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Application of an online ion chromatography-based instrument for gradient flux measurements of speciated nitrogen and sulfur
Citation:
Rumsey, I. C. AND J. T. WALKER. Application of an online ion chromatography-based instrument for gradient flux measurements of speciated nitrogen and sulfur. Atmospheric Measurement Techniques. Copernicus Publications, Katlenburg-Lindau, Germany, 9(6):2581-2592, (2016).
Impact/Purpose:
We describe for the first time the operating characteristics of the 2S MARGA system applied for gradient flux measurements. The instrument demonstrates lower detection limits relative to other similar wet chemistry systems and is sufficiently precise for measurement of air-surface exchange fluxes in remote environments where model derived deposition budgets require confirmation.
Description:
In North America, the dry component of total nitrogen and sulfur deposition remains uncertain due to a lack of measurements of sufficient chemical speciation and temporal extent to develop complete annual mass budgets or of sufficient process level detail to improve current air-surface exchange models. Over the past decade, significant advances have been made in the development of continuous chemical measurement techniques with sufficient sensitivity and temporal resolution to directly quantify air-surface exchange of nitrogen and sulfur compounds. However, their applicability is generally restricted to only one or a few of the compounds within the deposition budget. We characterize for the first time the performance of the Monitor for AeRosols and GAses in ambient air (MARGA), an on-line ion chromatography-based analyzer, as applied for air-surface exchange measurements of HNO3, NH3, NH4+, NO3-, SO2 and SO42-. Analytical accuracy and precision are assessed under field conditions and total uncertainty of fluxes measured by the aerodynamic gradient method are assessed for a representative 3-week period in the fall of 2012. During this period, percentages of hourly chemical gradients larger than the corresponding gradient detection limit were 86%, 55%, 81%, 74%, 77%, and 71% for NH3, NH4+, HNO3, NO3-, SO2, and SO42-, respectively. As expected, percentages were lowest for aerosol species, owing to their relatively low deposition velocities and correspondingly smaller gradients relative to gas phase species. When both day and night periods are considered, median total flux uncertainties are 38%, 91%, 52%, 46%, 69%, and 58% for NH3, NH4+, HNO3, NO3-, SO2, and SO42-. Total flux uncertainty is dominated by uncertainty in the chemical gradients during the day but uncertainty in the chemical gradients and transfer velocity are of the same order at night at our study site. While the characteristics of the analytical system reported here should be generally applicable to the MARGA 2S, the assessment of gradient precision and flux uncertainty will vary to some extent, though not dramatically, for different meteorological and atmospheric chemical conditions. We find that the overall uncertainties are similar to other wet chemical methods and that the instrument is sufficiently precise for flux gradient applications.
