We report enhancements of glyoxal and methylglyoxal relative to carbon monoxide and formaldehyde in agricultural biomass burning plumes intercepted by the NOAA WP-3D aircraft during the 2013 Southeast Nexus and 2015 Shale Oil and Natural Gas Nexus campaigns. Glyoxal and methylglyoxal were measured using broadband cavity enhanced spectroscopy, which for glyoxal provides a highly selective and sensitive measurement. While enhancement ratios of other species such as methane and formaldehyde were consistent with previous measurements, glyoxal enhancements relative to carbon monoxide averaged 0.0016 ± 0.0009, a factor of 4 lower than values used in global models. Glyoxal enhancements relative to formaldehyde were 30 times lower than previously reported, averaging 0.038 ± 0.02. Several glyoxal loss processes such as photolysis, reactions with hydroxyl radicals, and aerosol uptake were found to be insufficient to explain the lower measured values of glyoxal relative to other biomass burning trace gases, indicating that glyoxal emissions from agricultural biomass burning may be significantly overestimated. Methylglyoxal enhancements were three to six times higher than reported in other recent studies, but spectral interferences from other substituted dicarbyonyls introduce an estimated correction factor of 2 and at least a 25% uncertainty, such that accurate measurements of the enhancements are difficult.

Cavity enhanced spectroscopy, CES, is a high sensitivity direct absorption method that has seen increasing utility in the last decade, a period also marked by increasing requirements for understanding human impacts on atmospheric composition. This paper describes the current NOAA six channel cavity ring-down spectrometer (CRDS, the most common form of CES) for measurement of nitrogen oxides and O3. It further describes the results from measurements from a tower 300 m above the urban area of Seoul in late spring of 2015. The campaign demonstrates the performance of the CRDS instrument and provides new data on both photochemistry and nighttime chemistry in a major Asian megacity. The instrument provided accurate, high time resolution data for N2O5, NO, NO2, NOy and O3, but suffered from large wall loss in the sampling of NO3, illustrating the requirement for calibration of the NO3 inlet transmission. Both the photochemistry and nighttime chemistry of nitrogen oxides and O3 were rapid in this megacity. Sustained average rates of O3 buildup of 10 ppbv h−1 during recurring morning and early afternoon sea breezes led to a 50 ppbv average daily O3 rise. Nitrate radical production rates, P(NO3), averaged 3–4 ppbv h−1 in late afternoon and early evening, much greater than contemporary data from Los Angeles, a comparable U. S. megacity. These P(NO3) were much smaller than historical data from Los Angeles, however. Nighttime data at 300 m above ground showed considerable variability in high time resolution nitrogen oxide and O3, likely resulting from sampling within gradients in the nighttime boundary layer structure. Apparent nighttime biogenic VOC oxidation rates of several ppbv h−1 were also likely influenced by vertical gradients. Finally, daytime N2O5 mixing ratios of 3–35 pptv were associated with rapid daytime P(NO3) and agreed well with a photochemical steady state calculation.

•A new method for measurement of vehicle NO2/NOx using fast response Ox/NOx in plumes.•Statistics from a sample of >1800 exhaust plumes measured on-road near Denver, CO.•Plume average NO2/NOx is 5.3%, not weighted for NOx mass emissions.Nitrogen oxides (NOx = NO + NO2) emitted by on-road combustion engines are important contributors to tropospheric ozone production. The NOx fraction emitted as nitrogen dioxide (NO2) is usually presumed to be small but can affect ozone production and distribution, and this fraction is generally not reported in emissions inventories. We have developed an accurate method for determination of this primary NO2 emission and demonstrated it during measurement of on-road vehicle emission plumes from a mobile laboratory during July and August 2014 in the region between Denver and Greeley in Colorado. During a total of approximately 90 h of sampling from an instrumented mobile laboratory, we identified 1867 vehicle emission plumes, which were extracted using an algorithm that looks for rapid and large increases in measured NOx. We find a distribution of NO2/NOx emissions similar to a log-normal profile, with an average emission ratio of 0.053 ± 0.002 per sampled NOx plume. The average is not weighted by the total NOx emissions from sampled vehicles, which is not measured here, and so may not represent the NO2/NOx ratio of the total NOx emission if this ratio is a function of NOx itself. Although our current data set does not distinguish between different engine types (e.g., gasoline, light duty diesel and heavy duty diesel), the ratio is on the low end of recent reports of vehicle fleet NO2 to NOx emission ratios in Europe.

We present a sensitive, compact detector that measures total reactive nitrogen (NOy), as well as NO2, NO, and O3. In all channels, NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 405 nm. Ambient O3 is converted to NO2 in excess NO for the O3 measurement channel. Likewise, ambient NO is converted to NO2 in excess O3. Ambient NOy is thermally dissociated at ∼700 °C to form NO2 or NO in a heated quartz inlet. Any NO present in ambient air or formed from thermal dissociation of other reactive nitrogen compounds is converted to NO2 in excess O3 after the thermal converter. We measured thermal dissociation profiles for six of the major NOy components and compared ambient measurements with other instruments during field campaigns in Utah and Alabama. Alabama measurements were made in a rural location with high biogenic emissions, and Utah measurements were made in the wintertime in unusual conditions that form high ozone levels from emissions related to oil and gas production. The NOy comparison in Alabama, to an accepted standard measurement method (a molybdenum catalytic converter/chemiluminescence instrument), agreed to within 12%, which we define as an upper limit to the accuracy of the NOy channel. The 1σ precision is <30 pptv at 1 s and <4 pptv at 1 min time resolution for all measurement channels. The accuracy is 3% for the NO2 and O3 channels and 5% for the NO channel. The precision and accuracy of this instrument make it a versatile alternative to standard chemiluminescence-based NOy instruments.

The nitrate radical, NO3, is photochemically unstable but is one of the most chemically important species in the nocturnal atmosphere. It is accompanied by the presence of dinitrogen pentoxide, N2O5, with which it is in rapid thermal equilibrium at lower tropospheric temperatures. These two nitrogen oxides participate in numerous atmospheric chemical systems. NO3 reactions with VOCs and organic sulphur species are important, or in some cases even dominant, oxidation pathways, impacting the budgets of these species and their degradation products. These oxidative reactions, together with the ozonolysis of alkenes, are also responsible for the nighttime production and cycling of OH and peroxy (HO2 + RO2) radicals. In addition, reactions of NO3 with biogenic hydrocarbons are particularly efficient and are responsible for the production of organic nitrates and secondary organic aerosol. Heterogeneous chemistry of N2O5 is one of the major processes responsible for the atmospheric removal of nitrogen oxides as well as the cycling of halogen species though the production of nitryl chloride, ClNO2. The chemistry of NO3 and N2O5 is also important to the regulation of both tropospheric and stratospheric ozone. Here we review the essential features of this atmospheric chemistry, along with field observations of NO3, N2O5, nighttime peroxy and OH radicals, and related compounds. This review builds on existing reviews of this chemistry, and encompasses field, laboratory and modelling work spanning more than three decades.

We report near-ultraviolet and visible absorption cross sections of hydrogen peroxide (H2O2) using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS), a recently developed, high-sensitivity technique. The measurements reported here span the range of 353–410 nm and extend published electronic absorption cross sections by 60 nm to absorption cross sections below 1 × 10–23 cm2 molecule–1. We have calculated photolysis rate constants for H2O2 in the lower troposphere at a range of solar zenith angles by combining the new measurements with previously reported data at wavelengths shorter than 350 nm. We predict that photolysis at wavelengths longer than those included in the current JPL recommendation may account for up to 28% of the total hydroxyl radical (OH) production from H2O2 photolysis under some conditions. Loss of H2O2 via photolysis may be of the same order of magnitude as reaction with OH and dry deposition in the lower atmosphere; these processes have very different impacts on HOx loss and regeneration.

A sensitive, small detector was developed for atmospheric NO2 and NOx concentration measurements. NO2 is directly detected by laser diode based cavity ring-down spectroscopy (CRDS) at 404 nm. The sum of NO and NO2 (=NOx) is simultaneously measured in a second cavity by quantitative conversion of ambient NO to NO2 in excess ozone. Interferences due to absorption by other trace gases at 404 nm, such as ozone and water vapor, are either negligible or small and are easily quantified. The limit of detection is 22 pptv (2σ precision) for NO2 at 1 s time resolution. The conversion efficiency of NO to NO2 is 99% in excess O3. The accuracy of the NO2 measurement is mainly limited by the NO2 absorption cross section to ±3%. Because of the formation of undetectable higher nitrogen oxides in subsequent reactions of NO2 with ozone in the NOx channel, the (1σ) accuracy of the NOx measurement is increased to approximately ±5% depending on the level of NOx. The new instrument was designed to be easily deployed in the field with respect to size, weight and consumables. Measurements were validated against a photolysis/chemiluminescence detector during six days of sampling ambient air with colocated inlets. The data sets for NO2, NO and NOx exhibit high correlation and good agreement within the combined accuracies of both methods. Linear fits for all three species give similar slopes of 0.99 in ambient air.

Pulsed cavity ring-down spectroscopy is a highly sensitive method for direct absorption spectroscopy that has been applied to in situ detection of NO3, N2O5 and NO2 in the atmosphere from a variety of platforms, including ships, aircraft, and towers. In this paper, we report the development of schemes to significantly improve the accuracy of these measurements. This includes the following: (1) an overall improvement in the inlet transmission efficiencies (92 ± 2% for NO3 and 97 ± 1% for N2O5) achieved primarily through a reduction in the inlet residence time; and (2) the development of a calibration procedure that allows regular determination of these efficiencies in the field by addition of NO3 or N2O5 to the inlet from a portable source followed by conversion of NO3 to NO2. In addition, the dependence of the instrument’s sensitivity and accuracy to a variety of conditions encountered in the field, including variations in relative humidity, aerosol loading, and VOC levels, was systematically investigated. The rate of degradation of N2O5 transmission efficiency on the inlet and filter system due to the accumulation of inorganic aerosol was determined, such that the frequency of filter changes required for accurate measurements could be defined. In the absence of aerosol, the presence of varying levels of relative humidity and reactive VOC were found to be unimportant factors in the instrument’s performance. The 1 σ accuracy of the NO3, N2O5, and NO2 measured with this instrument are −9/+12, −8/+11, ± 6%, respectively, where the ∓ signs indicate that the actual value is low/high relative to the measurement. The largest contribution to the overall uncertainty is now due to the NO3 absorption cross section rather than the inlet transmission efficiency.

Abstract

Nitrogen oxides in the lower troposphere catalyze the photochemical production of ozone (O₃) pollution during the day but react to form nitric acid, oxidize hydrocarbons, and remove O₃ at night. A key nocturnal reaction is the heterogeneous hydrolysis of dinitrogen pentoxide, N₂O₅. We report aircraft measurements of NO₃ and N₂O₅, which show that the N₂O₅ uptake coefficient, g(N₂O₅), on aerosol particles is highly variable and depends strongly on aerosol composition, particularly sulfate content. The results have implications for the quantification of regional-scale O₃ production and suggest a stronger interaction between anthropogenic sulfur and nitrogen oxide emissions than previously recognized.

Absorption cross sections for the à 2A″ (0,90,0) ← 2A′ (0,01,0) band of HCO were determined at 295 K using pulsed laser photolysis combined with cavity ring-down spectroscopy. Formyl radicals (HCO) were produced from the reaction of atomic chlorine, generated by photolysis of Cl2 at 335 nm, with formaldehyde. The concentration of HCO was calibrated using two independent photochemical methods. The peak cross section of the P(8) line was determined to be (1.98 ± 0.36) × 10−18 cm2, and the intensity of the entire band was normalized to this line. The quoted 2σ uncertainty includes estimated systematic errors. Comparisons to previously reported values of HCO cross sections in this band are discussed.

The nitrate radical, NO3, is photochemically unstable but is one of the most chemically important species in the nocturnal atmosphere. It is accompanied by the presence of dinitrogen pentoxide, N2O5, with which it is in rapid thermal equilibrium at lower tropospheric temperatures. These two nitrogen oxides participate in numerous atmospheric chemical systems. NO3 reactions with VOCs and organic sulphur species are important, or in some cases even dominant, oxidation pathways, impacting the budgets of these species and their degradation products. These oxidative reactions, together with the ozonolysis of alkenes, are also responsible for the nighttime production and cycling of OH and peroxy (HO2 + RO2) radicals. In addition, reactions of NO3 with biogenic hydrocarbons are particularly efficient and are responsible for the production of organic nitrates and secondary organic aerosol. Heterogeneous chemistry of N2O5 is one of the major processes responsible for the atmospheric removal of nitrogen oxides as well as the cycling of halogen species though the production of nitryl chloride, ClNO2. The chemistry of NO3 and N2O5 is also important to the regulation of both tropospheric and stratospheric ozone. Here we review the essential features of this atmospheric chemistry, along with field observations of NO3, N2O5, nighttime peroxy and OH radicals, and related compounds. This review builds on existing reviews of this chemistry, and encompasses field, laboratory and modelling work spanning more than three decades.