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

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 degrees N, 71.4 degrees 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 parts per thousand for delta N-15-NO2 and -8.3 +/- 0.9 parts per thousand for delta N-15-NO2 based on duplicate seven, fourteen, twenty-one, and twenty-eight day exposures (n = 8) in summertime, and -11.9 +/- 0.7 parts per thousand for delta N-15-NOx and -6.4 +/- 1.4 parts per thousand for delta N-15-NO2 based on seven, fourteen, twenty-one, and twenty-eight day exposures (n = 7) in wintertime. The oxygen isotopic composition (delta O-18, Delta O-17) was also determined with averages of: 36.3 +/- 2.4 parts per thousand for delta O-18-NOx (n = 8), 50.5 +/- 3.2 parts per thousand for delta O-18-NO2 (n = 8), 6.2 +/- 0.8 parts per thousand for Delta O-17-NOx and 10.8 +/- 0.6 parts per thousand for Delta O-17-NO2 (n = 4) in summer; and 38.7 +/- 2.9 parts per thousand for delta O-18-NOx and 47.4 +/- 1.2 parts per thousand for delta O-18-NO2 (n = 7), 7.7 +/- 1.5 parts per thousand for Delta O-17-NO2 (n = 4) and 5.3 +/- 1.2 parts per thousand for Delta O-17-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 delta N-15 and delta O-18; 50 nmol N for Delta O-17) 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 similar to 25%o. 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. (C) 2016 Elsevier Ltd. All rights reserved.