Evaluation of Laser Absorption Spectroscopic Techniques for Eddy Covariance Flux Measurements of Ammonia

Abstract

An intercomparison was made between eddy covariance flux measurements of ammonia by a quantum cascade laser absorption spectrometer (QCLAS) and a lead-salt tunable diode laser absorption spectrometer (TDLAS). The measurements took place in September 2004 and again in April 2005 over a managed grassland site in Southern Scotland, U.K. These were also compared with a flux estimate derived from an "Ammonia Measurement by ANnular Denuder with online Analysis" (AMANDA), using the aerodynamic gradient method (AGM). The concentration and flux measurements from the QCLAS correlated well with those of the TDLAS and the AGM systems when emissions were high, following slurry application to the field. Both the QCLAS and TDLAS, however, underestimated the flux when compared with the AMANDA system, by 64%. A flux loss of 41% due to chemical reaction of ammonia in the QCLAS (and 37% in the TDLAS) sample tube walls was identified and characterized using laboratory tests but did not fully accountforthis difference. Recognizing these uncertainties, the agreement between the systems was nevertheless very close (R2 = 0.95 between the QCLAS and the TDLAS; R2 = 0.84 between the QCLAS and the AMANDA) demonstrating the suitability of the laser absorption methods for quantifying the temporal dynamics of ammonia fluxes.

Open-path, quantum cascade-laser-based sensor for high-resolution atmospheric ammonia measurements

“…It is important to note that surface sampling artifacts in closed-path sensors are typically reduced by minimizing inlet length, optimizing inlet coatings, inlet heating and maintaining high flow rates through small sampling volumes or other special sampling systems (Whitehead et al, 2008;Bianchi et al, 2012). Some of these measures increase power demands and limit portability for a variety of field platforms.…”

“…Although these closedpath systems are highly sensitive (ppbv or sub-ppbv), they suffer from sampling artifacts that limit precision and response time and complicate calibration methods (Whitehead et al, 2008;von Bobrutzki et al, 2010). In addition, these systems are large (usually > 100 kg) and require pumps and temperature control systems with high power consumption (usually hundreds of watts).…”

“…Ammonia surface adsorption and desorption effects in closed-path sensors typically limit the response time (for several hundred ppbv NH 3 changes) to a few seconds, resulting in attenuation of high-frequency concentration fluctuations (Yokelson et al, 2003;Nowak et al, 2007;McManus et al, 2008;Whitehead et al, 2008;Ellis et al, 2010;Gong et al, 2011). The time response of closed-path sensors is often defined by the initial time constant for gas exchange and the slower surface adsorption time constant (e.g., ∼ 0.4 s and ∼ 15 s, respectively, for a QC-TDLAS instrument) .…”

Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia

“…Several studies have investigated the adsorption of NH 3 to different types of sample inlet materials at various temperatures (Yokelson et al, 2003;Shah et al, 2006;Whitehead et al, 2008). In our laboratory intercomparison between the QC-TILDAS and the TDLAS, we examined the effect of heat and humidity on NH 3 adsorption to sampling lines.…”

Metrology for ammonia in ambient air – concept and first results of the EMRP project MetNH3

“…Historically there have been a number of field intercomparisons of both research and commercial NH3 instrumentation [1,8,44]. These studies have demonstrated that under field conditions additional complications arise which are not observed under controlled conditions in the laboratory.…”