Triple oxygen isotopes indicate urbanization affects sources of nitrate in wet and dry atmospheric deposition

Abstract

Abstract. Atmospheric nitrate deposition resulting from anthropogenic activities negatively affects human and environmental health. Identifying deposited nitrate that is produced locally vs. that originating from long-distance transport would help inform efforts to mitigate such impacts. However, distinguishing the relative transport distances of atmospheric nitrate in urban areas remains a major challenge since it may be produced locally and/or be transported from upwind regions. To address this uncertainty we assessed spatiotemporal variation in monthly weighted-average 17 O and δ 15 N values of wet and dry nitrate deposition during one year at urban and rural sites along the western coast of the northern Japanese island of Hokkaido, downwind of the East Asian continent. 17 O values of nitrate in wet deposition at the urban site mirrored those of wet and dry deposition at the rural site, ranging between ∼ +23 and +31 ‰ with higher values during winter and lower values in summer, which suggests the greater relative importance of oxidation of NO 2 by O 3 during winter and OH during summer. In contrast, 17 O values of nitrate in dry deposition at the urban site were lower (+19 -+25 ‰) and displayed less distinct seasonal variation. Furthermore, the difference between δ 15 N values of nitrate in wet and dry nitrate deposition was, on average, 3 ‰ greater at the urban than rural site, and 17 O and δ 15 N values were correlated for both forms of deposition at both sites with the exception of dry deposition at the urban site. These results suggest that, relative to nitrate in wet and dry deposition in rural environments and wet deposition in urban environments, nitrate in dry deposition in urban environments forms from relatively greater oxidation of NO by peroxy radicals and/or oxidation of NO 2 by OH. Given greater concentrations of peroxy radicals and OH in cities, these results imply that dry nitrate deposition results from local NO x emissions more so than wet deposition, which is transported longer distances. These results illustrate the value of stable isotope data for distinguishing the transport distances and reaction pathways of atmospheric nitrate pollution.

Influences of Atmospheric Pollution on the Contributions of Major Oxidation Pathways to PM2.5 Nitrate Formation in Beijing

“…These results clearly illustrated that air pollution reduced the importance of the OH· pathway relative to the O 3 pathways for NO 3 − production. Our results were supported by a previous study that showed the importance of the OH pathway decreased and the O 3 pathways increased in city environments compared to in rural environments (Nelson et al, ). Taking all together, atmospheric pollution substantially influenced the pathways of NO 2 oxidation to NO 3 − in city environments.…”

“…The Δ 17 O‐NO 3 − values of PM 2.5 (17.0–32.8‰; Figure S1) averaged 26.3 ± 3.4‰ in Beijing of 2014, which were comparable to those of precipitation at Sado Island in Japan (18.6–32.4‰, mean = 26.3‰; Tsunogai et al, ) but higher than that of dry deposition in an urban site at Sapporo in Japan (19–25‰; Nelson et al, ) and total suspended particulates on Mt. Lulin in central Taiwan (2.7–31.4‰, mean = 17‰; Guha et al, ).…”

“…The influence of air pollution on oxidizing pathways to atmospheric NO 3 − is important information for understanding atmospheric oxidizing capacity. So far, direct evidence on changing oxidizing capacity with anthropogenic emissions of pollutants is limited (Nelson et al, ). Anthropogenic NO x emissions can increase both tropospheric O 3 and OH· production with OH· having higher oxidizing capacity than O 3 (Alexander & Mickley, ).…”

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: a possible new index for nitrogen saturation

“…Kamisako et al (2008) determined the annual discharge rate at site KJ to be 1439 mm during the observations undertaken between 2002 and 2007 using the same method as this study and estimated that approximately 61 % of the precipitation becomes stream outflow in this catchment. Because the evapotranspiration loss from forested catchments in Japan was estimated to be 30 % to 50 % of deposition for the annual deposition rate from 2000 to 2500 mm (Ogawa, 2003), we concluded that the estimated annual discharge via the stream was highly reliable at the site, within the error range of 10 %. The determined export fluxes of nitrate in stream water (F total ) ranged from 74.7 to 698.4 µmol m −2 day −1 , and the determined export fluxes of NO − 3 atm in stream water (F atm ) ranged from 3.3 to 46.1 µmol m −2 day −1 (Fig.…”

“…where the constant β is 0.5279 (Kaiser et al, 2007;Miller, 2002). Continuous monitoring of the 17 O value of NO − 3 atm deposited at the midlatitudes of East Asia has clarified that the annual average 17 O values of NO − 3 atm are almost constant at 26.6 ‰ ± 0.9 ‰ (the average and the 1σ variation range; Nelson et al, 2018;Tsunogai et al, 2010Tsunogai et al, , 2016. In addition, 17 O is stable during the mass-dependent isotope fractionation processes within surface ecosystems (Miller, 2002;Thiemens et al, 2001).…”