Isotopic advances in understanding reactive nitrogen deposition and atmospheric processing

Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using oxygen isotopes

Tracking of reactive nitrogen (N) sources is important for the effective mitigation of N emissions. By combining the N and oxygen (O) isotopes of atmospheric NO₃^-, stable isotope mixing models were recently applied to evaluate the relative contributions of major NO_x sources. However, it has long been unresolved how to accurately constrain the δ¹⁵N differences between NO₃^- and corresponding NO_x (ε_(NO2→NO3-) values). Here, we first incorporated the HC oxidation (NO₂ → NO₃^-) pathway by using Δ¹⁷O values to evaluate the ε_(NO2→NO3-) values, performed on NO₃^- in PM_2.5 collected during the day and at night from January 4-13, 2015 at an urban site in Beijing. We found that the Δ¹⁷O-based ε values (ε_{17O-based(NO2→NO3-)}) (15.6 ± 7.4‰) differed distinctly from δ¹⁸O-based ε values (ε_{18O-based(NO2→NO3-)}) (33.0 ± 9.5‰) so did not properly incorporate the isotopic effects of the HC oxidation (NO₂ → NO₃^-) pathway. Based on the ε_(NO2→NO3-) values, δ¹⁵N values of NO_x from coal combustion (CC), vehicle exhausts (VE), biomass burning (BB), and the microbial N cycle (MC), as well as NO₃^- in PM_2.5, we further quantified the source contributions by using Stable Isotope Analysis in R (the SIAR model). We found that the respective fractional contributions of CC-NO_x and MC-NO_x were underestimated by 64% and were overestimated by 216% by using ε_{18O-based(NO2→NO3-)} values. We concluded that the new ε_{17O-based(NO2→NO3-)} values reduced uncertainties in contribution analysis and the evaluation method for atmospheric NO₃^- sources.

Persistent Nonagricultural and Periodic Agricultural Emissions Dominate Sources of Ammonia in Urban Beijing: Evidence from 15N Stable Isotope in Vertical Profiles

Ammonia (NH₃) emission reduction is key to limiting the deadly PM_2.5 pollution globally. However, studies of long-term source apportionment of vertical NH₃ are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of δ¹⁵N-NH₃ at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH₃ concentrations generally remained stable with height. δ¹⁵N-NH₃ increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable δ¹⁵N-NH₃ near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured δ¹⁵N-NH₃ only would overestimate the contribution of agricultural emissions to NH₃. By using an estimation of initial δ¹⁵N-NH₃, we found that nonagricultural sources contributed ∼72% of NH₃ on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH₃ concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured δ¹⁵N-NH₃ only likely underestimates fossil fuel source contribution, if the combined NH_x isotope effects are not considered.

Toward the improvement of total nitrogen deposition budgets in the United States

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Nitrogen Isotope Fractionation from Ammonia Gas to Ammonium in Particulate Ammonium Chloride

The nitrogen stable isotope ratios of NH₃ gas (δ¹⁵N-NH_3(g)) and particulate NH₄⁺ (δ¹⁵N-NH₄⁺_(p)) have been used to determine the sources and elucidate the environmental dynamics of NH_3(g) and NH₄⁺_(p). Some studies have determined the sources from δ¹⁵N-NH₄⁺_(p) in the ambient environment by using the nitrogen isotope fractionation value (Δδ¹⁵N). Studies of Δδ¹⁵N are limited and estimated the Δδ¹⁵N by theoretical calculation or laboratory experiments in closed systems. Here, we produced actual submicron-sized particles and exposed them to NH_3(g) in a dynamic chamber under various experimental conditions of temperature and NH_3(g) turnover rate. Δδ¹⁵N was lower when temperature was higher. For low turnover rates (about 0.9 per day), the Δδ¹⁵N was 31.6 ± 2.0‰ (mean ± 1SD), almost equal to the Δδ¹⁵N from theoretical calculations and laboratory tests in closed systems. Also, the relationship between Δδ¹⁵N, the temperature, and the turnover rate can be described in the equation form. This relationship can help to explain the discrepancies between Δδ¹⁵N values in closed and open systems. Our results could help to clarify the sources and mechanisms of particle formation.