A reliable and precise in situ CO2 and CO analysis system has been developed and deployed at eight sites in the NOAA Earth System Research Laboratory's (ESRL) Global Greenhouse Gas Reference Network. The network uses very tall (> 300 m) television and radio transmitter towers that provide a convenient platform for mid-boundary-layer trace-gas sampling. Each analyzer has three sample inlets for profile sampling, and a complete vertical profile is obtained every 15 min. The instrument suite at one site has been augmented with a cavity ring-down spectrometer for measuring CO2 and CH4. The long-term stability of the systems in the field is typically better than 0.1 ppm for CO2, 6 ppb for CO, and 0.5 ppb for CH4, as determined from repeated standard gas measurements. The instrumentation is fully automated and includes sensors for measuring a variety of status parameters, such as temperatures, pressures, and flow rates, that are inputs for automated alerts and quality control algorithms. Detailed and time-dependent uncertainty estimates have been constructed for all of the gases, and the uncertainty framework could be readily adapted to other species or analysis systems. The design emphasizes use of off-theshelf parts and modularity to facilitate network operations and ease of maintenance. The systems report high-quality data with > 93% uptime. Recurrent problems and limitations of the current system are discussed along with general recommendations for high-accuracy trace-gas monitoring. The network is a key component of the North American Carbon Program and a useful model for future research-grade operational greenhouse gas monitoring efforts.

Abstract. A reliable and precise in situ CO2 and CO analysis system has been developed and deployed at eight sites in the NOAA Earth System Research Laboratory's (ESRL) Global Greenhouse Gas Reference Network. The network uses very tall (> 300 m) television and radio transmitter towers that provide a convenient platform for mid-boundary-layer trace-gas sampling. Each analyzer has three sample inlets for profile sampling, and a complete vertical profile is obtained every 15 min. The instrument suite at one site has been augmented with a cavity ring-down spectrometer for measuring CO2 and CH4. The long-term stability of the systems in the field is typically better than 0.1 ppm for CO2, 6 ppb for CO, and 0.5 ppb for CH4, as determined from repeated standard gas measurements. The instrumentation is fully automated and includes sensors for measuring a variety of status parameters, such as temperatures, pressures, and flow rates, that are inputs for automated alerts and quality control algorithms. Detailed and time-dependent uncertainty estimates have been constructed for all of the gases, and the uncertainty framework could be readily adapted to other species or analysis systems. The design emphasizes use of off-the-shelf parts and modularity to facilitate network operations and ease of maintenance. The systems report high-quality data with > 93% uptime. Recurrent problems and limitations of the current system are discussed along with general recommendations for high-accuracy trace-gas monitoring. The network is a key component of the North American Carbon Program and a useful model for future research-grade operational greenhouse gas monitoring efforts.

Atmospheric radiocarbon (14C) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of 14C in fossil fuel reservoirs. The high sensitivity and precision that accelerator mass spectrometry (AMS) affords in atmospheric 14C analysis has greatly increased the potential for using such measurements to evaluate bottom-up emissions inventories of fossil fuel CO2 (CO2ff), as well as those for other co-emitted species. Here we use observations of 14CO2 and a series of primary hydrocarbons and combustion tracers from discrete air samples collected between June 2009 and September 2010 at the National Oceanic and Atmospheric Administration Boulder Atmospheric Observatory (BAO; Lat: 40.050° N, Lon: 105.004° W) to derive emission ratios of each species with respect to CO2ff. The BAO tower is situated at the boundary of the Denver metropolitan area to the south and a large industrial and agricultural region to the north and east, making it an ideal location to study the contrasting mix of emissions from the activities in each region. The species considered in this analysis are carbon monoxide (CO), methane (CH4), acetylene (C2H2), benzene (C6H6), and C3–C5 alkanes. We estimate emissions for a subset of these species by using the Vulcan high resolution CO2ff emission data product as a reference. We find that CO is overestimated in the 2008 National Emissions Inventory (NEI08) by a factor of ~2. A close evaluation of the inventory suggests that the ratio of CO emitted per unit fuel burned from on-road gasoline vehicles is likely over-estimated by a factor of 2.5. Using a wind-directional analysis of the data, we find enhanced concentrations of CH4, relative to CO2ff, in air influenced by emissions to the north and east of the BAO tower when compared to air influenced by emissions in the Denver metro region to the south. Along with enhanced CH4, the strongest enhancements of the C3–C5 alkanes are also found in the north and east wind sector, suggesting that both the alkane and CH4 enhancements are sourced from oil and gas fields located to the northeast, though it was not possible to rule out the contribution of non oil and gas CH4 sources.

A robust in situ CO2 and CO analysis system has been developed and deployed at eight sites in the NOAA Earth System Research Laboratory's (ESRL) Tall Tower Greenhouse Gas Observing Network. The network uses very tall (> 300 m) television and radio transmitter towers that provide a convenient platform for mid-boundary layer trace gas sampling. Each analyzer has three sample inlets for profile sampling, and a complete vertical profile is obtained every 15 min. The instrument suite at one site has been augmented with a cavity ring-down spectrometer for measuring CO2 and CH4. The long-term stability of the systems in the field is typically better than 0.1 ppm for CO2, 6 ppb for CO, and 0.5 ppb for CH4, as determined from repeated standard gas measurements. The instrumentation is fully automated and includes sensors for measuring a variety of status parameters, such as temperatures, pressures and flow rates that are inputs for automated alerts and quality control algorithms. These algorithms provide detailed and time-dependent uncertainty estimates for all of the gases and could be adapted to other species or analysis systems. The design emphasizes use of off the shelf parts and modularity to facilitate network operations and ease of maintenance. The systems report high-quality data with > 93% uptime. Recurrent problems and limitations of the current system are discussed along with general recommendations for high accuracy trace-gas monitoring. The network is a key component of the North American Carbon Program and a useful model for future research-grade operational greenhouse gas monitoring efforts.

Introduction Conclusions References

An algorithm to retrieve cloud optical thickness and effective radius (reff) from spectral transmittance was applied to radiance and irradiance observations of the Solar Spectral Flux Radiometer (SSFR) during the Research at the Nexus of Air Quality and Climate Change Campaign (CalNex). Data from an overcast day, 16 May 2010, was used to validate the algorithm. Retrievals from the SSFR, deployed on the Woods Hole Oceanic Institute R/V Atlantis, were compared to retrievals made from an airborne SSFR, the Geostationary Operations Environmental Satellite (GOES), an Atlantis‐based microwave radiometer, and the Moderate Resolution Imaging Spectroradiometer. In situ observations of reffduring a flight over the Atlantis were compared to the Atlantis SSFR and GOES retrievals. The cloud statistics for the CalNex campaign were compared to previous studies. The agreement between the different retrievals, quantified by determining the number of coincident observations when retrieval uncertainty overlapped, improved as the difference between the field‐of‐views (FOV) of the instruments decreased. It is shown that averaging the 1 Hz SSFR observations to the 15 minute GOES interval cannot fully account for the impact of the different FOVs. The average in situ reff (7.7 μm) fell between the average reffretrieved using the Atlantis‐based SSFR radiance (5.7μm) and irradiance (9.5 μm). The CalNex clouds showed a diurnal pattern observed in previous studies of marine boundary layer clouds in the region. The distribution of cloud optical thickness and liquid water path during CalNex was shown to be a gamma distribution, consistent with previous studies of high cloud fraction marine boundary layer clouds.

Abstract. We introduce a new spectral method for the retrieval of optical thickness and effective radius from cloud transmittance that relies on the spectral slope of the normalized transmittance between 1565 nm and 1634 nm, and on cloud transmittance at a visible wavelength. The standard dual-wavelength technique, which is traditionally used in reflectance-based retrievals, is ill-suited for transmittance because it lacks sensitivity to effective radius, especially for optically thin clouds. Using the spectral slope rather than the transmittance itself enhances the sensitivity of transmittance observations with respect to the effective radius. This is demonstrated by applying it to the moderate spectral resolution observations from the Solar Spectral Flux Radiometer (SSFR) and Shortwave Spectroradiometer (SWS), and by examining the retrieval uncertainties of the standard and the spectral method for data from the DOE ARM Southern Great Plains (SGP) site and a NOAA ship cruise (ICEALOT). The liquid water path (LWP) is derived from the retrieved optical thickness and effective radius, based on two different assumptions about the cloud vertical profile, and compared to the simultaneous observations from a microwave radiometer. Optical thickness and effective radius is also compared to MODIS retrievals. In general, the effective radius uncertainties were much larger for the standard retrieval than for the spectral retrieval, particularly for thin clouds. When defining 2 μm as upper limit for the tolerable uncertainty of the effective radius, the standard method returned only very few valid retrievals for clouds with an optical thickness below 25. For the analyzed ICEALOT data (mean optical thickness 23), the spectral method provided valid retrievals for 84 % of the data (24 % for the standard method). For the SGP data (mean optical thickness 44), both methods provided a high return of 90 % for the spectral method and 78 % for the standard method.