Emma L. Yates

Revisiting the evidence of increasing springtime ozone mixing ratios in the free troposphere over western North America

We present a 20 year time series of in situ free tropospheric ozone observations above western North America during springtime and interpret results using hindcast simulations (1980–2014) conducted with the Geophysical Fluid Dynamics Laboratory global chemistry-climate model (GFDL AM3). Revisiting the analysis of Cooper et al. (2010), we show that sampling biases can substantially influence calculated trends. AM3 cosampled in space and time with observations reproduces the observed ozone trend (0.65 ± 0.32 ppbv yr−1) over 1995–2008 (in simulations either with or without time-varying emissions), whereas AM3 “true median” with continuous temporal and spatial sampling indicates an insignificant trend (0.25 ± 0.32 ppbv yr−1). Extending this analysis to 1995–2014, we find a weaker ozone trend of 0.31 ± 0.21 ppbv yr−1 from observations and 0.36 ± 0.18 ppbv yr−1 from AM3 “true median.” Rising Asian emissions and global methane contribute to this increase. While interannual variability complicates the attribution of ozone trends, multidecadal hindcasts can aid in the estimation of robust confidence limits for trends based on sparse observational records.

A Comparison of In Situ Aircraft Measurements of Carbon Dioxide and Methane to GOSAT Data Measured Over Railroad Valley Playa, Nevada, USA

In this paper, we report the vertical profiles of CO2 and CH4 measured with a cavity ring-down spectrometer (CRDS) on a research aircraft from near-ground level to 8 km above mean sea level. The airborne platform employed in this paper is an Alpha Jet aircraft operated from NASAs Ames Research Center. Flights were undertaken to Railroad Valley, NV, USA, to coincide with overpasses of the Greenhouse Gases Observing Satellite (GOSAT). Ground-based CO2 and CH4 were simultaneously measured using CRDS, at the time and location of the airborne and satellite measurements. Results of three GOSAT coordinated aircraft profiles and ground-based measurements in June 2011 are presented and discussed in this paper. The accuracy of the CO2 and CH4 measurements has been determined based upon laboratory calibrations (World Meteorological Organisation traceable standard) and pressure/temperature flight simulations in a test chamber. The overall uncertainty for the airborne measurements ranged from 0.31 to 0.39 ppm for CO2 and from 3.5 to 5.6 ppb for CH4. Our column-averaged CO2 and CH4 measurements, which include about 61% of the total atmospheric mass, are extrapolated, using different techniques, to include the remainder of the tropospheric and stratospheric CO2 and CH4. The CO2 data are then analyzed using the Atmospheric CO2 Observations from Space 2.9 and 3.3 algorithms. For methane data, the RemoTeC v2.1 algorithm was used in its full physics setup. Column-averaged CO2 and XCO2, measured by GOSAT and analyzed from our data, ranged from 388.1 to 396.4 ppm, and XCH4 ranged from 1.743 to 1.822 ppm. The agreement of the satellite and aircraft CO2 mixing ratios, as well as ground measurements, falls within the uncertainties of the methods employed to acquire these numbers.

Photochemistry and Photophysics of n-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal: Experimental and Theoretical Study

Dilute mixtures of n-butanal, 3-methylbutanal, and 3,3-dimethylbutanal in synthetic air, different N(2)/O(2) mixtures, and pure nitrogen (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were ethene (n-butanal), propene (3-methylbutanal) or i-butene (3,3-dimethylbutanal), CO, vinylalcohol, and ethanal. The photolysis rates and the absolute quantum yields were found to be dependent on the total pressure of synthetic air but not of nitrogen. At 100 Torr, the total quantum yield Φ(100) = 0.45 ± 0.01 and 0.49 ± 0.07, whereas at 700 Torr, Φ(700) = 0.31 ± 0.01 and 0.36 ± 0.03 for 3-methylbutanal and 3,3-dimethybutanal, respectively. Quantum yield values for n-butanal were reported earlier by Tadić et al. (J. Photochem. Photobiol. A2001143, 169-179) to be Φ(100) = 0.48 ± 0.02 and Φ(700) = 0.32 ± 0.01. Two decomposition channels were identified: the radical channel RCHO → R + HCO (Norrish type I) and the molecular channel CH(3)CH(CH(3))CH(2)CHO → CH(2)CHCH(3) + CH(2)═CHOH or CH(3)C(CH(3))(2)CH(2)CHO → CHC(CH(3))CH(3) + CH(2)═CHOH, (Norrish type II) having the absolute quantum yields of 0.123 and 0.119 for 3-methybutanal and 0.071 and 0.199 for 3,3-dimethylbutanal at 700 Torr of synthetic air. The product ethenol CH(2)═CHOH tautomerizes to ethanal. We have performed ab initio and density functional quantum (DFT) chemical computations of both type I and type II processes starting from the singlet and triplet excited states. We conclude that the Norrish type I dissociation produces radicals from both singlet and triplet excited states, while Norrish type II dissociation is a two-step process starting from the triplet excited state, but is a concerted process from the singlet state.

A New Instrumented Airborne Platform for Atmospheric Research

AbstractThe NASA Ames Research Center operates a new research platform for atmospheric studies: an instrumented Alpha Jet. The present complement of instruments allows for the determination of carbon dioxide, ozone, water vapor, and methane concentrations as well as measurements of three-dimensional wind speeds, temperature, and pressure. Planned future instrumentation includes an Air-Core sampler and an instrument to measure formaldehyde. We give examples of measurements that have been made, including measurements carried out during a downward spiral over an expected methane source. An attractive property of this airborne system is its ability to respond rapidly to unexpected atmospheric events such as large forest fires or severe air quality events.

Two-Year Comparison of Airborne Measurements of CO 2 and CH 4 With GOSAT at Railroad Valley, Nevada

The Alpha Jet Atmospheric eXperiment (AJAX) is a project to measure the atmospheric profiles of greenhouse gases (GHGs) and ozone (O3) regularly over California and Nevada. Airborne instruments measuring GHGs and O3 are installed in a wing pod of an Alpha Jet aircraft and operated from the National Aeronautics and Space Administration Ames Research Center at Moffett Field, CA. The instruments yield precise and accurate in situ vertical profiles of atmospheric carbon dioxide (CO2), methane (CH4), and O3. Measurements of vertical profiles of GHGs and O3 over Railroad Valley, NV have been conducted directly under the Greenhouse gases Observing SATellite (GOSAT) over passes on a monthly basis as part of the AJAX project since June 2011. The purpose of this work is to calculate aircraft-based dry-air mole fractions of the GHGs for the validation of GOSAT data products. This study expands and improves our previous comparisons by evaluating three algorithms against 24 months of in situ data collected over a Gain-M target. We used three different algorithms: Atmospheric CO2 Observations from Space (ACOS v3.4r3), Remote Sensing of Greenhouse Gases for Carbon Cycle Modeling (RemoteC v2.3.5FP), and National Institute for Environmental Studies (NIES v2.11). We find that the CO2 average differences of ACOS and RemoteC from AJAX are 0.26% and 0.24%, respectively. The difference between NIES and AJAX is 0.96%, which is higher than that of ACOS and RemoteC. The CH4 average differences for RemoteC and NIES are 2.1% and 1.7%, respectively.

Investigating the influence of long-range transport on surface O3 in Nevada, USA, using observations from multiple measurement platforms

The current United States (US) National Ambient Air Quality Standard (NAAQS) for O3 (75 ppb) is expected to be revised to between 60 and 70 ppb. As the NAAQS becomes more stringent, characterizing the extent of O3 and precursors transported into the US is increasingly important. Given the high elevation, complex terrain, and location in the Intermountain West, the State of Nevada is ideally situated to intercept air transported into the US. Until recently, measurements of O3 and associated pollutants were limited to areas in and around the cities of Las Vegas and Reno. In 2011, the Nevada Rural Ozone Initiative began and through this project 13 surface monitoring sites were established. Also in 2011, the NASA Ames Alpha Jet Atmospheric eXperiment (AJAX) began making routine aircraft measurements of O3 and other greenhouse gases in Nevada. The availability of aircraft and surface measurements in a relatively rural, remote setting in the Intermountain West presented a unique opportunity to investigate sources contributing to the O3 observed in Nevada. Our analyses indicate that stratosphere to troposphere transport, long-range transport of Asian pollution, and regional emissions from urban areas and wildfires influence surface observations. The complexity of sources identified here along with the fact that O3 frequently approaches the threshold being considered for a revised NAAQS indicate that interstate and international cooperation will be necessary to achieve compliance with a more stringent regulatory standard. Further, on a seasonal basis we found no significant difference between daily 1-h maximum O3 at surface sites, which ranged in elevation from 888 to 2307 m, and aircraft measurements of O3 <2500 m which suggests that similar processes influence daytime O3 across rural Nevada and indicates that column measurements from Railroad Valley, NV are useful in understanding these processes.

Investigating seasonal methane emissions in Northern California using airborne measurements and inverse modeling

Seasonal methane (CH4) emissions in northern California are evaluated during this study using airborne measurement data and inverse model simulations. This research applies Alpha Jet Atmospheric eXperiment (AJAX) measurements obtained during January – February 2013, July 2014, and October – November 2014 over the San Francisco Bay Area (SFBA) and northern San Joaquin Valley (SJV) in order to constrain seasonal CH4 emissions in northern California. The California Greenhouse Gas Emissions Measurement (CALGEM) a priori emission inventory was applied in conjunction with the Weather Research and Forecasting and Stochastic Time-Inverted Lagrangian Transport (WRF-STILT) model and inverse modeling techniques to optimize CH4 emissions. Comparing model-predicted CH4 mixing ratios with airborne measurements, we find substantial underestimates suggesting that CH4 emissions were likely larger than the year 2008 a priori CALGEM emission inventory in northern California. Using AJAX measurements to optimize a priori emissions resulted in CH4 flux estimates from the SFBA/SJV of 1.77 ± 0.41, 0.83 ± 0.31, and 1.06 ± 0.39 Tg yr-1 when using winter, summer, and fall flight data, respectively. Averaging seasonal a posteriori emission estimates (weighted by posterior uncertainties) results in SFBA/SJV annual CH4 emissions of 1.28 ± 0.38 Tg yr-1. A posteriori uncertainties are reduced more effectively in the SFBA/SJV region compared to state-wide values indicating that the airborne measurements are most sensitive to emissions in this region. A posteriori estimates during this study suggest that dairy livestock was the source with the largest increase relative to the a priori CALGEM emission inventory during all seasons.

Photochemical model evaluation of 2013 California wild fire air quality impacts using surface, aircraft, and satellite data

The Rim Fire was one of the largest wildfires in California history, burning over 250,000 acres during August and September 2013 affecting air quality locally and regionally in the western U.S. Routine surface monitors, remotely sensed data, and aircraft based measurements were used to assess how well the Community Multiscale Air Quality (CMAQ) photochemical grid model applied at 4 and 12 km resolution represented regional plume transport and chemical evolution during this extreme wildland fire episode. Impacts were generally similar at both grid resolutions although notable differences were seen in some secondary pollutants (e.g., formaldehyde and peroxyacyl nitrate) near the Rim fire. The modeling system does well at capturing near-fire to regional scale smoke plume transport compared to remotely sensed aerosol optical depth (AOD) and aircraft transect measurements. Plume rise for the Rim fire was well characterized as the modeled plume top was consistent with remotely sensed data and the altitude of aircraft measurements, which were typically made at the top edge of the plume. Aircraft-based lidar suggests O3 downwind in the Rim fire plume was vertically stratified and tended to be higher at the plume top, while CMAQ estimated a more uniformly mixed column of O3. Predicted wildfire ozone (O3) was overestimated both at the plume top and at nearby rural and urban surface monitors. Photolysis rates were well characterized by the model compared with aircraft measurements meaning aerosol attenuation was reasonably estimated and unlikely contributing to O3 overestimates at the top of the plume. Organic carbon was underestimated close to the Rim fire compared to aircraft data, but was consistent with nearby surface measurements. Periods of elevated surface PM2.5 at rural monitors near the Rim fire were not usually coincident with elevated O3.

An Assessment of Ground Level and Free Tropospheric Ozone Over California and Nevada

Increasing free tropospheric ozone (O3), combined with the high elevation and often deep boundary layers at western US surface stations, poses challenges in attaining the more stringent 70 ppb O3 National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O3 is increasingly important. This work analyzes surface and vertical O3 observations over California and Nevada from 1995 to 2015. Over this period, the number of high O3 events (95th percentile) at US EPA CASTNET sites has decreased during summer, as a result of decreasing US emissions. In contrast, an increase in springtime 5th percentile O3 indicates a general increase of baseline O3. During 2012 there was a peak in exceedances and in the average spring-summer O3 mixing ratios at CASTNET sites. GEOS-Chem results show that the surface O3 attributable to transport from the upper troposphere and stratosphere were increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O3 measurements from aircraft, ozonesondes and lidar show distinct seasonal trends, with a high percentage of elevated O3 laminae (O3 >70 ppb, 3-8 km) during spring and summer. Analysis of the timing of high O3 surface events and correlation between surface and vertical O3 data is used to discuss varying sources of western US surface O3.

Elliptic Cylinder Airborne Sampling and Geostatistical Mass Balance Approach for Quantifying Local Greenhouse Gas Emissions

In this study, we explore observational, experimental, methodological, and practical aspects of the flux quantification of greenhouse gases from local point sources by using in situ airborne observations, and suggest a series of conceptual changes to improve flux estimates. We address the major sources of uncertainty reported in previous studies by modifying (1) the shape of the typical flight path, (2) the modeling of covariance and anisotropy, and (3) the type of interpolation tools used. We show that a cylindrical flight profile offers considerable advantages compared to traditional profiles collected as curtains, although this new approach brings with it the need for a more comprehensive subsequent analysis. The proposed flight pattern design does not require prior knowledge of wind direction and allows for the derivation of an ad hoc empirical correction factor to partially alleviate errors resulting from interpolation and measurement inaccuracies. The modified approach is applied to a use-case for quantifying CH4 emission from an oil field south of San Ardo, CA, and compared to a bottom-up CH4 emission estimate.