FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

Fate of Mercury in the Arctic 45
From the quick connect at the top of the filter pack were connected 3.2 m long neoprene hoses into
3 fast response switching valves supplied by MetSupport. From behind the switches, the three
valves were connected into one sampling line using a simple 3 inlet manifold constructed with 2 T-
type locking copper hose connectors in series and 1 L type locking hose connector as the end piece.
Coming out of the manifold, a locking ball valve, was used to adjust and fix the flow, as a back
up to the mass flow controller. Between the pump and the valve, a Tylan mass flow controller was
used to ensure a flow prior to the manifold of just over 10 litre min -1 , so that the flow was measured
as 10 litre min -1 at the denuder inlet. Pressure loss was minimal in the manifold and through the
sampling lines. Once the system was running, the lag time from when the switch opened to when
the flow started out the denuder was very small compared to the air sampling switching frequency
of 1 Hz.
Normally flux systems operate at air sampling switching rates of 10 Hz, switching as fast as the
air flow is sampled with the sonic anemometer. The only lag time between the air and the switching
comes from the software and physical switching process, including flow development in systems
that do not have zero air induced into the sampling inlet system to stabilize the flow. This means
that within one second the flow can effectively be changed between one accumulator to another 10
times, sufficient for total air accumulators such as canisters or bags. Denuders are selective
accumulators, collecting only the trace gas of interest, and require laminar flow. Given the
geometry of the URG annular denuders, the 10 Hz sampling switching rate nominally allows for
full laminar flow development and escape of an air packet through the end of the annular denuder.
From initial measurements, it was immediately clear, that the annular denuders were not
collecting efficiently while switching at 10 Hz, with RGM mass just above detection limits being
collected. Therefore tests were carried out in Oak Ridge, on top of a 50 m tower at Walker Branch,
to determine if the switching rate required for good reproduction of total RGM ambient