J. A. Neuman

Surface and Lightning Sources of Nitrogen Oxides over the United States: Magnitudes, Chemical Evolution, and Outflow

[1] We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NOx. The boundary layer NOx data provide top-down verification of a 50% decrease in power plant and industry NOx emissions over the eastern United States between 1999 and 2004. Observed NOx concentrations at 8–12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NOx and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NOx observations with GEOS-Chem requires a factor of 4 increase in modeled NOx yield per flash (to 500 mol/ flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NOy-CO correlation analysis of the fraction f of North American NOx emissions vented to the free troposphere as NOy (sum of NOx and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO3 but at higher altitudes is mostly PAN. The model successfully simulates NOy export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export.

Design and initial characterization of an inlet for gas‐phase NOy measurements from aircraft

An understanding of gas-phase HNO3 transmission through an inlet is necessary to evaluate the quality of NOy measurements from an aircraft platform. A simple, inexpensive, low-volume Teflon inlet is described and its suitability as an aircraft inlet for gas-phase NOy is assessed. Aerosol transmission is not characterized, but inlet design and orientation probably discriminates against the majority of aerosol by mass. Laboratory data, in-flight HNO3 standard addition calibrations, and ambient NOy measurements from the 1997 North Atlantic Regional Experiment aircraft mission are used to characterize inlet transmission efficiencies and time constants. Laboratory tests show high transmission efficiencies for HNO3 which are relatively independent of ambient temperature and humidity. In-flight standard addition calibrations were carried out at ambient temperatures ranging from −20° to +8°C and relative humidities from 3% to 71%. These data suggest that nearly all the sampled air contacts an inlet surface, with 90% of added HNO3 being transmitted in ∼1.5 s. Ambient data are presented to demonstrate negligible hysteresis in 1-Hz NOy measurements, relative to variability observed in ozone data, from an air mass where HNO3 is expected to be a large fraction of the total NOy. Power spectra of ambient NOy (at temperatures from −35° to +35°C and relative humidities from 3% to 100%) and ozone measurements suggest an effective NOy instrument time constant of ∼2 s.

Why do models overestimate surface ozone in the Southeast United States

Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NOx ≡ NO + NO2) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC4RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NOx from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC4RS observations of NOx and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO2 columns. Our results indicate that NEI NOx emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NOx emissions. Upper tropospheric NO2 from lightning makes a large contribution to satellite observations of tropospheric NO2 that must be accounted for when using these data to estimate surface NOx emissions. We find that only half of isoprene oxidation proceeds by the high-NOx pathway to produce ozone; this fraction is only moderately sensitive to changes in NOx emissions because isoprene and NOx emissions are spatially segregated. GEOS-Chem with reduced NOx emissions provides an unbiased simulation of ozone observations from the aircraft, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NOx oxidation products. However, the model is still biased high by 8±13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

Quantifying atmospheric methane emissions from the Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions

We present measurements of methane (CH4) taken aboard a NOAA WP-3D research aircraft in 2013 over the Haynesville shale region in eastern Texas/northwestern Louisiana, the Fayetteville shale region in Arkansas, and the northeastern Pennsylvania portion of the Marcellus shale region, which accounted for the majority of Marcellus shale gas production that year. We calculate emission rates from the horizontal CH4 flux in the planetary boundary layer downwind of each region after subtracting the CH4 flux entering the region upwind. We find 1 day CH4 emissions of (8.0 ± 2.7) × 107 g/h from the Haynesville region, (3.9 ± 1.8) × 107 g/h from the Fayetteville region, and (1.5 ± 0.6) × 107 g/h from the Marcellus region in northeastern Pennsylvania. Finally, we compare the CH4 emissions to the total volume of natural gas extracted from each region to derive a loss rate from production operations of 1.0–2.1% from the Haynesville region, 1.0–2.8% from the Fayetteville region, and 0.18–0.41% from the Marcellus region in northeastern Pennsylvania. The climate impact of CH4 loss from shale gas production depends upon the total leakage from all production regions. The regions investigated in this work represented over half of the U.S. shale gas production in 2013, and we find generally lower loss rates than those reported in earlier studies of regions that made smaller contributions to total production. Hence, the national average CH4 loss rate from shale gas production may be lower than values extrapolated from the earlier studies.

A comparison of observations and model simulations of NOx/NOy in the lower stratosphere

Extensive airborne measurements of the reactive nitrogen reservoir (NO_(y)) and its component nitric oxide (NO) have been made in the lower stratosphere. Box model simulations that are constrained by observations of radical and long-lived species and which include heterogeneous chemistry systematically underpredict the NO_x (= NO + NO_2) to NO_y ratio. The model agreement is substantially improved if newly measured rate coefficients for the OH + NO_2 and OH + HNO_3 reactions are used. When included in 2-D models, the new rate coefficients significantly increase the calculated ozone loss due to NO_x and modestly change the calculated ozone abundances in the lower stratosphere. Ozone changes associated with the emissions of a fleet of supersonic aircraft are also altered.

Subsidence, mixing, and denitrification of Arctic polar vortex air measured during POLARIS

We determine the degree of denitrification that occurred during the 1996-1997 Arctic winter using a technique that is based on balloon and aircraft borne measurements of NO y , N 2 O, and CH 4 . The NO 3 /N 2 O relation can undergo significant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transport related reductions in NO y can be difficult to distinguish from the effects of denitrification caused by sedimentation of condensed HNO 3 . In this study, high-altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled by the ER-2 aircraft (i.e., ∼20 km) and mix isentropically with extravortex air. Observed correlations of CH 4 and N 2 O are used to quantify the degree of subsidence and mixing for individual air masses. On the basis of these results the expected mixing ratio of NO y resulting from subsidence and mixing, defined here as NO y ** , is calculated and compared with the measured mixing ratio of NO y . Values of NO y and NO y ** agree well during most parts of the flights. A slight deficit of NO y versus NO y ** is found only for a limited region during the ER-2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (∼2-3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic-scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively brief encounters with temperatures below the nitric acid trihydrate equilibrium temperature. Much larger degrees of denitrification would have been inferred if mixing effects had been ignored, which is the traditional approach to diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NO y /N 2 O relation with respect to denitrification.

Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

Stratospheric measurements of NO, NO_(2), O_(3), ClO, and HO_(2) were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO_(2) and NO are in steady state through NO2 photolysis and reactions involving O_(3), ClO, BrO, and HO_(2). By combining observations of O_(3), ClO, and HO_(2), observed and modeled values of the NO_(2) photolysis rate coefficient (JNO_(2)), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO_(2) and JNO_(2). An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO_(2)/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O_(3) → NO_(2) + O_(2) would aid future analyses of these or similar atmospheric observations.

Instrumentation and Measurement Strategy for the NOAA SENEX Aircraft Campaign as Part of the Southeast Atmosphere Study 2013

Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO2 measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.

A fast-response chemical ionization mass spectrometer for in situ measurements of HNO3 in the upper troposphere and lower stratosphere

A chemical ionization mass spectrometer instrument has been developed for in situ measurements of nitric acid (HNO3) in the upper troposphere and lower stratosphere from the NASA WB-57 aircraft. Fast and sensitive measurements of HNO3 are achieved by using a low surface area heated Teflon sampling inlet and detection techniques that employ ion-molecule reactions. Sensitivity to HNO3 is determined in flight by adding HNO3 from a calibrated HNO3 permeation source into the sample air flow, and instrument background is measured by displacing ambient air from the sampling inlet with a flow of dry nitrogen in the sampling inlet. Instrument temperatures, pressures, and gas flows are controlled in flight to maintain a constant detection sensitivity in a changing ambient operating environment. The initial performance of this new instrument is evaluated using HNO3 and ozone data obtained between 6 and 19 km during the 1999 NASA Atmospheric Chemistry of Combustion Emissions Near the Tropopause mission. The data reve...

Modeling the weekly cycle of NOx and CO emissions and their impacts on O3 in the Los Angeles‐South Coast Air Basin during the CalNex 2010 field campaign

We developed a new nitrogen oxide (NOx) and carbon monoxide (CO) emission inventory for the Los Angeles-South Coast Air Basin (SoCAB) expanding the Fuel-based Inventory for motor-Vehicle Emissions and applied it in regional chemical transport modeling focused on the California Nexus of Air Quality and Climate Change (CalNex) 2010 field campaign. The weekday NOx emission over the SoCAB in 2010 is 620 t d−1, while the weekend emission is 410 t d−1. The NOx emission decrease on weekends is caused by reduced diesel truck activities. Weekday and weekend CO emissions over this region are similar: 2340 and 2180 t d−1, respectively. Previous studies reported large discrepancies between the airborne observations of NOx and CO mixing ratios and the model simulations for CalNex based on the available bottom-up emission inventories. Utilizing the newly developed emission inventory in this study, the simulated NOx and CO mixing ratios agree with the observations from the airborne and the ground-based in situ and remote sensing instruments during the field study. The simulations also reproduce the weekly cycles of these chemical species. Both the observations and the model simulations indicate that decreased NOx on weekends leads to enhanced photochemistry and increase of O3 and Ox (=O3 + NO2) in the basin. The emission inventory developed in this study can be extended to different years and other urban regions in the U.S. to study the long-term trends in O3 and its precursors with regional chemical transport models.