Improved methods for signal processing in measurements of mercury by Tekran<sup>®</sup> 2537A and 2537B instruments

Abstract

Abstract. Atmospheric Hg measurements are commonly carried out using Tekran ® Instruments Corporation's model 2537 Hg vapor analyzers, which employ gold amalgamation preconcentration sampling and detection by thermal desorption (TD) and atomic fluorescence spectrometry (AFS). A generally overlooked and poorly characterized source of analytical uncertainty in those measurements is the method by which the raw Hg atomic fluorescence (AF) signal is processed. Here I describe new software-based methods for processing the raw signal from the Tekran ® 2537 instruments, and I evaluate the performances of those methods together with the standard Tekran ® internal signal processing method. For test datasets from two Tekran ® instruments (one 2537A and one 2537B), I estimate that signal processing uncertainties in Hg loadings determined with the Tekran ® method are within ±[1 % + 1.2 pg] and ±[6 % + 0.21 pg], respectively. I demonstrate that the Tekran ® method can produce significant low biases (≥ 5 %) not only at low Hg sample loadings (< 5 pg) but also at tropospheric background concentrations of gaseous elemental mercury (GEM) and total mercury (THg) (∼ 1 to 2 ng m −3 ) under typical operating conditions (sample loadings of 5-10 pg). Signal processing uncertainties associated with the Tekran ® method can therefore represent a significant unaccounted for addition to the overall ∼ 10 to 15 % uncertainty previously estimated for Tekran ® -based GEM and THg measurements. Signal processing bias can also add significantly to uncertainties in Tekran ® -based gaseous oxidized mercury (GOM) and particle-bound mercury (PBM) measurements, which often derive from Hg sample loadings < 5 pg. In comparison, estimated signal processing uncertainties associated with the new methods described herein are low, ranging from within ±0.053 pg, when the Hg thermal desorption peaks are defined manually, to within ±[2 % + 0.080 pg] when peak definition is automated. Mercury limits of detection (LODs) decrease by 31 to 88 % when the new methods are used in place of the Tekran ® method. I recommend that signal processing uncertainties be quantified in future applications of the Tekran ® 2537 instruments.

Mercury distribution in the upper troposphere and lowermost stratosphere according to measurements by the IAGOS-CARIBIC observatory: 2014–2016

“…The DOHGS took part in the Reno Atmospheric Mercury Intercomparison eXperiment (RAMIX) whose results have been controversially discussed by Gustin et al (2013), Ambrose et al (2013) and Hynes et al (2017). According to Gustin et al (2013) and Ambrose et al (2013) GOM measurements by the DOHGS technique agreed well with nylon and cation exchange filters while yielding substantially higher values than the Tekran speciation system. Hynes et al (2017), on the other hand, found a good agreement of GOM concentrations measured by two-photon laser-induced fluorescence (2P-LIF) with and without a pyrolyzer (the 2P-LIF technique can measure GEM directly without using any filter) and by the Tekran speciation system.…”

“…Despite an additional pump the nominal flow of 0.5 L (STP) min −1 could not be sustained at the highest flight levels. Limited air flow, the short sampling time and low concentrations resulted in only ∼ 2 pg of mercury, which is much smaller than 10 pg considered as minimum for biasfree internal default integration of the signal by the Tekran instrument (Swartzendruber et al, 2009;Slemr et al, 2016;Ambrose, 2017). For this reason, as previously mentioned, the raw analyzer signals were processed post-flight using a manual integration procedure described in detail by Slemr et al (2016).…”

“…In order to get information about the GEM fraction, sample air was passed through a quartz wool scrubber for GOM during the outbound flights between August 2014 and February 2016. The method using quartz wool to capture GOM in order to measure GEM, called DOHGS (Detector for Oxidized Hg Species), is described by Lyman and Jaffe (2012) and Ambrose et al (2013), and the measurements made by this technique have been reported by Lyman and Jaffe (2012), Ambrose et al (2013Ambrose et al ( , 2015, Gratz et al (2015) and Shah et al (2016). The DOHGS took part in the Reno Atmospheric Mercury Intercomparison eXperiment (RAMIX) whose results have been controversially discussed by Gustin et al (2013), Ambrose et al (2013) and Hynes et al (2017).…”

“…The method using quartz wool to capture GOM in order to measure GEM, called DOHGS (Detector for Oxidized Hg Species), is described by Lyman and Jaffe (2012) and Ambrose et al (2013), and the measurements made by this technique have been reported by Lyman and Jaffe (2012), Ambrose et al (2013Ambrose et al ( , 2015, Gratz et al (2015) and Shah et al (2016). The DOHGS took part in the Reno Atmospheric Mercury Intercomparison eXperiment (RAMIX) whose results have been controversially discussed by Gustin et al (2013), Ambrose et al (2013) and Hynes et al (2017). According to Gustin et al (2013) and Ambrose et al (2013) GOM measurements by the DOHGS technique agreed well with nylon and cation exchange filters while yielding substantially higher values than the Tekran speciation system.…”

“…Most mercury data were obtained using the Tekran internal default signal integration procedure but since April 2014 we manually integrated the Tekran raw signal. This postflight integration of the raw signal substantially improved the detection limit and precision of the mercury measurements and removed negative bias of the default integration leading to the occasional occurrence of zero concentrations in the data before April 2014 (Slemr et al, 2016;Ambrose, 2017). Raw signal data are available only since April 2014 and older data cannot be reintegrated.…”