Isotopic Composition Of Atmospheric Nitrate Research Articles

Speciated Collection of Nitric Acid and Fine Particulate Nitrate for Nitrogen and Oxygen Stable Isotope Determination.

Stable isotopic composition of atmospheric nitrate (nitric acid (HNO3) + particulate nitrate (pNO3-)) provides a higher-order dimensional analysis of critical atmospheric components, enabling a process-level understanding of precursor emissions, oxidation chemistry, aerosol acidity, and depositional patterns. Current methods have not been evaluated for their ability to accurately speciate and determine nitrogen (δ15N) and oxygen (δ18O and Δ17O) isotope compositions for gaseous and particle phases. Suitability of a denuder-filter sampling system for the collection of speciated HNO3(g) and pNO3- for off-line concentration and isotopic determination was tested using both laboratory and field collections. Honeycomb denuders coated with either NaCl or Na2CO3 solutions were used to collect HNO3(g). Laboratory experiments found that both coating solutions quantitatively collected HNO3(g), with the Na2CO3 solution demonstrating a higher operative capacity (>1470 μg of HNO3; n = 25) compared to the NaCl solution (∼750 μg of HNO3; n = 25). The precision values for laboratory-tested HNO3(g) collections are ±0.6‰ and ±1.2‰ for δ15N and δ18O for the NaCl solution and ± 0.8‰ and ±1.2‰ for the Na2CO3 solution. Replicate (urban) samples indicate that the Na2CO3 solution is significantly less selective for HNO3(g) collection than the NaCl solution. Nylon filters were found to collect efficiently and retain laboratory-generated NaNO3 and NH4NO3 particles, with maximum standard deviations for δ15N and δ18O of ±0.3‰ and ±0.3‰, respectively. Field replicates, while predictably more variable, also show consistency for δ15N and δ18O of ±0.6‰ and ±1.3‰ for particulate species, respectively. Recommended methods for field collections of speciated HNO3(g) and pNO3- for isotopic measurements would best utilize the NaCl solution and Nylon filters.

Isotopic evidence that recent agriculture overprints climate variability in nitrogen deposition to the Tibetan Plateau

The stable isotopes of nitrogen in nitrate archived in polar ice have been interpreted as reflecting a shift in reactive nitrogen sources or changes in atmospheric chemical reactivity. Here, we present a novel concentration and isotopic record of nitrate (δ15N-NO3−) from a central Tibetan Plateau ice core over the last ~200 years. We find that nitrate concentration increased from 6.0 ± 2.3 μeq/L (mean ± 1σ) in the preindustrial period (prior to 1900s) to 7.3 ± 2.7 μeq/L in post-1950. Over the same time period, the δ15N-NO3− decreased from 8.7 ± 3.7‰ to 4.2 ± 3.1‰, with much larger interannual variation in δ15N-NO3− during the preindustrial period. We present a useful framework for quantifying the sensitivity of the isotopic composition of atmospheric nitrate to changes in both sources and chemistry (gas and aerosol phase). After 1950, nitrogen deposition is primarily driven by fertilizer use, leading to significant increases in concentration and decreases in δ15N-NO3−. The large interannual variability of ice core δ15N-NO3− in the preindustrial reflects natural processes, namely the El Niño Southern Oscillation (ENSO) and dust events. Our results highlight a new connection between the nitrogen cycle and ENSO, and the overprinting of natural climate signals by recent anthropogenic increases in reactive nitrogen release.

The observation of isotopic compositions of atmospheric nitrate in Shanghai China and its implication for reactive nitrogen chemistry

The occurrence of PM2.5 pollution in China is usually associated with the formation of atmospheric nitrate, the oxidation product of nitrogen oxides (NOX = NO + NO2). The oxygen-17 excess of nitrate (Δ17O(NO3−)) can be used to reveal the relative importance of nitrate formation pathways and get more insight into reactive nitrogen chemistry. Here we present the observation of isotopic composition of atmospheric nitrate (Δ17O and δ15N) collected from January to June 2016 in Shanghai China. Concentrations of atmospheric nitrate ranged from 1.4 to 24.1 μg m−3 with the mean values being (7.6 ± 4.4 (1SD)), (10.2 ± 5.8) and (4.1 ± 2.4) μg m−3 in winter, spring and summer respectively. Δ17O(NO3−) varied from 20.5‰ to 31.9‰ with the mean value being (26.9 ± 2.8) ‰ in winter, followed by (26.6 ± 1.7) ‰ in spring and the lowest (23.2 ± 1.6) ‰ in summer. Δ17O(NO3−)-constrained estimates suggest that the conversion of NOX to nitrate is dominated by NO2 + OH and/or NO2 + H2O, with the mean possible contribution of 55–77% in total and even higher (84–92%) in summer. A diurnal variation of Δ17O(NO3−) featured by high values at daytime (28.6 ± 1.2‰) and low values (25.4 ± 2.8‰) at nighttime was observed during our diurnal sampling period. This trend is related to the atmospheric life of nitrate (τ) and calculations indicate τ is around 15 h during the diurnal sampling period. In terms of δ15N(NO3−), it changed largely in our observation, from −2.9‰ to 18.1‰ with a mean of (6.4 ± 4.4) ‰. Correlation analysis implies that the combined effect of NOX emission sources and isotopic fractionation processes are responsible for δ15N(NO3−) variations. Our observations with the aid of model simulation in future study will further improve the understanding of reactive nitrogen chemistry in urban regions.

Theoretical calculation of oxygen equilibrium isotope fractionation factors involving various NOy molecules, [rad]OH, and H2O and its implications for isotope variations in atmospheric nitrate

The oxygen stable isotope composition (δ18O) of nitrogen oxides [NOx=nitric oxide (NO)+nitrogen dioxide (NO2)] and their oxidation products (NOy=NOx+nitric acid (HNO3)+particulate nitrate (p-NO3−)+nitrate radical (NO3)+dinitrogen pentoxide (N2O5)+nitrous acid (HONO)+…) have been shown to be a useful tool for inferring the proportion of NOx that is oxidized by ozone (O3). However, isotopic fractionation processes may have an influence on δ18O of various NOy molecules and other atmospheric O-bearing molecules pertinent to NOx oxidation chemistry. Here we have evaluated the impacts of O isotopic exchange involving NOy molecules, the hydroxyl radical (OH), and water (H2O) using reduced partition function ratios (xβ) calculated by hybrid density functional theory. Assuming atmospheric isotopic equilibrium is achieved between NO and NO2 during the daytime, and NO2, NO3, and N2O5 during the nighttime, δ18O–δ15N compositions were predicted for the major atmospheric nitrate formation pathways using our calculated exchange fractionation factors and isotopic mass-balance. Our equilibrium model predicts that various atmospheric nitrate formation pathways, including NO2+OH→HNO3, N2O5+H2O+surface→2HNO3, and NO3+R→HNO3+R will yield distinctive δ18O–δ15N compositions. Our calculated δ18O–δ15N compositions match well with previous atmospheric nitrate measurements, and will potentially help better understand the role oxidation chemistry plays on the N and O isotopic composition of atmospheric nitrate.

Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign

Abstract. Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO2 = NOx). The unique and distinctive 17O excess (Δ17O = δ17O − 0.52 × δ18O) of ozone, which is transferred to NOx via oxidation, is a particularly useful isotopic fingerprint in studies of NOx transformations. Constraining the propagation of 17O excess within the NOx cycle is critical in polar areas, where there exists the possibility of extending atmospheric investigations to the glacial–interglacial timescale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic Plateau) during the austral summer of 2011/2012. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NOx, O3, OH, HO2, RO2, etc.) and direct observations of the transferrable Δ17O of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO2 pathway and using concurrent measurements of OH and NO2, we calculated a Δ17O signature for nitrate on the order of (21–22 ± 3) ‰. These values are lower than the measured values that ranged between 27 and 31 ‰. This discrepancy between expected and observed Δ17O(NO3−) values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this East Antarctic region. However, systematic errors or false isotopic balance transfer functions are not totally excluded.

Spatial and diurnal variability in reactive nitrogen oxide chemistry as reflected in the isotopic composition of atmospheric nitrate: Results from the CalNex 2010 field study

AbstractHere we present measurements of the size‐resolved concentration and isotopic composition of atmospheric nitrate (NO3−) collected during a cruise in coastal California. Significant differences in air mass origin and atmospheric chemistry were observed in the two main regions of this cruise (South and Central Coast) with corresponding differences in NO3− concentration and isotope ratios. Measurements of the 17O‐excess (Δ17O) of NO3− suggest that nocturnal chemistry played an important role in terms of total NO3− production (~ 50%) in the coastal Los Angeles region (South Coast), where NO3− concentrations were elevated due to the influence of sea breeze / land breeze recirculation and Δ17O(NO3−) averaged (25.3 ± 1.6)‰. Conversely, Δ17O(NO3−) averaged (22.3 ± 1.8)‰ in the Central Coast region, suggesting that the daytime OH + NO2 reaction was responsible for 60–85% of NO3− production in the marine air sampled in this area. A strong diurnal signal was observed for both the Δ17O and δ15N of NO3−. In the case of Δ17O, this trend is interpreted quantitatively in terms of the relative proportions of daytime and nighttime production and the atmospheric lifetime of NO3−. For δ15N, which had an average value of (0.0 ± 3.2)‰, the observed diurnality suggests a combined effect of isotopic exchange between gas‐phase precursors and variability in reactive nitrogen sources. These findings represent a significant advance in our understanding of the isotope dynamics of nitrate and its precursor molecules, with potentially important implications for air quality modeling.

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

Long-term observations of the reactive chemical composition of the tropical marine boundary layer (MBL) are rare, despite its crucial role for the chemical stability of the atmosphere. Recent observations of reactive bromine species in the tropical MBL showed unexpectedly high levels that could potentially have an impact on the ozone budget. Uncertainties in the ozone budget are amplified by our poor understanding of the fate of NOx (= NO + NO2), particularly the importance of nighttime chemical NOx sinks. Here, we present year-round observations of the multiisotopic composition of atmospheric nitrate in the tropical MBL at the Cape Verde Atmospheric Observatory. We show that the observed oxygen isotope ratios of nitrate are compatible with nitrate formation chemistry, which includes the BrNO3 sink at a level of ca. 20 ± 10% of nitrate formation pathways. The results also suggest that the N2O5 pathway is a negligible NOx sink in this environment. Observations further indicate a possible link between the NO2/NOx ratio and the nitrogen isotopic content of nitrate in this low NOx environment, possibly reflecting the seasonal change in the photochemical equilibrium among NOx species. This study demonstrates the relevance of using the stable isotopes of oxygen and nitrogen of atmospheric nitrate in association with concentration measurements to identify and constrain chemical processes occurring in the MBL.

Evaluating source, chemistry and climate change based upon the isotopic composition of nitrate in ice cores

A clear negative trend is found in the nitrogen isotopic ratios (15N/14N) of nitrate over the industrial period, based on a 100-meter ice core from Summit, Greenland. This record indicates that isotopic composition of ice core nitrate reflects changes in nitrogen oxide (NOx) source emissions and that post-industrial emissions of NOx have resulted in a 12‰ (vs. air N2) decline in δ15N of atmospheric nitrate over the last ~300 years. Interestingly, over the last glacial period (as recorded in the Greenland GISP2 ice core), the δ15N of nitrate changes by ~20‰ from a typical pre-industrial Holocene value of 10‰ to a mean glacial value of 28‰, despite the lack of a significant change in nitrate concentration. The more recent ice core record clearly indicates an influence of NOx emission sources, therefore suggesting that the glacial-interglacial change in δ15N may be a record of significant variations in the contribution of NOx sources, such as lightning, biomass burning, biogenic soil emissions and oxidation of nitrous oxide in the stratosphere. In contrast to the source changes recorded by the nitrogen isotopes, the oxygen isotopic record of atmospheric nitrate (18O/16O, 17O/16O) in ice cores has implications for reconstruction of past atmospheric oxidant levels. This is because the oxygen isotopic composition of nitrate reflects oxidation of NOx to nitrate in the atmosphere by oxidants such as ozone and hydroxy and peroxy radicals. Measurements of variations in the isotopic composition of atmospheric nitrate inferred from ice cores could be used to improve interpretation of other records of δ15N such as those from tree rings and/or sediments.

Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65°S to 79°N

The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship‐borne latitudinal transect. In nonpolar areas, the δ15N of nitrate mostly deriving from anthropogenically emitted NOx is found to be significantly different (from 0 to 6‰) from nitrate sampled in locations influenced by natural NOx sources (−4 ± 2)‰. The effects on δ15N(NO3−) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Δ17O = δ17O − 0.52 × δ18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different Δ17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low δ15N values (down to −40‰) are found in air masses exposed to snow‐covered areas, showing that snowpack emissions of NOx from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.

Major influence of BrO on the NOx and nitrate budgets in the Arctic spring, inferred from Δ17O(NO3 - ) measurements during ozone depletion events

Environmental context. Ozone depletion events (ODEs) in the Arctic lower atmosphere drive profound changes in the chemistry of nitrogen oxides (NOx) because of the presence of bromine oxide (BrO). These are investigated using the isotopic composition of atmospheric nitrate (NO3–), which is a ubiquitous species formed through the oxidation of nitrogen oxides. Since BrO is speculated to play a key role in the atmospheric chemistry of marine regions and in the free troposphere, our studies contribute to the improvement of the scientific knowledge on this new topic in atmospheric chemistry. Abstract. The triple oxygen isotopic composition of atmospheric inorganic nitrate was measured in samples collected in the Arctic in springtime at Alert, Nunavut and Barrow, Alaska. The isotope anomaly of nitrate (Δ17O = δ17O–0.52δ18O) was used to probe the influence of ozone (O3), bromine oxide (BrO), and peroxy radicals (RO2) in the oxidation of NO to NO2, and to identify the dominant pathway that leads to the production of atmospheric nitrate. Isotopic measurements confirm that the hydrolysis of bromine nitrate (BrONO2) is a major source of nitrate in the context of ozone depletion events (ODEs), when brominated compounds primarily originating from sea salt catalytically destroy boundary layer ozone. They also show a case when BrO is the main oxidant of NO into NO2.