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, <i>r</i>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.