UV absorption data

Lab Note #23
Jun 12, 2014
In my last lab note, I explained how we do the measurements of ultra-violet (UV) absorption.  The figure below shows Maria's absorption measurements from snowpit 9.  The y-axis is depth, the black line (lower x-axis) shows the "optical depth", and the red line (upper x-axis) shows the fraction of absorption due to brown (as opposed to black carbon) material.  Brown material consists of things like dust and organics.

This snowpit was collected just after fresh snowfall, so you can see that "dusty layer" between about 2 and 6 cm depth.  An optical depth of 5, which is the maximum value around 5 cm depth, means that about 99% of the UV radiation is being absorbed.  For comparison, an optical depth of 0.5 means that about 30% of the UV light is being absorbed.  The data demonstrates how effective this dusty layer is at absorbing UV radiation, which will limit snow chemistry within and below this layer.  This may explain why we only see a clear signature of snow chemistry just after fresh snowfall, before the dust had a chance to build up on the surface of the snow and absorb most of the UV light.  The red line shows that most of the absorption in that layer is due to brown material, as opposed to another important absorber, black carbon (a.k.a. soot).  Though we don't know for sure what the brown material is, we suspect it is dust based on its appearance and the large amount of oil-truck traffic on the dirt roads that surrounded the field site.

These optical depth measurements will be used in our snow chemistry model for calculating the recycling rates of nitrogen in the snow.
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