Tropospheric GOM at the Pic du Midi ObservatoryCorrecting Bias in Denuder Based Observations

Gaseous elemental mercury (GEM, Hg)
emissions are transformed to divalent reactive Hg (RM) forms throughout
the troposphere and stratosphere. RM is often operationally quantified
as the sum of particle bound Hg (PBM) and gaseous oxidized Hg (GOM).
The measurement of GOM and PBM is challenging and under mounting criticism.
Here we intercompare six months of automated GOM and PBM measurements
using a Tekran (TK) KCl-coated denuder and quartz regenerable particulate
filter method (GOMTK, PBMTK, and RMTK) with RMCEM collected on cation exchange membranes (CEMs)
at the high altitude Pic du Midi Observatory. We find that RMTK is systematically lower by a factor of 1.3 than RMCEM. We observe a significant relationship between GOMTK (but
not PBMTK) and Tekran flushTK blanks suggesting
significant loss (36%) of labile GOMTK from the denuder
or inlet. Adding the flushTK blank to RMTK results
in good agreement with RMCEM (slope = 1.01, r2 = 0.90) suggesting we can correct bias in RMTK and GOMTK. We provide a bias corrected (*) Pic du Midi
data set for 2012–2014 that shows GOM* and RM* levels in dry
free tropospheric air of 198 ± 57 and 229 ± 58 pg m–3 which agree well with in-flight observed RM and with
model based GOM and RM estimates.