T. B. Ryerson

An efficient photolysis system for fast‐response NO2 measurements

A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground-, and aircraft-based field data. Focusing the output of a 200-W short-arc Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. At lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within ±(40 pptv+7%) for a 10-s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.

Transport of Asian ozone pollution into surface air over the western United States in spring

and satellite measurements in May–June 2010 with a new global high-resolution (50 50 km 2 ) chemistry-climate model (GFDL AM3). We find that AM3 with full stratosphere-troposphere chemistry nudged to reanalysis winds successfully reproduces observed sharp ozone gradients above California, including the interleaving and mixing of Asian pollution and stratospheric air associated with complex interactions of midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts; at 800 hPa a maximum enhancement to ozone occurs over the southwestern U.S., including the densely populated Los Angeles Basin. During strong episodes, Asian emissions can contribute 8–15 ppbv ozone in the model on days when observed daily maximum 8-h average ozone (MDA8 O3) exceeds 60 ppbv. We find that in the absence of Asian anthropogenic emissions, 20% of MDA8 O3 exceedances of 60 ppbv in the model would not have occurred in the southwestern USA. For a 75 ppbv threshold, that statistic increases to 53%. Our analysis indicates the potential for Asian emissions to contribute to high-O3 episodes over the high-elevation western USA, with implications for attaining more stringent ozone standards in this region. We further demonstrate a proof-of-concept approach using satellite CO column measurements as a qualitative early warning indicator to forecast Asian ozone pollution events in the western U.S. with lead times of 1–3 days.

Trace gas signatures of the airstreams within North Atlantic cyclones: Case studies from the North Atlantic Regional Experiment (NARE ’97) aircraft intensive

This study reveals how airstreams within midlatitude cyclones draw and export trace gases from the polluted continental boundary layer, the midtroposphere, and the stratosphere of North America to the troposphere of the North Atlantic Ocean. The North Atlantic Regional Experiment (NARE) produced aircraft-based trace gas measurements from eight midlatitude cyclones during the autumn of 1997. Meteorological and back trajectory analysis identified the various component airstreams of several cyclones, including a cold conveyor belt, two warm conveyor belts, a dry airstream, a previously undefined post cold front air-stream, and a streamer fragment that originated in a dry airstream off the coast of California. O3, CO, and NOy mixing ratio distributions and relationships were determined for each airstream. Airstream chemical composition was related to the origin and transport history of the associated air mass. The lowest O3 values were associated with airstreams originating in Canada or the Atlantic Ocean marine boundary layer; the highest O3 values were associated with airstreams with a recent stratospheric component. The highest CO values were associated with lower tropospheric outflow from New England and a warm conveyor belt that advected boundary layer CO from the southeast United States to the mid and upper troposphere. The highest NOy values were also the result of lower troposphere polluted outflow from New England. Most NOy was removed from the airstreams that transported polluted boundary layer air to the free troposphere. A steep and positive O3/NOy slope was found for all airstreams in the free troposphere regardless of air mass origin.

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.

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.

Mixing of anthropogenic pollution with stratospheric ozone: A case study from the North Atlantic wintertime troposphere

As part of the North Atlantic Regional Experiment (NARE), instrumentation for the measurement of O3 and CO was included on research flights conducted by the National Oceanic and Atmospheric Administration WP-3D Orion aircraft from St. Johns, Newfoundland, Canada, and Keflavik, Iceland, from February 2 to 25, 1999. These flights sampled the lower troposphere over the western North Atlantic Ocean. One significant feature observed during these flights was the close proximity of air masses with contrasting source signatures: high levels of anthropogenic pollution immediately adjacent to elevated O3 of stratospheric origin. Here we present a case study showing the most pronounced example of this proximity, which was associated with a frontal passage across North America and out into the North Atlantic region. Trajectory analyses and satellite imagery are used to investigate the transport mechanisms that create the interleaving of air masses from the different sources. One important chemical feature was noted: in air masses with differing amounts of anthropogenic pollution admixed, O3 was negatively correlated with CO, which indicates that emissions from surface anthropogenic sources had reduced O3 in this wintertime period, even in air masses transported into the free troposphere.

Quantifying atmospheric methane emissions from oil and natural gas production in the Bakken shale region of North Dakota

We present in situ airborne measurements of methane (CH4) and ethane (C2H6) taken aboard a NOAA DHC-6 Twin Otter research aircraft in May 2014 over the Williston Basin in northwestern North Dakota, a region of rapidly growing oil and natural gas production. The Williston Basin is best known for the Bakken shale formation, from which a significant increase in oil and gas extraction has occurred since 2009. We derive a CH4 emission rate from this region using airborne data by calculating the CH4 enhancement flux through the planetary boundary layer downwind of the region. We calculate CH4 emissions of (36 ± 13), (27 ± 13), (27 ± 12), (27 ± 12), and (25 ± 10) × 103 kg/h from five transects on 3 days in May 2014 downwind of the Bakken shale region of North Dakota. The average emission, (28 ± 5) × 103 kg/h, extrapolates to 0.25 ± 0.05 Tg/yr, which is significantly lower than a previous estimate of CH4 emissions from northwestern North Dakota and southeastern Saskatchewan using satellite remote sensing data. We attribute the majority of CH4 emissions in the region to oil and gas operations in the Bakken based on the similarity between atmospheric C2H6 to CH4 enhancement ratios and the composition of raw natural gas withdrawn from the region.

Fugitive Emissions from the Bakken Shale Illustrate Role of Shale Production in Global Ethane Shift

Ethane is the second most abundant atmospheric hydrocarbon, exerts a strong influence on tropospheric ozone, and reduces the atmospheres oxidative capacity. Global observations showed declining ethane abundances from 1984 to 2010, while a regional measurement indicated increasing levels since 2009, with the reason for this subject to speculation. The Bakken shale is an oil and gas-producing formation centered in North Dakota that experienced a rapid increase in production beginning in 2010. We use airborne data collected over the North Dakota portion of the Bakken shale in 2014 to calculate ethane emissions of 0.23 ± 0.07 (2σ) Tg/yr, equivalent to 1–3% of total global sources. Emissions of this magnitude impact air quality via concurrent increases in tropospheric ozone. This recently developed large ethane source from one location illustrates the key role of shale oil and gas production in rising global ethane levels.

Quantifying sources and sinks of reactive gases in the lower atmosphere using airborne flux observations

Atmospheric composition is governed by the interplay of emissions, chemistry, deposition, and transport. Substantial questions surround each of these processes, especially in forested environments with strong biogenic emissions. Utilizing aircraft observations acquired over a forest in the southeast U.S., we calculate eddy covariance fluxes for a suite of reactive gases and apply the synergistic information derived from this analysis to quantify emission and deposition fluxes, oxidant concentrations, aerosol uptake coefficients, and other key parameters. Evaluation of results against state-of-the-science models and parameterizations provides insight into our current understanding of this system and frames future observational priorities. As a near-direct measurement of fundamental process rates, airborne fluxes offer a new tool to improve biogenic and anthropogenic emissions inventories, photochemical mechanisms, and deposition parameterizations.

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.