Use of multiple tools including lead isotopes to decipher sources of ozone and reactive mercury to urban and rural locations in Nevada, USA

Comparison and calibration of methods for ambient reactive mercury quantification

Gaseous oxidized mercury (GOM) is the dominant form of atmospheric mercury (Hg) deposited and sequestered within ecosystems. Thus, accurate, calibrated measurements of GOM are needed. Here, two active membrane-based collection systems (RMAS) were used to determine GOM and particulate-bound Hg (PBM), as well as reactive Hg (RM = GOM + PBM), and compared with two dual-channel systems (DCS) and a Tekran 2537/1130 speciation system. The DCS measured operationally defined GOM by difference, using concentrations of gaseous elemental Hg (GEM) and total gaseous Hg. One DCS was linked to a custom-built, automated calibration system that permeated GEM, HgBr₂, or HgCl₂. The five systems were co-located for one-year to develop a dataset that would allow for understanding limitations of each system, and assessing measurement accuracy and long-term precision of the calibrator. The Tekran system measured ~14.5 % of the GOM measured by the other systems. The USU and UNR DCS and RMAS were significantly correlated, but the DCS was 50 and 30 % higher, respectively, than the RMAS. The calibrator performed consistently in the field and lab, and the DCS fully recovered GOM injected by the calibrator. Since the uncalibrated DCS measured the same concentrations as the calibrated DCS, they are both accurate methods for measuring RM and/or GOM. Some loss occurred from the RMAS membranes. SYNOPSIS: Accurate and calibrated measurements of atmospheric reactive mercury using membranes and two dual-channel systems.

Anthropogenic Asian aerosols provide Fe to the North Pacific Ocean

Fossil-fuel emissions may impact phytoplankton primary productivity and carbon cycling by supplying bioavailable Fe to remote areas of the ocean via atmospheric aerosols. However, this pathway has not been confirmed by field observations of anthropogenic Fe in seawater. Here we present high-resolution trace-metal concentrations across the North Pacific Ocean (158°W from 25°to 42°N). A dissolved Fe maximum was observed around 35°N, coincident with high dissolved Pb and Pb isotope ratios matching Asian industrial sources and confirming recent aerosol deposition. Iron-stable isotopes reveal in situ evidence of anthropogenic Fe in seawater, with low δ⁵⁶Fe (-0.23‰ > δ⁵⁶Fe > -0.65‰) observed in the region that is most influenced by aerosol deposition. An isotope mass balance suggests that anthropogenic Fe contributes 21-59% of dissolved Fe measured between 35° and 40°N. Thus, anthropogenic aerosol Fe is likely to be an important Fe source to the North Pacific Ocean.

Use of Multiple Lines of Evidence to Understand Reactive Mercury Concentrations and Chemistry in Hawai'i, Nevada, Maryland, and Utah, USA

To advance our understanding of the mercury (Hg) biogeochemical cycle, concentrations and chemistry of gaseous oxidized Hg (GOM), particulate-bound Hg (PBM), and reactive Hg (RM = GOM + PBM) need to be known. The UNR-RMAS 2.0 provides a solution that will advance knowledge. From 11/2017 to 02/2019, the RMAS 2.0 was deployed in Hawai'i, Nevada, Maryland, and Utah to test system performance and develop an understanding of RM at locations impacted by different atmospheric oxidants. Mauna Loa Observatory, Hawai'i, impacted by the free troposphere and the marine boundary layer, had primarily -Br/Cl RM compounds. The Nevada location, directly adjacent to a major interstate highway and experiences inputs from the free troposphere, exhibited -Br/Cl, -N, -S, and organic compounds. In Maryland, compounds observed were -N, -S, and organic-Hg. This site is downwind of coal-fired power plants and located in a forested area. The location in Utah is in a basin impacted by oil and natural gas extraction, multiday wintertime inversion episodes, and inputs from the free troposphere. Compounds were -Br/Cl or -O, -N, and -Br/Cl. The chemical forms of RM identified were consistent with the air source areas, predominant ion chemistry, criterion air pollutants, and meteorology.

Phosphorus Speciation in Atmospherically Deposited Particulate Matter and Implications for Terrestrial Ecosystem Productivity

Chemical forms of phosphorus (P) in airborne particulate matter (PM) are poorly known and do not correlate with solubility or extraction measurements commonly used to infer speciation. We used P X-ray absorption near-edge structure (XANES) and ³¹P nuclear magnetic resonance (NMR) spectroscopies to determine P species in PM collected at four mountain sites (Colorado and California). Organic P species dominated samples from high elevations, with organic P estimated at 65-100% of total P in bulk samples by XANES and 79-88% in extracted fractions (62-84% of total P) by NMR regardless of particle size (≥10 or 1-10 μm). Phosphorus monoester and diester organic species were dominant and present in about equal proportions, with low fractions of inorganic P species. By comparison, PM from low elevation contained mixtures of organic and inorganic P, with organic P estimated at 30-60% of total P. Intercontinental PM transport determined from radiogenic lead (Pb) isotopes varied from 0 to 59% (mean 37%) Asian-sourced Pb at high elevation, whereas stronger regional PM inputs were found at low elevation. Airborne flux of bioavailable P to high-elevation ecosystems may be twice as high as estimated by global models, which will disproportionately affect net primary productivity.