Inferring &amp;lt;sup&amp;gt;222&amp;lt;/sup&amp;gt;Rn soil fluxes from ambient &amp;lt;sup&amp;gt;222&amp;lt;/sup&amp;gt;Rn activity and eddy covariance measurements of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Sander Van Der Laan,Swagath Manohar,Harro Meijer,Ingrid Van Der Laan-Luijkx,Michiel Van Der Molen,Alex Vermeulen,Fred Bosveld,Andrew Manning

doi:10.5194/amt-9-5523-2016

Abstract

Abstract. We present a new methodology, which we call Single Pair of Observations Technique with Eddy Covariance (SPOT-EC), to estimate regional-scale surface fluxes of 222Rn from tower-based observations of 222Rn activity concentration, CO2 mole fractions and direct CO2 flux measurements from eddy covariance. For specific events, the regional (222Rn) surface flux is calculated from short-term changes in ambient (222Rn) activity concentration scaled by the ratio of the mean CO2 surface flux for the specific event to the change in its observed mole fraction. The resulting 222Rn surface emissions are integrated in time (between the moment of observation and the last prior background levels) and space (i.e. over the footprint of the observations). The measurement uncertainty obtained is about ±15 % for diurnal events and about ±10 % for longer-term (e.g. seasonal or annual) means. The method does not provide continuous observations, but reliable daily averages can be obtained. We applied our method to in situ observations from two sites in the Netherlands: Cabauw station (CBW) and Lutjewad station (LUT). For LUT, which is an intensive agricultural site, we estimated a mean 222Rn surface flux of (0.29 ± 0.02) atoms cm−2 s−1 with values > 0.5 atoms cm−2 s−1 to the south and south-east. For CBW we estimated a mean 222Rn surface flux of (0.63 ± 0.04) atoms cm−2 s−1. The highest values were observed to the south-west, where the soil type is mainly river clay. For both stations good agreement was found between our results and those from measurements with soil chambers and two recently published 222Rn soil flux maps for Europe. At both sites, large spatial and temporal variability of 222Rn surface fluxes were observed which would be impractical to measure with a soil chamber. SPOT-EC, therefore, offers an important new tool for estimating regional-scale 222Rn surface fluxes. Practical applications furthermore include calibration of process-based 222Rn soil flux models, validation of atmospheric transport models and performing regional-scale inversions, e.g. of greenhouse gases via the SPOT 222Rn-tracer method.

Highlights

222Rn is a radioactive noble gas that is produced at a constant rate from 226Ra, which is relatively uniformly distributed in all soils
For Lutjewad station (LUT), we find a mean 222Rn surface flux of (0.43 ± 0.05) atoms cm−2 s−1 and a median of 0.17 atoms cm−2 s−1 based on 209 events between January 2008 and January 2010 (Fig. 5a and Table 1)
The statistical distribution of the 222Rn surface fluxes is shown in Fig. 6, and from this we find that the mean value is much higher than the median because of a few exceptionally large 222Rn surface fluxes

Summary

Introduction

When released into the atmosphere, 222Rn is transported and mixed in the atmosphere similar to all other gases emitted from, or close to, the surface These features make 222Rn an important tracer in atmospheric sciences. S. van der Laan et al.: 222Rn soil fluxes from ambient measurements port models (Dentener et al, 1999; Gupta et al, 2004; Zahorowski et al, 2004). Another highly useful application of 222Rn is the direct inversion method commonly referred to as the 222Rn tracer method (Levin, 1987; Schmidt et al, 1996; van der Laan et al, 2014). The ratio of the 222Rn surface flux to a measured 222Rn activity concentration difference over time at a certain observation height can be applied to calculate the surface flux of another constituent

Methods

Results

Discussion

Conclusion