Atmospheric and snow nitrate isotope systematics at Summit, Greenland: the reality of the post-depositional effect

The effect of post–depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland remains a subject of debate which hinders the interpretations of ice–core nitrate concentrations and isotope records. Here we present the first year–round observations of atmospheric aerosol nitrate and its isotopic compositions at Summit, and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3–) remained negative throughout the year, ranging from –3.1 ‰ to –47.9 ‰ with a mean of (–14.8 ± 7.3) ‰, and displayed no apparent seasonality that is different from the distinct seasonal δ15N(NO3–) variations observed in snowpack. The spring average aerosol δ15N(NO3–) was (–17.9 ± 8.3) ‰, significantly depleted compared to snowpack spring average of (4.6 ± 2.1) ‰, with surface snow δ15N(NO3–) of (–6.8 ± 0.5) ‰ that is in between. The differences in aerosol, surface snow and snowpack δ15N(NO3–) are best explained by the photo-driven post–depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3–), the atmospheric Δ17O(NO3–) was of similar seasonal pattern and magnitude of change to that in snowpack, suggesting little to no changes in Δ17O(NO3–) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3–) varied similarly as atmospheric Δ17O(NO3–), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3–) was larger than that of Δ17O(NO3–), and the linear relationships between δ18O/Δ17O(NO3–) were different for atmospheric and snowpack samples. This suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3–) is larger than that of Δ17O(NO3–) given that photolysis is a mass-dependent process.

Ozone response to emission reductions in the southeastern United States

Ozone (O3) formation in the southeastern US is studied in relation to nitrogen oxide (NOx) emissions using long-term (1990s–2015) surface measurements of the Southeastern Aerosol Research and Characterization (SEARCH) network, U.S. Environmental Protection Agency (EPA) O3 measurements, and EPA Clean Air Status and Trends Network (CASTNET) nitrate deposition data. Annual fourth-highest daily peak 8 h O3 mixing ratios at EPA monitoring sites in Georgia, Alabama, and Mississippi exhibit statistically significant (p < 0.0001) linear correlations with annual NOx emissions in those states between 1996 and 2015. The annual fourth-highest daily peak 8 h O3 mixing ratios declined toward values of ∼ 45–50 ppbv and monthly O3 maxima decreased at rates averaging ∼ 1–1.5 ppbv yr−1. Mean annual total oxidized nitrogen (NOy) mixing ratios at SEARCH sites declined in proportion to NOx emission reductions. CASTNET data show declining wet and dry nitrate deposition since the late 1990s, with total (wet plus dry) nitrate deposition fluxes decreasing linearly in proportion to reductions of NOx emissions by ∼ 60 % in Alabama and Georgia. Annual nitrate deposition rates at Georgia and Alabama CASTNET sites correspond to 30 % of Georgia emission rates and 36 % of Alabama emission rates, respectively. The fraction of NOx emissions lost to deposition has not changed. SEARCH and CASTNET sites exhibit downward trends in mean annual nitric acid (HNO3) concentrations. Observed relationships of O3 to NOz (NOy–NOx) support past model predictions of increases in cycling of NO and increasing responsiveness of O3 to NOx. The study data provide a long-term record that can be used to examine the accuracy of process relationships embedded in modeling efforts. Quantifying observed O3 trends and relating them to reductions in ambient NOy species concentrations offers key insights into processes of general relevance to air quality management and provides important information supporting strategies for reducing O3 mixing ratios.

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

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 Δ17O and δ15N 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. Δ17O 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 NO2 by O3 during winter and OH during summer. In contrast, Δ17O values of nitrate in dry deposition at the urban site were lower (+19 – +25 ‰) and displayed less distinct seasonal variation. Furthermore, the difference between δ15N values of nitrate in wet and dry nitrate deposition was, on average, 3 ‰ greater at the urban than rural site, and Δ17O and δ15N 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 NO2 by OH. Given greater concentrations of peroxy radicals and OH in cities, these results imply that dry nitrate deposition results from local NOx 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.

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

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 come from upwind regions. To address this uncertainty we assessed spatiotemporal variation in monthly weighted-average Δ17O and δ15N 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. Δ17O values of nitrate in wet deposition at the urban site mirrored those of wet and dry deposition at the rural site, ranging between ~ +22 and +30 ‰ with higher values during winter and lower values in summer, which suggests greater relative importance of oxidation of NO2 by O3 during winter and OH during summer. In contrast, Δ17O values of nitrate in dry deposition at the urban site were lower (+19–+25 ‰) and displayed less distinct seasonal variation. Furthermore, the difference between δ15N values of nitrate in wet and dry nitrate deposition was, on average, 3 ‰ greater at the urban than rural site, and Δ17O and δ15N 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 deposition in urban environments and wet and dry deposition in rural environments, nitrate in dry deposition in urban environments forms from relatively greater oxidation of NO by peroxy radicals and/or oxidation of NO2 by OH. Given greater concentrations of peroxy radicals and OH in cities, these results imply that dry nitrate deposition results from local NOx 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.