Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence.

Atmospheric nitrate is preserved in Antarctic snow firn and ice. However, at low snow accumulation sites, post-depositional processes induced by sunlight obscure its interpretation. The goal of these studies (see also Paper I by Meusinger et al.

Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry.

Post-depositional processes alter nitrate concentration and nitrate isotopic composition in the top layers of snow at sites with low snow accumulation rates, such as Dome C, Antarctica. Available nitrate ice core records can provide input for studying past atmospheres and climate if such processes are understood. It has been shown that photolysis of nitrate in the snowpack plays a major role in nitrate loss and that the photolysis products have a significant influence on the local troposphere as well as on other species in the snow.

Analysis of oxygen-17 excess of nitrate and sulfate at sub-micromole levels using the pyrolysis method.

Rationale: The oxygen-17 excess (Δ(17)O) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure Δ(17)O requires large sample amounts (>4 µmol for nitrate and >1 µmol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples.

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer.

Long-term observations of the reactive chemical composition of the tropical marine boundary layer (MBL) are rare, despite its crucial role for the chemical stability of the atmosphere. Recent observations of reactive bromine species in the tropical MBL showed unexpectedly high levels that could potentially have an impact on the ozone budget. Uncertainties in the ozone budget are amplified by our poor understanding of the fate of NOx (= NO + NO2), particularly the importance of nighttime chemical NOx sinks.

Oxygen isotope exchange with quartz during pyrolysis of silver sulfate and silver nitrate.

Rationale: Triple oxygen isotopes of sulfate and nitrate are useful metrics for the chemistry of their formation. Existing measurement methods, however, do not account for oxygen atom exchange with quartz during the thermal decomposition of sulfate. We present evidence for oxygen atom exchange, a simple modification to prevent exchange, and a correction for previous measurements.

Measurement of the 17O-excess (Δ17O) of tropospheric ozone using a nitrite-coated filter.

Rationale: The (17)O-excess (Δ(17)O) of tropospheric ozone (O(3)) serves as a useful marker in studies of atmospheric oxidation pathways; however, due to the complexity and expense of currently available analytical techniques, no systematic sampling campaign has yet been undertaken and natural variations in Δ(17)O(O(3)) are therefore not well constrained.

Methods: The nitrite-coated filter method is a new technique for O(3) isotope analysis that employs the aqueous phase NO(2)(-) + O(3) → NO(3)(-) + O(2) reaction to obtain quantitative information on O(3) via the oxygen atom transfer to nitrate (NO(3)(-)). The triple-oxygen isotope analysis of the NO(3)(-) produced during this reaction, achieved in this study using the bacterial denitrifier method followed by isotope-ratio mass spectrometry (IRMS), directly yields the Δ(17)O value transferred from O(3).