Mercury vapor air–surface exchange measured by collocated micrometeorological and enclosure methods – Part I: Data comparability and method characteristics

Abstract

Abstract. Reliable quantification of air-biosphere exchange flux of elemental mercury vapor (Hg 0 ) is crucial for understanding the global biogeochemical cycle of mercury. However, there has not been a standard analytical protocol for flux quantification, and little attention has been devoted to characterize the temporal variability and comparability of fluxes measured by different methods. In this study, we deployed a collocated set of micrometeorological (MM) and dynamic flux chamber (DFC) measurement systems to quantify Hg 0 flux over bare soil and low standing crop in an agricultural field. The techniques include relaxed eddy accumulation (REA), modified Bowen ratio (MBR), aerodynamic gradient (AGM) as well as dynamic flux chambers of traditional (TDFC) and novel (NDFC) designs. The five systems and their measured fluxes were cross-examined with respect to magnitude, temporal trend and correlation with environmental variables.Fluxes measured by the MM and DFC methods showed distinct temporal trends. The former exhibited a highly dynamic temporal variability while the latter had much more gradual temporal features. The diurnal characteristics reflected the difference in the fundamental processes driving the measurements. The correlations between NDFC and TDFC fluxes and between MBR and AGM fluxes were significant (R>0.8, p<0.05), but the correlation between DFC and MM fluxes were from weak to moderate (R = 0.1-0.5). Statistical analysis indicated that the median of turbulent fluxes estimated by the three independent MM techniques were not significantly different. Cumulative flux measured by TDFC is considerably lower (42 % of AGM and 31 % of MBR fluxes) while those measured by NDFC, AGM and MBR were similar (<10 % difference). This suggests that incorporating an atmospheric turbulence property such as friction velocity for correcting the DFC-measured flux effectively bridged the gap between the Hg 0 fluxes measured by enclosure and MM techniques. Cumulated flux measured by REA was ∼ 60 % higher than the gradient-based fluxes. Environmental factors have different degrees of impacts on the fluxes observed by different techniques, possibly caused by the underlying assumptions specific to each individual method. Recommendations regarding the application of flux quantification methods were made based on the data obtained in this study.

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

“…is found to be insignificantly variant. Since Hg 0 air concentration at this site generally followed pronounced diurnal patterns (Part I, Zhu et al, 2015), the C REA detection limit was on average 10-15 % higher during the peak in the late morning hours compared to the minimum level. Hg 0 flux observations can be identified as insignificant from 0 when the corresponding C fall below its detection limit.…”

“…1 included a larger proportion of turbulence data with poor quality (see Fig. 4 in Zhu et al, 2015), contributing to a higher overall uncertainty in δ H H (Table 1).…”

“…Possible reasons for the data disparity include (1) the difference in the light transmission properties of the two chamber materials, and (2) the difference in soil temperature inside the chamber. The TDFC was manufactured from quartz glass, while the NDFC was assembled from polycarbonate (PC) sheets (Lin et al, 2012;Zhu et al, 2013bZhu et al, , 2015. Quartz has exceptional transmission properties for UV light down to 250 nm, while PC does not allow transmission of UV light that plays an important role in promoting Hg II photo-reduction in the substrate Lin et al, 2012).…”

“…This yielded an additional 4 % of the data classified by Flag 2. The representatives of 20 min fluxes were checked by footprint analysis (Zhu et al, 2015), but occasioned no additional flags, since the sampling tower is predominately surrounded by continuous farmlands within a ∼ 2 km radius. Hard-flagged data denote periods of greater uncertainty in the turbulent fluxes and the uncertainty quantification itself may become questionable (Mauder et al, 2013).…”

“…The results are presented in two companion papers. In Part I, the five systems and their measured fluxes were cross-examined with respect to magnitude, temporal trend and correlation with environmental variables (Zhu et al, 2015). In this Part II, the objective is to investigate the quality of the flux data by quantifying measurement error under varying meteorological conditions.…”