This work presents measurements of sulfur dioxide (SO2) emission fluxes from point sources using airborne near‐ultraviolet (UV) spectroscopy. A Czerny‐Turner spectrograph has been optimized to ...measure SO2 and the oxygen collision complex (O4) in the wavelength region of 286–408 nm from an aircraft platform. The spectrograph was deployed aboard the NOAA WP‐3D Orion aircraft during the New England Air Quality Study during the summer of 2004. The spectrograph has zenith and nadir field of views, allowing for measurements of pollution plumes when the aircraft is in or above the planetary boundary layer. The near‐UV spectra are analyzed using the differential optical absorption spectroscopy (DOAS) method to retrieve SO2 and O4 differential slant column densities (DSCDs). The SO2 DSCDs are used to identify point source plumes and are converted to vertical column densities (VCDs), which are needed to calculate emissions of SO2 from point sources. The conversion to VCDs requires knowledge of the photon optical path length or the air mass factor (AMF). We present a novel approach to calculate the AMF using observations of the absorption of solar radiation by O4. The SO2 VCDs, wind speed and direction, and aircraft speed are then used to obtain emission fluxes from power plants. The measured SO2 power plant emission fluxes are compared to the reported emissions from the power plants. The measured and reported SO2 emission fluxes are in good agreement.
We carried out a spectroscopic field experiment designed to measure water vapor continuum absorption in the visible and near‐infrared spectral regions. Atmospheric spectra at ∼1 nm resolution were ...recorded using direct sunlight at high solar zenith angles during sunrise. Simultaneously radiosonde soundings and a network of geodetic Global Positioning System (GPS) receivers were deployed to constrain the water vapor amount along the absorption path. The solar spectra were analyzed using the Differential Optical Absorption Spectroscopy technique, while the GPS and radiosonde observations were used as input data to a line‐by‐line radiative transfer model to compute theoretical differential absorption spectra. The difference between the measurements and the simulated spectra provides information regarding the additional absorption owing to the H2O continuum. The data are compared to predictions of the widely used Clough‐Kneizys‐Davies continuum model as well as with theoretically derived spectra of water dimer. The results show that continuum absorption contributes significantly to solar absorption even in highly saturated H2O bands. The comparisons provide the needed observations to improve future continuum parameterizations.
Ground‐based visible absorption spectroscopy of zenith‐scattered sky light was used to measure changes in the column abundance of NO2 during the passage of a thunderstorm over Boulder, Colorado, on ...September 12, 2002. The measurements showed a 10‐fold rise in slant column NO2 within the thunderstorm cell, consistent with an increase of about 7 ppbv in the lower core updrafts and main precipitation shaft. Mean mixing ratios of about 3 and 1.5 ppbv were inferred for the upper cell and anvil, respectively. These observations, together with measurements from the Denver NEXRAD and the Colorado State University (CSU) CHILL radars, and cloud‐to‐ground (CG) lightning flash data from the National Lightning Detection Network (NLDN), are used to estimate the flash mean production of NOx by CG lightning. The measurements suggest a mean production rate of (5.8 ± 2.9) × 1026 NOx molecules per CG flash, intermediate between the values derived from previous spectroscopic studies and in good agreement with the widely used value of 6.7 × 1026 NOx molecules per CG flash estimated by Price et al. 1997. This agreement may be fortuitous, however, since the lightning in the present study was about a factor of two less energetic than the mean assumed by Price et al. and the production of (2.3 ± 1.4) × 1017 NOx/J inferred here is significantly larger than the value of 1 × 1017 NOx/J assumed in their calculations. These differences can be reduced, however, if the estimates of Price et al. are recalculated using newer peak current data from the NLDN, and more recently published flash energy production rates of Wang et al. 1998.
Measurements of gaseous ammonia above a montane-subalpine forest in the Colorado mountains show that the role of the forest as a source or sink depends on the atmospheric concentrations. The canopy ...appeared to be an ammonia source when exposed to air containing low concentrations, but a sink when exposed to air enriched by nearby agricultural sources. The forest-averaged compensation point was 0.8 part per billion by volume at 20 degrees C. The net burden of ammonia and other nitrogen species of anthropogenic origin at this site was much less than at forest sites in the eastern United States and Europe and may provide a valuable resource for this nitrogen-limited ecosystem
The Tropospheric Ozone Lidar Network (TOLNet) is a unique network of lidar systems that measure high-resolution atmospheric profiles of ozone. The accurate characterization of these lidars is ...necessary to determine the uniformity of the network calibration. From July to August 2014, three lidars, the TROPospheric OZone (TROPOZ) lidar, the Tunable Optical Profiler for Aerosol and oZone (TOPAZ) lidar, and the Langley Mobile Ozone Lidar (LMOL), of TOLNet participated in the Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) mission and the Front Range Air Pollution and Photochemistry Experiment (FRAPPA) to measure ozone variations from the boundary layer to the top of the troposphere. This study presents the analysis of the intercomparison between the TROPOZ, TOPAZ, and LMOL lidars, along with comparisons between the lidars and other in situ ozone instruments including ozonesondes and a P-3B airborne chemiluminescence sensor. The TOLNet lidars measured vertical ozone structures with an accuracy generally better than +/-15 % within the troposphere. Larger differences occur at some individual altitudes in both the near-field and far-field range of the lidar systems, largely as expected. In terms of column average, the TOLNet lidars measured ozone with an accuracy better than +/-5 % for both the intercomparison between the lidars and between the lidars and other instruments. These results indicate that these three TOLNet lidars are suitable for use in air quality, satellite validation, and ozone modeling efforts.
Ground‐based lidar measurements of ozone mixing ratio and aerosol backscatter profiles above Fritz Peak Observatory near Boulder, Colorado (39.9° N, 105.3°W), are used in conjunction with nearby ...radiosonde profiles and GOES‐9 satellite water vapor imagery to describe the mixing of a dry, ozone‐rich streamer of stratospheric air into the upper troposphere over the western United States on June 30 and July 1, 1997. The satellite images show that the streamer extended over more than 2000 km from the eastern Pacific Ocean to the central United States beneath the subtropical jet with a width of ∼200 km over Colorado. The streamer persisted with little change for more than 6 hours of lidar observations and then abruptly disappeared to be replaced by a region of unusually low ozone. The disappearance of the streamer is attributed to localized convective activity induced by the incursion of a moist, ozone‐poor subtropical air mass between the streamer and the convective boundary layer. This activity disrupted the streamer and, together with shear‐induced mixing beneath the descending subtropical jet, merged ∼6.5 × 1031 molecules of ozone into the free troposphere over Colorado between 8 and 10 km. These results illustrate the effectiveness of small‐scale processes for the irreversible transfer of stratospheric air into the upper troposphere.
The microphysical properties of an optically thin (τ ≤ 0.12) cirrus layer formed by dispersing contrails above Boulder, Colorado, have been examined in a ground‐based study. Backscatter and ...depolarization lidar measurements at 0.532 μm were used to characterize the cloud morphology, while near‐infrared (0.9 to 1.7 μm) spectroscopy was used to measure zenith scattered sunlight from the ice particles. The spectra are used to infer the particle effective diameter from published size and shape dependent scattering calculations. The measurements indicate growth in particle size from average effective diameters less than 20 μm in isolated contrails, to more than 40 μm as the contrails coalesced to form a 1–2 km thick cirrus layer. The retrieved size distributions are consistent with published measurements of contrail‐cirrus size distributions from in situ observations, and suggest that the lidar/near‐infrared spectroscopy combination can provide a useful method for deriving radiatively important information regarding optically thin (τ ≤ 0.1) cirrus under conditions where the single‐scattering approximation is valid.
Forecasts of Pacific jet variability are used to predict
stratosphere-to-troposphere transport (STT) and tropical-to-extratropical
moisture export (TME) during boreal spring over the Pacific–North ...American
region. A retrospective analysis first documents the regionality of STT and
TME for different Pacific jet patterns. Using these results as a guide,
Pacific jet hindcasts, based on zonal-wind forecasts from the European Centre
for Medium-Range Weather Forecasting Integrated Forecasting System, are
utilized to test whether STT and TME over specific geographic regions may be
predictable for subseasonal forecast leads (3–6 weeks ahead of time). Large
anomalies in STT to the mid-troposphere over the North Pacific, TME to the
west coast of the United States, and TME over Japan are found to have the best
potential for subseasonal predictability using upper-level wind forecasts. STT
to the planetary boundary layer over the intermountain west of the United
States is also potentially predictable for subseasonal leads but likely only
in the context of shifts in the probability of extreme events. While STT and
TME forecasts match verifications quite well in terms of spatial structure and
anomaly sign, the number of anomalous transport days is underestimated
compared to observations. The underestimation of the number of anomalous
transport days exhibits a strong seasonal cycle, which becomes steadily worse
as spring progresses into summer.
A five‐year time series of lidar profiles from a site in Colorado is used to investigate the sources and scales of variability for ozone in the free troposphere. Analysis of 475 daily mean profiles ...derived from more than 25,000 individual profiles shows that ∼66% of the total variance is due to fluctuations on time scales from 1 day to 1 month, ∼30% on time scales from 1 month to 1 year, and ∼4% on time scales greater than 1 year. The analysis also identifies low‐frequency fluctuations correlated with the El Niño/Southern Oscillation (ENSO) (following a ∼6‐month lag) that may account for as much as 5% of the total variance in the data. The ENSO‐related changes can also lead to apparent ozone trends that range from −0.2 to +0.5 ppbv/yr for different 10‐year intervals between 1970 and 1990.
A theoretical approach is used to quantify the information available to retrieve cloud physical properties from data taken by a ground‐based spectrometer measuring scattered sunlight in the ...near‐infrared wavelength region. Three wavelength regions between 0.9 and 1.7 μm, each containing water vapor, liquid, and ice absorption features, are examined using a differential optical absorption spectroscopy optimal estimation retrieval technique. Cloud properties that can be retrieved include path‐integrated liquid water path and path‐integrated ice water path (PLWP and PIWP), cloud liquid and ice temperatures, and the second moment of the photon path distribution. The accuracy of these cloud property retrievals is estimated for a variety of simulated conditions, with key analysis assumptions identified. The sensitivity of the measurements in the longest wavelength region to liquid water and ice is high, allowing for accurate estimates of PLWP and PIWP under optically thin clouds, while the shorter two wavelength bands provide more information under optically thicker clouds. Observations of mixed‐phase clouds over Barrow, Alaska, are used to illustrate the practicality of the technique. Retrieved LWP values (inferred from PLWP) are compared to LWP estimates from a microwave radiometer and an atmospheric emitted radiance interferometer; PIWP estimates are compared to IWP estimates from a millimeter‐wave cloud radar. Cloud liquid temperature and photon path distribution information retrieved from these data are also presented. Furthermore, we suggest a technique for combining near‐infrared spectral PLWP measurements with microwave radiometer observations to estimate cloud droplet effective radius.
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