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.