Estimation of Uncertainty in Air-Water Exchange Flux and Gross Volatilization Loss of PCBs: A Case Study Based on Passive Sampling in the Lower Great Lakes.

Ying Liu,Derek Muir,Rainer Lohmann,Carrie A. McDonough,Mohammed Khairy,Siyao Wang

doi:10.1021/acs.est.6b02891

Abstract

Compared with dry and wet deposition fluxes, air-water exchange flux cannot be directly measured experimentally. Its model-based calculation contains considerable uncertainty because of the uncertainties in input parameters. To capture the inherent variability of air-water exchange flux of PCBs across the lower Great Lakes and to calculate their annual gross volatilization loss, 57 pairs of air and water samples from 19 sites across Lakes Erie and Ontario were collected using passive sampling technology during 2011-2012. Error propagation analysis and Monte Carlo simulation were applied to estimate uncertainty in the air-water exchange fluxes. Results from both methods were similar, but error propagation analysis estimated a smaller uncertainty than Monte Carlo simulation in cases of net deposition. Maximum likelihood estimations (MLE) of wind speed and air temperature were recommended to quantify the site-specific air-water exchange flux. An assumed 30-40% of relative uncertainty in overall air-water mass transfer velocity was confirmed. MLEs of volatilization fluxes of total PCBs across Lakes Erie and Ontario were 0.78 and 0.53 ng m-2 day-1, respectively, and gross volatilization losses of total PCBs over the whole lakes were 74 kg year-1 for Lake Erie and 63 kg year-1 for Lake Ontario. Mass balance analysis across Lake Ontario indicated that volatilization was the uppermost loss process of aqueous PCBs.

Highlights

Polychlorinated biphenyls (PCBs) are a class of persistent toxic chemical substances of concern in the Great Lakes.1-‐3 Atmospheric deposition was considered as a significant source of PCBs to the lower Great Lakes, including dry deposition, wet deposition and air-‐water diffusive fluxes.[4, 5] Atmospheric processes accounted for 80-‐90% of total loadings of PCBs to the oceans and 65% of total atmospheric deposition of PCBs was attributed to gas transfer.[6]
The general method is to quantifiy uncertainty associated with incomplete data by model-‐fitting probability distribution functions (PDF) of the incomplete data that are used as input to Monte Carlo simulations
For lighter PCBs, the relative uncertainty (RU) in %equilibrium were lower, as congeners equilibrated in the field (~100% of %equilibrium), whereas for heavier PCBs, the RUs in %equilibrium reached up to 51%, as those congeners attained

Summary

21 ABSTRACT

22 Compared with dry and wet deposition fluxes, air-‐water exchange flux cannot. 25 capture inherent variability of air-‐water exchange flux of PCBs across the lower. 22 Compared with dry and wet deposition fluxes, air-‐water exchange flux cannot. 25 capture inherent variability of air-‐water exchange flux of PCBs across the lower. 26 Great Lakes and calculate their annual gross volatilization loss, 57 pairs of air 27 and water samples from 19 sites across Lakes Erie and Ontario were collected. 29 analysis and Monte Carlo simulation were applied to estimate uncertainty in the. Results from both methods were similar, but error. 31 propagation analysis estimated smaller uncertainty than Monte Carlo simulation. MLEs on volatilization fluxes of total PCBs. across Lakes Erie and Ontario were 0.78 ng m-‐2 day-‐1 and 0.53 ng m-‐2 day-‐1, respectively, and gross volatilization losses of total PCBs over the whole lakes. analysis across Lake Ontario indicated that volatilization was an uppermost loss process of aqueous PCBs

INTRODUCTION

RESULTS AND DISCUSSION

487 ACKNOWLEDGEMENTS

499 REFERENCES