidity of the snow.  In addition to UV, snow chemistry requires acidity, as the reactions are acid-catalyzed.  Note that ozone was fairly low on this day.

However, there were several snow pits that did show signs of active recycling of nitrogen.  The image below shows one example.  These all occurred within a day or two of fresh snow fall.  You can see that the dusty layer is now buried below the fresh snowfall.  In this layer of fresh snow, the nitrogen isotopes increase rapidly with depth (over the first 2 cm, up to the dusty layer).  The surface values are quite light compared to the rest of the campaign, and the nitrogen isotopes of the aerosol collected from the atmospheric samples are also relatively "light" compared to most of the rest of the field campaign.  This suggest that active recycling of nitrogen in snow only occurs in fresh, acidic snow, which is actually consistent with our expectations.  


We have many more measurements to do, including the optical properties of the snow, where we will have a more quantitative estimate of the effect of this dusty layer.  We will also want to take a look at measurements (such as ozone) from other groups during the field campaign, to see if the periods of active recycling of nitrogen in the snow were associated with the few high ozone events that occurred during the campaign.

I will keep you updated on our progress.

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https://experiment.com/projects/does-fracking-cont...
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