Measurements Of Reactive Nitrogen Species Research Articles

Unraveling Urban NOx Emission Sources in Polluted Arctic Wintertime Using NO2 Nitrogen Isotopes

AbstractNitrogen (N) isotopic fractionation during nitrogen oxides (NOx) cycling and conversion into atmospheric nitrate alters the original N isotopic composition (δ15N) of NOx emissions. Limited quantification of these isotopic effects in urban settings hampers the δ15N‐based identification and apportionment of NOx sources. δ15N of nitrogen dioxide (NO2) measured during winter in downtown Fairbanks, Alaska, displayed a large temporal variability, from −10.2 to 24.1‰. δ15N(NO2) records are found to be driven by equilibrium isotopic fractionation, at a rate in very close agreement with theoretical predictions. This result confirms that N isotopic partitioning between NO and NO2 can be accurately predicted over a wide range of conditions. This represents an important step for inferring NOx emission sources from isotopic composition measurement of reactive nitrogen species. After correcting our δ15N(NO2) measurements for N fractionation effects, a δ15N‐based source apportionment analysis identifies vehicle and space heating oil emissions as the dominant sources of breathing‐level NOx at this urban site. Despite their large NOx emissions, coal‐fired power plants with elevated chimney stacks (>26 m) appear to make a small contribution to surface NOx levels in downtown Fairbanks (likely less than 18% on average). The combined uncertainties of the δ15N of NOx from heating oil combustion and of the influence of low temperatures on the δ15N of NOx emitted by vehicle exhaust prevent a more detailed partitioning of surface NOx sources in Fairbanks.

The Michigan–Ontario Ozone Source Experiment (MOOSE): An Overview

The Michigan–Ontario Ozone Source Experiment (MOOSE) is an international air quality field study that took place at the US–Canada Border region in the ozone seasons of 2021 and 2022. MOOSE addressed binational air quality issues stemming from lake breeze phenomena and transboundary transport, as well as local emissions in southeast Michigan and southern Ontario. State-of-the-art scientific techniques applied during MOOSE included the use of multiple advanced mobile laboratories equipped with real-time instrumentation; high-resolution meteorological and air quality models at regional, urban, and neighborhood scales; daily real-time meteorological and air quality forecasts; ground-based and airborne remote sensing; instrumented Unmanned Aerial Vehicles (UAVs); isotopic measurements of reactive nitrogen species; chemical fingerprinting; and fine-scale inverse modeling of emission sources. Major results include characterization of southeast Michigan as VOC-limited for local ozone formation; discovery of significant and unaccounted formaldehyde emissions from industrial sources; quantification of methane emissions from landfills and leaking natural gas pipelines; evaluation of solvent emission impacts on local and regional ozone; characterization of the sources of reactive nitrogen and PM2.5; and improvements to modeling practices for meteorological, receptor, and chemical transport models.

Evaluation of the accuracy of thermal dissociation CRDS and LIF techniques for atmospheric measurement of reactive nitrogen species

Abstract. The sum of all reactive nitrogen species (NOy) includes NOx (NO2 + NO) and all of its oxidized forms, and the accurate detection of NOy is critical to understanding atmospheric nitrogen chemistry. Thermal dissociation (TD) inlets, which convert NOy to NO2 followed by NO2 detection, are frequently used in conjunction with techniques such as laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) to measure total NOy when set at > 600 °C or speciated NOy when set at intermediate temperatures. We report the conversion efficiency of known amounts of several representative NOy species to NO2 in our TD-CRDS instrument, under a variety of experimental conditions. We find that the conversion efficiency of HNO3 is highly sensitive to the flow rate and the residence time through the TD inlet as well as the presence of other species that may be present during ambient sampling, such as ozone (O3). Conversion of HNO3 at 400 °C, nominally the set point used to selectively convert organic nitrates, can range from 2 to 6 % and may represent an interference in measurement of organic nitrates under some conditions. The conversion efficiency is strongly dependent on the operating characteristics of individual quartz ovens and should be well calibrated prior to use in field sampling. We demonstrate quantitative conversion of both gas-phase N2O5 and particulate ammonium nitrate in the TD inlet at 650 °C, which is the temperature normally used for conversion of HNO3. N2O5 has two thermal dissociation steps, one at low temperature representing dissociation to NO2 and NO3 and one at high temperature representing dissociation of NO3, which produces exclusively NO2 and not NO. We also find a significant interference from partial conversion (5–10 %) of NH3 to NO at 650 °C in the presence of representative (50 ppbv) levels of O3 in dry zero air. Although this interference appears to be suppressed when sampling ambient air, we nevertheless recommend regular characterization of this interference using standard additions of NH3 to TD instruments that convert reactive nitrogen to NO or NO2.

Enhanced concentrations of reactive nitrogen species in wildfire smoke

During the summer of 2012 the Hewlett Gulch and High Park wildfires burned an area of 400 km2 northwest of Fort Collins, Colorado. These fires both came within 20 km of the Department of Atmospheric Science at Colorado State University, allowing for extensive measurements of smoke-impacted air masses over the course of several weeks. In total, smoke plumes were observed at the measurement site for approximately 125 h. During this time, measurements were made of multiple reactive nitrogen compounds, including gas phase species NH3, NOx, and HNO3, and particle phase species NO3− and NH4+, plus an additional, unspeciated reactive nitrogen component that is measured by high temperature conversion over a catalyst to NO. Concurrent measurements of CO, levoglucosan and PM2.5 served to confirm the presence of smoke at the monitoring site. Significant enhancements were observed for all of the reactive nitrogen species measured in the plumes, except for NH4+ which did not show enhancements, likely due to the fresh nature of the plume, the presence of sufficient regional ammonia to have already neutralized upwind sulfate, and the warm conditions of the summer measurement period which tend to limit ammonium nitrate formation. Excess mixing ratios for NH3 and NOx relative to excess mixing ratios of CO in the smoke plumes, ΔNH3/ΔCO (ppb/ppb) and ΔNOx/ΔCO (ppb/ppb), were determined to be 0.027 ± 0.002 and 0.0057 ± 0.0007, respectively. These ratios suggest that smoldering combustion was the dominant source of smoke during our plume interceptions. Observations from prior relevant laboratory and field measurements of reactive nitrogen species are also briefly summarized to help create a more comprehensive picture of reactive nitrogen and fire.

The enigma of reactive nitrogen in volcanic emissions

Nitrogen fixation has been reported at numerous active volcanoes. Where it occurs, it should enhance nitrogen bioavailability in the local environment and reactive nitrogen chemistry in the troposphere. Measurements of reactive nitrogen species in volcanic emissions are highly variable, in part due to an incomplete assessment of a diverse speciation (e.g., NO, NO2, HNO3, NO3− and NO2HO2), but suggest typical molar ratios of NOy/H2O∼10−5. These high levels of reactive nitrogen species may be explained by equilibrium in high temperature (>1000°C) mixtures of magmatic and atmospheric gases. However, using a kinetic model for a simplified magmatic and atmospheric gas mixture (i.e., the C–O–N–H–Ar system), we show that the rate of thermal nitrogen fixation is too slow for equilibrium to be attained in transient mixtures forming at volcanic vents. Other mechanisms for high NOy/H2O should therefore be considered including the catalysis of nitrogen fixation (e.g., by metal salts or silicate particles). Our results also suggest that H2 and CO should oxidise extremely rapidly (<10−3s at 1100°C) at volcanic vents, conflicting with recent measurements indicating the stability of these reduced species. We therefore propose that high temperature oxidation at volcanic vents is a sporadic process due to variability in the mixture lifetime (with time and across the mixing front between magmatic and atmospheric gases). This variability would result in minimal losses of H2 and CO while still allowing oxidised trace species (e.g., NO, OH, Br, Cl) to form in significant concentrations and thus influence the chemical evolution of the volcanic plume at ambient temperatures.

Measurement of trace atmospheric species by chemical ionization mass spectrometry: Speciation of reactive nitrogen and future directions

Chemical ionization mass spectrometry (CIMS) has proven to be a powerful method for sensitive, fast time response (t approximately 1 sec) measurements of various atmospheric compounds with limits of detection (LOD) of the order of tens of pptv and lower. The rapid time response of CIMS is particularly well suited for airborne measurements and its application has largely grown out of airborne measurements in the stratosphere and upper troposphere. This work reviews some of the advances in CIMS technology that have occurred in the past decade. In particular, CIMS methods for selective measurement of reactive nitrogen species (e.g., HNO3, HO2NO2, PAN, and NH3) in the lower atmosphere (altitudes approximately 0-8 km) are described. In addition, recent developments in CIMS technology for the selective measurement of gas-phase hydroperoxides and aerosol chemical composition are briefly described.

An assessment of aircraft as a source of particles to the upper troposphere

Condensation nuclei measurements are examined in conjunction with measurements of reactive nitrogen species (NOy) to identify aircraft plumes in primary air traffic corridors over the North Atlantic. Several hundred plumes exhibiting ≥100 pptv enhancements in NOy mixing ratio were observed. The plumes were typically a few hundred meters wide, exhibited high NO/NOy ratios, and ranged in age from ∼10 minutes to ∼10 hours. Assuming the sampled aircraft emitted ∼12 g NOx (as NO2) kg−1 fuel burned and that the loss of NOy to the particle phase was negligible, we calculate median aerosol emission indices in terms of number of particles kg−1 of fuel burned of ∼120×1015 for CN ≥8 nm in size; ∼50×1015 for CN ≥ 17 nm; and ∼3×1015 for the nonvolatile CN ≥ 17 nm. Using published fuel burn statistics, background aerosol concentrations, and a 10 day particle lifetime, we conclude that present aviation sources enhance global averaged upper‐tropospheric fine and nonvolatile aerosol number densities by ∼6% and ∼3%, respectively.

Measurements and analysis of reactive nitrogen species in the rural troposphere of Southeast United States: Southern oxidant study site SONIA

Ambient concentrations of reactive nitrogen compounds as well as total NOY were measured during June and early July 1992 at a rural site, site SONIA, in the central Piedmont region of North Carolina as a part of the Southern Oxidants Study. The measurements of reactive nitrogen species were made in an effort to provide a comprehensive understanding of nitrogen chemistry and to investigate the total nitrogen budget at the site. NO y , NO 2 and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NO y concentration reached was ∼ 14.5 ppbv, and the maximum concentrations of NO and NO 2 were 5.4 and 7.8 ppbv, respectively. The mean NO y mixing ratio was found to be 2.88 ± 1.58 ppbv ( n = 743) with an average daily maximum of 3.6 ppbv. The mean mixing ratios of NO and N0 2 were found to be 0.15 ± 0.29 ppbv ( n = 785) and 1.31 ± 0.99 ppbv ( n = 769). Average daily maxima of NO and N0 2 were 0.4 and 2.0 ppbv, respectively. HNO 3 and PAN showed diurnal variation with maxima in the afternoon and minimum in the night, and mean mixing ratios were found to be 0.67 ± 0.33 ppbv ( n = 250) and 0.40 ± 0.24 ppbv ( n = 578). The fractions of individual reactive nitrogen species to total NO y , were investigated and contrasted to the results from a remote marine site and rural continental sites. As in two other rural continental sites in the U.S., NO x was found to be the most abundant constituent (∼45%) of NOY; while HNO 3 was the most abundant compound in NO y measured at a remote marine site. The discrepancy between the NO y , partitioning at site SONIA and the marine site is attributed to the influence of local and regional anthropogenic sources of NO x and the continental origin of the majority of air masses encountered at the site. The NO x NO y ratio and NO z ( = NO y − NO x ) were used as an indicator of the chemical age of airmasses. The NO x NO y ratio showed strong positive correlations with the photochemical oxidants HNO 3 ( r = 0.76), PAN ( r = 0.68) and 0 3 ( r = 0.79) measured at the site. Positive correlations were found between surface wind direction and both the magnitude of NO y and the NO x NO y ratio. These correlations suggest that synoptic meteorological conditions and transport of NO x are important in the distribution of NO y and its relationship with photochemical oxidants at the site. The ozone production efficiency was illustrated by correlation of O 3 and NO z and compared with other published measurements made in the Southeast U.S., and published results from a 3D Eulerian model simulation.