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 (GOM _TK , PBM _TK , and RM _TK ) with RM _CEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RM _TK is systematically lower by a factor of 1.3 than RM _CEM . We observe a significant relationship between GOM _TK (but not PBM _TK ) and Tekran flush _TK blanks suggesting significant loss (36%) of labile GOM _TK from the denuder or inlet. Adding the flush _TK blank to RM _TK results in good agreement with RM _CEM (slope = 1.01, r ² = 0.90) suggesting we can correct bias in RM _TK and GOM _TK . 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.