Sources of uncertainty in eddy covariance ozone flux measurements made by dry chemiluminescence fast response analysers

Abstract

Abstract. We present a systematic intercomparison study of eddy covariance ozone flux measurements made using two fast response dry chemiluminescence analysers. Ozone deposition was measured over a well characterised managed grassland near Edinburgh, Scotland, during August 2007. A data quality control procedure specific to these analysers is introduced. Absolute ozone fluxes were calculated based on the relative signals of the dry chemiluminescence analysers using three different methods and the results are compared for both analysers. It is shown that the error in the fitted analyser calibration parameters required for the flux calculations provides a substantial source of uncertainty in the fluxes. The choice of the calculation method itself can also constitute an uncertainty in the flux as the calculated fluxes by the three methods do not agree within error at all times. This finding highlights the need for a consistent and rigorous approach for comparable datasets, such as e.g. in flux networks. Ozone fluxes calculated by one of the methods were then used to compare the two analysers in more detail. This systematic analyser comparison reveals half-hourly flux values differing by up to a factor of two at times with the difference in mean hourly flux ranging from 0 to 23% with an error in the mean daily flux of ± 12%. The comparison of analysers shows that the agreement in fluxes is excellent for some days but that there is an underlying uncertainty as a result of variable analyser performance and/or non-linear sensitivity.

A fast and precise chemiluminescence ozone detector for eddy flux and airborne application

“…The instrument signal is certainly proportional to the sensitivity c of the sensor disc (Eq. 18) and the instrument precision (1-σ noise) is proportional to the square root of c. With the actual sensor disc sensitivity c, which depends on the disc material, age, and manufacturing (which may cause significant inter-disc variability, Muller et al, 2010), the actual measurement precision can easily be inferred from the Eqs. (19) and (20).…”

“…Muller et al (2010) addressed the calibration of dry CI instruments in detail. They suggest inferring the instrument sensitivity based on 15 or 30 min averages, in which three approaches differing in the way of data treatment are analyzed (e.g.…”

“…The sensitivity dependence on the O 3 mixing ratio of the sample air. Fresh sensor discs are quite insensitive to O 3 and have to pre-activated with an O 3 dose of 100-500 ppbv h −1 (Speusser, 1989;Muller et al, 2010, and references therein). Thereafter, still a weak and slowly varying dependence of the sensor sensitivity on the ozone mixing remains.…”

“…Moreover, two clones of the GFAS sensor are in use, i.e. the Rapid Ozone Flux Instrument (ROFI) described in Coyle (2005) and Muller et al (2010), and the 10 Hzanalyzer built by Jim Womack at NOAA-ATDD (Kurpius et al, 2002). (3) FOZAN I and II uses Coumarin-307 and was developed for the deployment on board the high-flying Russian aircraft Geophysica (Yushkov et al, 1999;Ulanovsky et al, 2001).…”

Predicting and partitioning ozone fluxes to maize crops from sowing to harvest: the Surfatm-O<sub>3</sub> model

“…Specifically for ozone flux, the ratio method described in Muller et al (2010) which provided measurements of deposition velocity (the flux divided by concentration) owing to the very small and constant offset of the fast ozone analyser (ATDD, NOAA, USA), was applied. The ozone flux was obtained by multiplying the deposition velocity by the absolute ozone concentration measured with slow response sensor (Environnement SA, model 41M, France).…”