Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity

Abstract

Abstract. The operation of a laser-based sensor for gasphase elemental mercury, Hg(0), is described. It utilizes sequential two-photon laser excitation with detection of blueshifted laser-induced fluorescence (LIF) to provide a highly specific detection scheme that precludes detection of anything other than atomic mercury. It has high sensitivity, fast temporal resolution, and can be deployed for in situ measurements in the open atmosphere with essentially no perturbation of the environment. An ambient sample can also be pulled through a fluorescence cell, allowing for standard addition calibrations of the concentration. No type of preconcentration is required and there appears to be no significant interferences from other atmospheric constituents, including gas-phase oxidized mercury species. As a consequence, it is not necessary to remove oxidized mercury, commonly referred to as reactive gaseous mercury (RGM), from the air sample. The instrument has been deployed as part of an instrument intercomparison and compares well with conventional instrumentation that utilizes preconcentration on gold followed by analysis using cold-vapor atomic fluorescence spectroscopy (CVAFS). Currently, the achievable detection sensitivity is ∼ 15 pg m −3 (∼ 5 × 10 4 atoms cm −3 , ∼ 2 ppq) at a sampling rate of 0.1 Hz, i.e., averaging 100 shots with a 10 Hz laser system. Preliminary results are described for a 50 Hz instrument that utilizes a modified excitation sequence and has monitored ambient elemental mercury with an effective sampling rate of 10 Hz. Additional work is required to produce the precision necessary to perform eddy correlation measurements. Addition of a pyrolysis channel should allow for the measurement of total gaseous mercury (TGM) and hence RGM (by difference) with good sensitivity and time resolution.

Measuring and modeling mercury in the atmosphere: a critical review

“…Two laser systems have been developed for measurement of GEM (Faïn et al, 2010;Pierce et al, 2013;Bauer et al, 2002Bauer et al, , 2010Bauer et al, , 2014. One is a cavity ring-down system, and the other operates on the principle of laser-induced fluorescence.…”

“…The longest period of continuous sampling lasted for 26 h. During RAMIX they sampled directly from the manifold and, in addition, at the end of the campaign sampled ambient air independently, including true in situ sampling on the roof of their mobile lab. They also attempted to measure GOM by pyrolyzing the sample air and measuring the difference between Hg(0) and TGM (Bauer et al, 2014; A. Hynes, personal communication, 2015).…”

“…Model-observation comparisons consistently suggest models overestimate GOM surface concentrations, sometimes by as much as an order of magnitude (Amos et al, 2012;Kos et al, 2013;Holloway et al, 2012;Bieser et al, 2014). The measurement-model mis- Shia et al (1999); Seigneur et al (2001Seigneur et al ( , 2003Seigneur et al ( , 2004Seigneur et al ( , 2006; Lohman et al (2008 Syrakov et al (1995) * The standard GEOS-Chem has a global domain with the option to have a nested high-resolution simulation over North America .…”

Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

“…More recently, a high frequency (25 Hz) cavity ring-down spectroscopy (CRDS) sensor has been deployed for Hg 0 concentration measurement, but it has a higher detection limit (> 0.35 ng m −3 ) and suffers from the sensor's baseline drifting and interferences with O 3 (Faïn et al, 2010;Pierce et al, 2013). Another laser technique, the laser-induced fluorescence sensor, has been designed and successfully applied for up to 1 day of continuous measurement with improved detection limit (∼ 15 pg m −3 ) (Bauer et al, 2002(Bauer et al, , 2014. However, both methods have not yet been proven for application in long-term field measurements.…”

Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part II: Bias and uncertainty analysis

“…Although newly developed fast instruments for Hg 0 have been tested and validated, their precision needs improvements to perform regular Hg 0 -EC flux measurements (Pierce et al, 2013;Bauer et al, 2014). MM techniques used to quantify Hg 0 (turbulent) flux include relaxed eddy accumulation (REA), a modified Bowen ratio (MBR), and the aerodynamic gradient method (AGM).…”