Variation in isotopologues of atmospheric nitric acid in passively collected samples along an air pollution gradient in southern California

Abstract

The sources and oxidation pathways of atmospheric nitric acid (HNO₃) can be evaluated using the isotopic signatures of oxygen (O) and nitrogen (N). This study evaluated the ability of Nylasorb nylon filters to passively collect unbiased isotopologues of atmospheric HNO₃ under controlled and field conditions. Filters contained in passive samplers were exposed in continuous stirred tank reactors (CSTRs) at high (16 μg/m³) and moderate (8 μg/m³) HNO₃ concentrations during 1–4 week deployment times. Filters were concurrently exposed at high and low N deposition sites along a gradient in the Sonoran Desert. Filters deployed in CSTRs at moderate HNO₃ concentrations for 1–2 weeks had greater variation of ä18O relative to the 3–4 week deployments, while high concentration samples were consistent across weeks. All deployment means were within 2‰ of the source solution. The δ¹⁵N of all weekly samples were within 0.5‰ of the source solution. Thus, when deployed for longer than 3 weeks, Nylasorb filters collected an isotopically unbiased sample of atmospheric HNO₃. The initial HNO₃ samples at the high deposition field sites had higher δ¹⁵N and δ¹⁸O values than the low deposition sites, suggesting either two independent sources of HNO₃ were mixing or that heavier isotopologues of HNO₃ were preferentially lost from the gas phase through physical deposition or equilibrium chemical reactions. Subsequent HNO₃ samples were subject to summer monsoon conditions leading to variation of isotopic signatures of N and O following 2-source mixing dynamics. Both sites mixed with a source that dominated during the two discrete precipitation events. The high number of lightning strikes near the samplers during the monsoon events suggested that lightning-created HNO₃ was one of the dominant mixing sources with an approximate isotopic signature of 21.6‰ and −0.6‰ for δ¹⁸O andδ¹⁵N respectively. Two-source mixing models suggest that lightning-created HNO₃ made up between 40 and 42% of atmospheric HNO₃ at the high deposition sites and 59–63% at the low deposition during the 4 week exposure.

Citation

Bell, M.D.; Sickman, J.O.; Bytnerowicz, A.; Padgett, P.E.; Allen, E.B. 2014. Variation in isotopologues of atmospheric nitric acid in passively collected samples along an air pollution gradient in southern California. Atmospheric Environment. 94: 287-296.