The capacity to make in situ geo-referenced measurements of methane concentration and stable isotopic composition (δ(13)C(CH4)) would greatly improve our understanding of the distribution and type of methane sources in the environment, allow refined determination of the extent to which microbial production and consumption contributes to methane cycling, and enable the testing of hypotheses about the sensitivity of methane cycling to changes in environmental conditions. In particular, characterizing biogeochemical methane cycling dynamics in the deep ocean is hampered by a number of challenges, especially in environments where high methane concentrations preclude intact recovery of undisturbed samples. To that end, we have developed an in situ analyzer capable of δ(13)C(CH4) measurements in the deep ocean. Here we present data from laboratory and field studies in which we characterize the instrument's analytical capabilities and performance and provide the first in situ stable isotope based characterization of the influence of anaerobic methane oxidation on methane flux from seep sediments. These data illustrate how in situ measurements can permit finer-scale analyses of variations in AOM activity, and facilitate advances in using δ(13)C(CH4) and other isotopic systems to interrogate biogeochemical cycles in the deep sea and other remote or challenging environments.

A new ammonia (NH3) analyzer was developed based on off-axis integrated cavity output spectroscopy. Its feasibility was demonstrated by making tropospheric measurements in flights aboard the Department of Energy Gulfstream-1 aircraft. The ammonia analyzer consists of an optical cell, quantum-cascade laser, gas sampling system, control and data acquisition electronics, and analysis software. The NH3 mixing ratio is determined from high-resolution absorption spectra obtained by tuning the laser wavelength over the NH3 fundamental vibration band near 9.67 μm. Excellent linearity is obtained over a wide dynamic range (0-101 ppbv) with a response rate (1/e) of 2 Hz and a precision of ±90 pptv (1σ in 1 s). Two research flights were conducted over the Yakima Valley in Washington State. In the first flight, the ammonia analyzer was used to identify signatures of livestock from local dairy farms with high vertical and spatial resolution under low wind and calm atmospheric conditions. In the second flight, the analyzer captured livestock emission signals under windy conditions. Our results demonstrate that this new ammonia spectrometer is capable of providing fast, precise, and accurate in situ observations of ammonia aboard airborne platforms to advance our understanding of atmospheric compositions and aerosol formation.

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Diurnal and vertical patterns of carbonyl sulfide (OCS) and CO 2 mixing ratios above and within a 60-m-tall old-growth temperate forest are presented. Canopy air from four different heights was sampled in situ using a continuous integrated cavity output spectroscopy analyzer during August-September 2014. Measurements revealed large vertical gradients in OCS, from which we inferred ecosystem fluxes. The diurnal cycle of OCS mixing ratios at all heights exhibited a typical pattern characterized by nighttime drawdown, an early morning minimum, and a maximum of OCS around midday. Daytime increase in the upper canopy is attributed to entrainment of planetary boundary layer air into the canopy. The ecosystem was found to be a large daytime sink of OCS (mean maximum daytime flux~À75 pmol · m À2 · s À1 ). Mean leaf relative uptake (concentration normalized uptake of OCS flux to CO 2 uptake) was found to be 6.9. We discuss this high leaf relative uptake in the context of the presence and distribution of epiphytes at the site. While epiphytic uptake of OCS has been studied before, we show for the first time that this may contribute significantly to ecosystem fluxes under humid or moist conditions. We test this theory using a chamber experiment measuring epiphytic fluxes for two species of lichen and one moss species (in situ and in a laboratory). We suggest that the role of epiphytes should be explicitly considered when using OCS as a tracer of ecosystem-scale photosynthesis in forest ecosystems with abundant epiphytic cover and biomass.Plain Language Summary The resilience of old growth forests in changing climates is less well understood, in part due to difficulties in measuring forest productivity. Here we apply a new method using measurements of carbonyl sulfide (OCS) to understand the same in a tall old growth forest in the Pacific northwestern United States. We find that OCS is taken up by plants, soil, and epiphytes (lichens and mosses) and provides biophysical controls on OCS uptake and its utility in estimating productivity in similar forests.

Developments in cavity-enhanced absorption spectrometry have made it possible to measure water isotopes using faster, more cost-effective field-deployable instrumentation. Several groups have attempted to extend this technology to measure water extracted from plants and found that other extracted organics absorb light at frequencies similar to that absorbed by the water isotopomers, leading to δ(2)H and δ(18)O measurement errors (Δδ(2)H and Δδ(18)O). In this note, the off-axis integrated cavity output spectroscopy (ICOS) spectra of stable isotopes in liquid water is analyzed to determine the presence of interfering absorbers that lead to erroneous isotope measurements. The baseline offset of the spectra is used to calculate a broadband spectral metric, m(BB), and the mean subtracted fit residuals in two regions of interest are used to determine a narrowband metric, m(NB). These metrics are used to correct for Δδ(2)H and Δδ(18)O. The method was tested on 14 instruments and Δδ(18)O was found to scale linearly with contaminant concentration for both narrowband (e.g., methanol) and broadband (e.g., ethanol) absorbers, while Δδ(2)H scaled linearly with narrowband and as a polynomial with broadband absorbers. Additionally, the isotope errors scaled logarithmically with m(NB). Using the isotope error versus m(NB) and m(BB) curves, Δδ(2)H and Δδ(18)O resulting from methanol contamination were corrected to a maximum mean absolute error of 0.93 [per thousand] and 0.25 [per thousand] respectively, while Δδ(2)H and Δδ(18)O from ethanol contamination were corrected to a maximum mean absolute error of 1.22 [per thousand] and 0.22 [per thousand]. Large variation between instruments indicates that the sensitivities must be calibrated for each individual isotope analyzer. These results suggest that the properly calibrated interference metrics can be used to correct for polluted samples and extend off-axis ICOS measurements of liquid water to include plant waters, soil extracts, wastewater, and alcoholic beverages. The general technique may also be extended to other laser-based analyzers including methane and carbon dioxide isotope sensors.