Comparison of reactive gaseous mercury measured by KCl-coated denuders and cation exchange membranes during the Pacific GEOTRACES GP15 expedition

Reactive gaseous mercury (RGHg) over the marine boundary layer (MBL) has been investigated in many oceans, such as the North Atlantic, Arctic and Antarctic using the KCl-coated denuder, as deployed in the Tekran instrument. Given recent concerns on the suitability of the denuder for capturing RGHg, we initiated a comparison study of RGHg concentrations measured by the KCl-coated denuder and with cation exchange membranes (CEM) during the Pacific GEOTRACES GP15 cruise between Alaska and Tahiti along 152°W from 18 September to November 24, 2018. RGHg concentrations measured by the KCl-coated denuder showed a strong variability along the cruise and ranged from 0.2 to 42.2 pg/m3 (average 7.2 pg/m3), while RGHg collected by the CEM similarly showed a large variability with a range from 10.7 to 143.3 pg/m3. However, a different pattern spatially and temporally was seen with the two measurement devices. Overall, RGHg concentrations measured by the CEM were, on average, 5 times higher than those measured by the KCl-coated denuder. In addition, the Tekran data suggest that occasional daily peaks of RGHg were associated with ozone depletion providing evidence for the formation of RGHg by reactions with reactive halogen species. A diurnal cycle in the RGHg concentration was observed in the low latitude tropical regions, likely caused by these photochemical reactions. Although the measurements by the denuder have better time resolution, and allow for examination of processes for RGHg formation, their efficiency of capture of RGHg needs to be further considered. This study suggested that using the KCl-coated denuder led to an underestimation of RGHg over the MBL, as found in previous studies. Therefore, further studies should be made to examine the measured RGHg concentrations over the MBL using different quantification approaches to further examine the distribution of RGHg. •Hg speciation in the MBL over the Pacific was quantified during 2018 GEOTRACES GP15.•A comparison measurement of RGHg by a KCl-coated denuder and CEM showed differences.•RGHg measured by CEM was on average 5 times higher than the value from the denuder.•This study suggested RGHg was underestimated using the KCl-coated denuder over MBL.•High RGHg was found with low ozone and there was a diurnal cycle in the tropics.