Technical considerations for the use of passive samplers to quantify the isotopic composition of NOx and NO2 using the denitrifier method

Abstract

The isotopic composition of nitrogen oxides (NOx = NO + NO2) hold potential to trace emissions sources, as well as chemistry in the atmosphere. Passive samplers that collect NOx and NO2 as nitrite could provide a simple and inexpensive means by which to collect samples in a variety of environments, and we report here on several methodological considerations when using Ogawa passive collectors of NOx and NO2 for concentration analysis via colorimetric methods and isotopic analysis via the denitrifier method. Using Ogawa samplers, NOx and NO2 were collected in Providence, RI, USA (41.8°N, 71.4°W) with exposure times of seven, fourteen, twenty-one, and twenty-eight days during summer and winter in an area dominated by vehicle emissions, particularly diesel delivery trucks. Average values were −9.7 ± 0.7‰ for δ15N-NOx and −8.3 ± 0.9‰ for δ15N-NO2 based on duplicate seven, fourteen, twenty-one, and twenty-eight day exposures (n = 8) in summertime, and −11.9 ± 0.7‰ for δ15N-NOx and −6.4 ± 1.4‰ for δ15N-NO2 based on seven, fourteen, twenty-one, and twenty-eight day exposures (n = 7) in wintertime. The oxygen isotopic composition (δ18O, Δ17O) was also determined with averages of: 36.3 ± 2.4‰ for δ18O-NOx (n = 8), 50.5 ± 3.2‰ for δ18O-NO2 (n = 8), 6.2 ± 0.8‰ for Δ17O-NOx and 10.8 ± 0.6‰ for Δ17O-NO2 (n = 4) in summer; and 38.7 ± 2.9‰ for δ18O-NOx and 47.4 ± 1.2‰ for δ18O-NO2 (n = 7), 7.7 ± 1.5‰ for Δ17O-NO2 (n = 4) and 5.3 ± 1.2‰ for Δ17O-NOx (n = 5), in winter. Our isotopic results differ from previous studies that utilized passive samplers and suggest specific nitrogen and oxygen isotopic signatures associated with vehicular emissions. To collect sufficient sample for multiple isotopic analyses (e.g., 10 nmol N for δ15N and δ18O; 50 nmol N for Δ17O) the exposure time for the samplers, even in urban areas, is at least days. This necessarily results in collection of a mixture of local emissions sources, as well as background air that can represent multiple NOx sources. Thus, passive samplers cannot be used to diagnose isotopic signatures unless only exposed to a single emissions source. The oxygen isotopic composition of nitrite (the collection analyte) results determined via the denitrifier method must be corrected in comparison to nitrate isotopic reference materials, resulting in an increase of ∼25‰. Even with this correction, the oxygen isotopic results are lower than what would be expected for ozone-dominated atmospheric chemistry in an urban area, and the oxygen isotopic results for NOx were consistently lower than that found for NO2. We suggest that the oxygen isotopic composition of NO and NO2 were modified upon capture and conversion to nitrite on the passive sampler pads, and does not directly reflect abundances in the atmosphere.