The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites that provide long-term measurements of climate-relevant properties, mobile ground- and ship-based facilities to conduct shorter field campaigns (6–12 months), and the ARM Aerial Facility (AAF). The airborne observations acquired by the AAF enhance the surface-based ARM measurements by providing high-resolution in situ measurements for process understanding, retrieval-algorithm development, and model evaluation that are not possible using surface-or satellite-based techniques.

Several ARM aerial efforts were consolidated to form AAF in 2006. With the exception of a small aircraft used for routine measurements of aerosols and carbon cycle gases, AAF at the time had no dedicated aircraft and only a small number of instruments at its disposal. AAF successfully carried out several missions contracting with organizations and investigators who provided their research aircraft and instrumentation. In 2009, AAF started managing operations of the Battelle-owned Gulfstream I (G-1) large twin-turboprop research aircraft. Furthermore, the American Recovery and Reinvestment Act of 2009 provided funding for the procurement of over twenty new instruments to be used aboard the G-1 and AAF contracted aircraft. Depending on the requested scope, AAF now executes campaigns using the G-1 or contracted aircraft, producing freely available datasets for studying gas, aerosol, cloud, and radiative processes and their interactions in the atmosphere. AAF is also engaged in the maturation and testing of newly developed airborne sensors to help foster the next generation of airborne instruments.

The Aerial Facility of the DOE Atmospheric Radiation Measurement program has produced climate-relevant datasets from a large number of field campaigns and recently upgraded its capabilities to measure gases, aerosols, and clouds.

The scientific community has long recognized that observing Earth's climate system requires the combination of surface-based, airborne, and space-borne measurements (e.g., Anderson et al. 2005). The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites in three important climatic regimes that provide long-term measurements of climate-relevant properties (Mather and Voyles 2013). ARM also operates mobile ground- and ship-based facilities to conduct shorter field campaigns (6–12 months) to investigate understudied climate regimes around the globe (e.g., Miller and Slingo 2007).

Airborne observations by ARM's Aerial Facility (AAF) enhance the surface-based ARM measurements by providing vertical and horizontal context for surface based measurements, validation of remote sensing measurements made from space or the surface, and information not currently accessible by surface- or space-based remote sensing methods (e.g., detailed cloud microphysics, composition, and morphology of aerosols).

HISTORY OF ARM AERIAL MEASUREMENTS.

Almost since its inception, ARM has used aircraft to enhance its surface-based measurements (which began in 1992 in Oklahoma). The separately funded ARM Unmanned Aerospace Vehicle (UAV) program (Stephens et al. 2000) carried out 12 missions between 1993 and 2006 relying on UAVs and piloted aircraft. Examples are the ARM Enhanced Shortwave Experiment (ARESE; e.g., Valero et al. 1997), the Mixed-Phase Arctic Cloud Experiment (MPACE; e.g., McFarquhar et al. 2007b), and the Tropical Warm Pool–International Cloud Experiment (TWP-ICE; e.g., McFarquhar et al. 2007a). ARM has also funded aircraft campaigns outside the UAV program such as the Aerosol Intensive Observation Period (AIOP; Ferrare et al. 2006a) and the Aerosol Lidar Validation Experiment (ALIVE; e.g., Schmid et al. 2009). Additionally, ARM started twice-per-week routine flights with a small aircraft measuring vertical profiles of aerosol optical properties and carbon cycle gases in 2000 and 2002, respectively (see discussion below).

THE ARM AERIAL FACILITY.

Approach.

The AAF (initially named ARM Aerial Vehicle Program) was formally established in October 2006 with the mandate of executing all ARM aircraft campaigns under one organizational umbrella. As with any of the ARM facilities, the use of the AAF is awarded to a principal investigator free of charge through a competitive proposal process following the requirements laid out by the ARM program (see Mather and Voyles 2013). Choice of aircraft and instrumentation are driven by scientific requirements and all resulting data are available free of charge through the ARM archive.

AAF tasks include identifying the aircraft, instrumentation, investigators, and airport best suited to meet the scientific requirements within the logistical constraints. AAF then places contracts with the operators of the aircraft, the institutions of the investigators, and fixed base operators for hangar and office space. Working with the principal investigator for a given mission, AAF develops the flight plans that best meet the scientific requirements and obtains approvals from civilian and military aviation authorities and other stakeholders. This includes interagency coordination for joint campaigns and particularly for coordinated flights. AAF also prepares all documentation for the required safety reviews. During a mission, AAF coordinates all operational and logistical aspects. After conclusion of a mission, AAF coordinates the data archival.

With the exception of the small aircraft used for the routine aerosol and carbon cycle gases flights, AAF did not initially have a dedicated aircraft. Also only a small number of aircraft instruments (inherited from the ARM UAV program) were owned by AAF. In this mode, AAF successfully carried out several missions working with organizations and investigators who provided their research aircraft and most of the instrumentation.

In October 2009, AAF started managing and executing operations of the Battelle Memorial Institute–owned Grumman G-159 Gulfstream I (G-1) research aircraft. Battelle acquired the twin-turboprop G-1 aircraft in 1988 to be converted and operated for atmospheric research by the Pacific Northwest National Laboratory (PNNL) in Richland, Washington (see Fig. 1). Between 1988 and 2009, with base funding from the U.S. Department of Energy (DOE) Atmospheric Chemistry and later the Atmospheric Science Programs, the G-1 carried out numerous domestic and international campaigns (e.g., Berg et al. 2009; Kleinman et al. 2009, 2012).

Fig. 1.

Front view of DOE/Battelle G-1 aircraft. Wing and nose pylons are fully populated with a total of eight cloud and aerosol probes.

Fig. 1.

Front view of DOE/Battelle G-1 aircraft. Wing and nose pylons are fully populated with a total of eight cloud and aerosol probes.

In 2010, the Atmospheric Science Program and the science component of ARM merged into the current Atmospheric System Research program. The broader focus of the new Atmospheric System Research program and the fact that ARM (and hence AAF) is serving an even broader science community required broadening the research focus of the G-1 from aerosol and trace gas measurements to include the measurement of cloud microphysical properties and radiative properties of the atmosphere. To facilitate this enhanced scope, the G-1 has been upgraded with wing pylons to carry six additional probes, new generators and invertors that provide increased payload power, more fuel-efficient engines that increase range and maximum altitude, extended range fuel tanks that allow ferrying to any location on the globe, and a retrofitted center roof hatch modified to carry radiometers. The current capabilities of the G-1 are listed in Table 1.

Table 1.

Current capabilities of the Battelle/DOE G-1 research aircraft

Current capabilities of the Battelle/DOE G-1 research aircraft
Current capabilities of the Battelle/DOE G-1 research aircraft

Instrumentation.

The instrumentation currently available through the AAF for use on the G-1 and other aircraft is listed in Table 2. The list includes instruments from the former ARM UAV and G-1 programs with significant new additions (21 instruments) funded by the American Recovery and Reinvestment Act of 2009. Over 50 AAF instruments are currently available to make airborne measurements of aircraft and atmospheric state parameters, aerosol and cloud properties, trace gas concentrations, and radiation. The AAF instruments include a comprehensive suite of aerosol and cloud probes measuring aerosol size distributions with diameters from 15 nm to 50 μm and cloud and precipitation particles size distributions from 2 μm to 2 cm.

Table 2.

AAF baseline measurements and instrumentation.

AAF baseline measurements and instrumentation.
AAF baseline measurements and instrumentation.
Table 2.

Continued.

Continued.
Continued.

A composite aerosol particle size distribution using three wing-mounted aerosol probes is shown in Fig. 2. The distribution measured off the coast of Cape Cod, Massachusetts, close to the ocean surface at an altitude of 160 m MSL, shows a significant amount of coarse mode sea salt aerosol. In the overlap region of the three probes (D = 0.5–2 μm), the size distributions show ambiguity due to instrumental artifacts. We are working on a product that will “harmonize” the size distributions based on our knowledge of the performance of each instrument.

Fig. 2.

Example composite aerosol particle size distribution using three wing-mounted aerosol probes during TCAP (1522–1532 UTC 25 Feb 2013; altitude of 160 m MSL) off the coast of Cape Cod, Massachusetts.

Fig. 2.

Example composite aerosol particle size distribution using three wing-mounted aerosol probes during TCAP (1522–1532 UTC 25 Feb 2013; altitude of 160 m MSL) off the coast of Cape Cod, Massachusetts.

A composite cloud particle size distribution using four wing-mounted cloud probes is shown in Fig. 3. Shown is an average over an entire flight over the Sierra Nevada in California on 2 March 2011. The distribution shows a water cloud mode (D ≈ 10 μm), an ice cloud mode (D ≈ 50 μm), and a precipitation mode (rain and snow) with D ≈ 2–20 mm. The first bin of the cloud-imaging probe (CIP) and the first three size bins of the two-dimensional stereo probe (2DS) should be ignored because of instrumental artifacts.

Fig. 3.

Example composite cloud particle size distribution using four wing-mounted cloud probes during CalWater. Shown is an average over an entire flight carried out on 2 Mar 2011 (1700–1953 UTC). The distribution shows a water cloud mode (D ≈ 10 μm), an ice cloud mode (D ≈ 50 μm), and a precipitation mode (rain and snow) with D ≈ 2–20 mm. The first bin of the CIP and the first three size bins of the 2DS should be ignored because of instrumental artifacts.

Fig. 3.

Example composite cloud particle size distribution using four wing-mounted cloud probes during CalWater. Shown is an average over an entire flight carried out on 2 Mar 2011 (1700–1953 UTC). The distribution shows a water cloud mode (D ≈ 10 μm), an ice cloud mode (D ≈ 50 μm), and a precipitation mode (rain and snow) with D ≈ 2–20 mm. The first bin of the CIP and the first three size bins of the 2DS should be ignored because of instrumental artifacts.

Instruments mounted inside the aircraft cabin are supplied with sample air using three different kinds of inlets: rear-facing inlets for gas-phase measurements, an isokinetic inlet for aerosols up to D = 5 μm, and a counterflow virtual impactor (CVI) inlet admitting cloud particles (D = 11–50 μm) that are subsequently dried to provide a sample stream of residual aerosols. Our recently procured instruments and our older instrumentation are continuously being upgraded, whenever possible, to remain state of the art. As an example, each of the cloud probes is now equipped with knife-edge tips (see Figs. 4, 5) designed to reduce shattering of ice crystals and droplets into the sample volume, a problem that has plagued the community for decades (Korolev et al. 2011, 2013; Lawson 2011). Another example is the humidigraph system designed, built, and tested by PNNL that delivers measurements of aerosol hygroscopicity with unprecedented fidelity (see Pekour et al. 2013). Also part of the AAF instrument suite is a rather extensive set of trace gas measurements (CO, CO2, CH4, NO, NO2, NOy, N2O, SO2, O3, and H2O) using various detection methods. A cavity ring-down system is measuring concentrations of CO2, CH4, and H2O. The accuracy and precision of the CO2 measurements the system provides were characterized recently in a ground-based setting by Flowers et al. (2012).

Fig. 4.

Cloud probes mounted on right wing of G-1. 2DS (top front), High Volume Precipitation Spectrometer, version 3 (HVPS-3; top rear), and CIP [bottom; upper part of Cloud Aerosol and Precipitation Spectrometer (CAPS)], all equipped with knife-edge tips to reduce shattering of cloud particles into the sample volume.

Fig. 4.

Cloud probes mounted on right wing of G-1. 2DS (top front), High Volume Precipitation Spectrometer, version 3 (HVPS-3; top rear), and CIP [bottom; upper part of Cloud Aerosol and Precipitation Spectrometer (CAPS)], all equipped with knife-edge tips to reduce shattering of cloud particles into the sample volume.

Fig. 5.

Cloud Droplet Probe, version 2 (CDP-2) with knife-edge tips to reduce shattering of cloud particles into the sample volume.

Fig. 5.

Cloud Droplet Probe, version 2 (CDP-2) with knife-edge tips to reduce shattering of cloud particles into the sample volume.

The extensive suite of AAF instruments means that AAF staff and associated scientists have to operate, calibrate, and maintain numerous instruments simultaneously. Therefore, highly labor-intensive instruments (such as mass spectrometers) are typically not acquired by AAF but are provided by guest investigators and the required “care and feeding” is provided by the guest investigators' deployment teams. A list of recent guest instruments is shown in Table 3.

In 2008, AAF sponsored a workshop discussing airborne instrumentation needs for climate and atmospheric research, issued a call for proposals, and subsequently funded five instrument maturation efforts (see McFarquhar et al. 2011b). One of these efforts supported the completion and testing of world's first airborne Spectrometer for Sun Tracking and Sky-Scanning Atmospheric Research (4STAR; see Dunagan et al. 2013; Kassianov et al. 2012a). Other completed maturation efforts are the further development of the Holographic Detector for Clouds 2 (HOLODEC 2; see Lu et al. 2012) and a calibration and performance assessment of the airborne Cloud Extinction Probe (see Korolev et al. 2014).

Table 3.

AAF measurements provided by recently used guest instruments.

AAF measurements provided by recently used guest instruments.
AAF measurements provided by recently used guest instruments.

AAF has also worked closely with several investigators funded by the Small Business Innovation Research program. Most recently the capabilities of a newly developed fast in situ airborne ammonia spectrometer (Leen et al. 2013) have been successfully demonstrated during dedicated test flights aboard the G-1.

Data infrastructure.

With the exception of the routine-flying small-aircraft campaign, AAF campaigns are episodic and require a data infrastructure that is different from the one employed for ARM routine measurements (see Mather and Voyles 2013). Data from many of the aircraft research instruments are usually collected by an onboard central data acquisition/ visualization system. The remaining instruments (typically the guest instruments) have their own data acquisition systems. On the G-1 (and similarly for other aircraft) the clocks of all the instruments are synchronized using an onboard time server. On the G-1 aircraft four onboard scientists monitor the instruments using displays attached directly, using the display of the central data system or using netbook computers that connect to the central data system and many instruments using an onboard Ethernet connection. This setup allows for initial quality control of the measurements and corrective actions in the air if needed. A satellite downlink streams a limited number of variables to a ground station and also allows interaction with researchers on the ground via a chat window. After landing, data are transferred to a local data share drive and are also uploaded to the ARM data share system. Automated quick looks are produced and disseminated through the ARM wiki site.

Preliminary data are available to campaign participants and approved collaborators. AAF also produces a quality-assured merged dataset of the aircraft and atmospheric state parameters following the format adopted by the Interagency Working Group for Airborne Data and Telemetry Systems (Webster and Freudinger 2009). The final quality-assured data are formatted in the International Consortium for Atmospheric Research on Transport and Transformation standard, which contains a common header section of metadata within each ASCII formatted data file. The data are uploaded to the ARM archive within 6–12 months after the campaign where they are freely available to the scientific community.

CAMPAIGNS.

In the following we describe campaigns carried out by AAF grouped by their scientific focus (i.e., aerosols, clouds, aerosol–cloud interactions, and carbon cycle gases).

Aerosols.

IAP.

Between March 2000 and June 2005, ARM operated a Cessna 172 from Ponca City, Oklahoma, that made routine airborne measurements of aerosol optical properties over the Southern Great Plains (SGP) site. Initially, measurements of this in situ aerosol profile (IAP) campaign were limited to scattering and absorption coefficients of dried aerosols with particle diameter D less than 1 μm (Andrews et al. 2004). Stepped profiles at nine altitudes up to ~3,700 m were flown twice a week.

In 2005 IAP was migrated to a more capable aircraft, a Cessna 206 (see Fig. 6). Gradually the number of altitudes was increased to 12 and extended to ~5,300 m. At the same time the Cessna 206 was equipped with an improved inlet allowing sampling of supermicron aerosols with diameters D < 7 μm (Andrews et al. 2011a). At the end of 2007, IAP was discontinued after 8 years of successful operation. The aerosol instruments were removed from the payload but the routine flights were continued to support the ongoing ARM Airborne Carbon Measurements (ACME) campaign (see section on carbon cycle gases).

Fig. 6.

DOE/Greenwood Cessna 206 as used in the ongoing ACME campaign.

Fig. 6.

DOE/Greenwood Cessna 206 as used in the ongoing ACME campaign.

Detailed findings from the 597-flights, 8-yr record of aerosol optical measurements have been presented by Andrews et al. (2011a). Validity of the measurements has been tested with interplatform comparisons and closure studies (Andrews et al. 2004, 2006; Hallar et al. 2006; Ferrare et al. 2006b; Kassianov et al. 2007; Schmid et al. 2009). The data record has also been used for the evaluation of modeled black carbon profiles (Skeie et al. 2011). A complete list of publications originating from the IAP campaign is shown in Table 4.

Table 4.

Campaigns carried out by AAF.

Campaigns carried out by AAF.
Campaigns carried out by AAF.
Table 4.

Continued.

Continued.
Continued.
Table 4.

Continued.

Continued.
Continued.

CARES.

The Carbonaceous Aerosols and Radiative Effects Study (CARES) aimed to study emissions of urban and the production of natural (biogenic) aerosols, their transformation and possible interactions between them, and the resulting radiative forcing (Zaveri et al. 2012). CARES was carried out in June 2010 in and around the central valley of California. The evolution of the Sacramento, California, urban plume was studied with two strategically located, heavily instrumented ground sites and with the G-1 aircraft performing flights along and across the plume twice a day on selected days. The National Aeronautics and Space Administration (NASA) B-200 aircraft flew at a higher altitude in close coordination with the G-1 measuring vertical profiles of aerosol optical properties along the G-1 flight track using a High Spectral Resolution Lidar (HSRL). CARES and the partially overlapping National Oceanic and Atmospheric Administration (NOAA) California Research at the Nexus of Air Quality and Climate Change (CalNex; Ryerson et al. 2013) campaign also carried out some joint operations: the G-1 overflew the R/V Atlantis and flew coordinated flights with the NOAA WP-3 and Twin Otter aircraft.

Zaveri et al. (2012) provide a comprehensive overview of the measurements carried out during CARES offering early insights by contrasting and comparing numerous datasets obtained from the surface and aloft. Some of the significant results are as follows:

  • Fast et al. (2012) have used data from the G-1 and the B-200 aircraft and the two ground sites to assess the capability of the Weather Research and Forecasting Model (WRF-Chem) to predict transport and mixing patterns of trace gases and aerosols in the Sacramento urban plume.

  • Using a high-resolution aerosol mass spectrometer, a proton transfer reaction mass spectrometer, and trace gas detectors aboard the G-1, Shilling et al. (2013) found that production of organic aerosol was enhanced when anthropogenic emissions from Sacramento mixed with isoprene-rich air from the foothills.

  • Measuring single-particle mixing state with the Aircraft Aerosol Time-of-Flight Mass Spectrometer, Cahill et al. (2012) found that the large majority of all measured carbonaceous particles were internally mixed with secondary species, mainly nitrate (in CalNex) and nitrate and sulfate (in CARES).

  • Laskin et al. (2012) performed offline microscopy of aerosol particles collected aboard the G-1 during CARES. They found substantial chloride depletion in aged sea salt particles, postulating that “chloride components in sea salt particles may effectively react with organic acids releasing HCl gas to the atmosphere” (Laskin et al. 2012, p. 1).

  • Kassianov et al. (2012b) have used G-1 and ground-based data to demonstrate the large impact of coarse mode aerosols on aerosol radiative forcing during CARES.

  • Cazorla et al. (2013) used aircraft and groundbased data from CARES, CalNex, and CalWater to relate aerosol absorption due to soot, organic carbon, and dust to emission sources.

  • B-200 HSRL data from CARES were used in a multicampaign aerosol classification study by Burton et al. (2012). Ottaviani et al. (2012) used data from the NASA Research Scanning Polarimeter aboard the B-200 to characterize the polarized reflectance of snow fields in the high Sierra.

TCAP

The Two-Column Aerosol Project (TCAP) aimed to characterize two columns of aerosol and cloud properties off the U.S. East Coast in two different seasons. The first column coincided with the location of the ARM Mobile Facility on Cape Cod (deployed for a 12-month period that started in July 2012). A second column of air was sampled about 150 km offshore with the G-1 aircraft during two intensive operation periods in July 2012 and February 2013, which climatologically are the months with maximum and minimum aerosol loading.

During the first intensive operation period the B-200 aircraft, carrying for the first time NASA's second generation airborne HSRL-2, flew at higher altitude in close coordination with the G-1 providing vertical profiles of aerosol optical properties along the G-1 flight track. TCAP also marked the first deployment ever of an airborne spectrometer capable of automated sun-tracking and sky-scanning measurements (4STAR; Dunagan et al. 2013). Shinozuka et al. (2013) performed radiative closure analyses between 4STAR and in situ aerosol measurements on the G-1 and also compared aerosol extinction and optical depth derived from 4STAR and HSRL-2.

Clouds.

CLASIC

The purpose of the Cloud and Land Surface Interaction Campaign (CLASIC) was to advance our understanding of cumulus convection and its controls, particularly those associated with land surface processes (Miller et al. 2007). CLASIC was carried out during June 2007 over SGP when large changes in the land surface (winter wheat harvest) lead to large changes in the surface albedo, latent heat flux, and sensible heat flux. Remote sensing measurements were carried out from the NASA ER-2 and P3, the Sky Research Jet Stream, and the International Twin Otter. In situ measurements of clouds, aerosols, and gases were carried out aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter, the Duke University Bell 206 helicopter, and the aforementioned Cessna 206 (which performed special flight patterns in support of CLASIC). CLASIC ran concurrently and collocated with the DOE ASP Cumulus Humilis Aerosol Processing Study (CHAPS; Berg et al. 2009), which brought the DOE G-1 and the NASA B-200 aircraft to SGP. Operationally, this required AAF to coordinate a total of up to nine research aircraft on an almost daily basis (see Table 4).

Unfortunately “the SGP experienced extremely high precipitation in June (and May) 2007, with the result that soil moisture saturation or near saturation prevailed across the SGP for much of CLASIC” (Lamb et al. 2012, p. 1736), making it impossible to fully address the science goals of CLASIC. However several papers have been published on retrieving surface properties with remote sensing instruments aboard the ER-2, the Jet Stream, and the International Twin Otter (Bindlish et al. 2009; Gatebe et al. 2010; Román et al. 2011). Two publications discuss retrievals of aerosol optical depth in the vicinity of broken clouds (Kassianov et al. 2009, 2010), combining data from CHAPS and CLASIC aircraft. This large dataset, however, has great potential to be used in future research.

RACORO

This routine-flying cloud campaign aimed to obtain an extended-term statistical characterization of continental, boundary layer, optically thin liquid water clouds (Vogelmann et al. 2012). The Routine AAF Clouds Optical Radiative Observations (RACORO) operated for over 5 months, from 22 January to 30 June 2009, over the SGP with the CIRPAS Twin Otter logging 260 research hours during 59 flights. The Twin Otter carried out comprehensive measurements of cloud, aerosol, radiation, and atmospheric state parameters. Building on AAF's experience with its small-aircraft routine-flying campaign described earlier, only robust instrumentation that could be operated fairly autonomously was used for RACORO.

Vogelmann et al. (2012) provide a comprehensive overview of the approach used and datasets obtained during RACORO. Lu et al. (2012a,b, 2013) have used RACORO data to analyze the impacts of vertical velocity on cloud microphysics, to estimate lateral entrainment rates in shallow cumuli, and to establish a relationship between cloud microphysics and entrainment rates. De Boer et al. (2013) used aircraft data from RACORO, AIOP and an ASP campaign to evaluate aerosol–cloud interaction in a global climate model. Additional publications from RACORO to date include a methodology paper for correcting for tilt from horizontal in downwelling shortwave irradiance measurements on moving platforms (Long et al. 2010). Small et al. (2011) compare an automated decision algorithm with the heuristic decision approach actually used to make go–no-go decisions during RACORO.

The B-200 aircraft carrying HSRL joined RACORO for the last month of the mission in June 2009. HSRL data obtained in RACORO contributed to an assessment of the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar attenuated backscatter profiles (Rogers et al. 2011) and to an aerosol classification obtained from 18 field campaigns that have been conducted with the HSRL over North America since 2006 (Burton et al. 2012).

SPartICus

During the Small Particles in Cirrus (SPartICus) routine-flying campaign (January–June 2010), the Stratton Park Engineering Company (SPEC) Incorporated Learjet 25 research aircraft flew approximately 200 hours devoted to measurements in ice clouds. The flights focused on cirrus clouds over the ARM SGP site as well as under the A-Train satellite constellation. The Learjet carried state-of-the art instrumentation (which included cloud probes with the above-mentioned knife-edge tips) to measure crystal size distributions and morphology, water vapor, and atmospheric state parameters. Cooper and Garrett (2011) and Deng et al. (2013) have compared these in situ measurements with several A-Train ice cloud retrieval products. Lawson (2011) has evaluated two techniques to reduce the errors in measurements of ice particle size distributions due to shattering at the inlets and tips of cloud particle probes. Liu et al. (2012) and Zhang et al. (2013) have used ice crystal number concentrations measured during SPartICus to test various parameterizations of ice nucleation mechanisms used in a global climate model. Mitchell et al. (2011a) have used data obtained in cirrus clouds from several campaigns, including ISDAC and SPartICus, to challenge the assumption used extensively in atmospheric radiation transfer, climate modeling, and remote sensing that particle size distribution optical properties can be uniquely described in terms of their effective diameter and their cloud water content.

Cloud–aerosol interactions.

ISDAC

The Indirect and Semi-Direct Aerosol Campaign (ISDAC) was carried out in April over Alaska during the International Polar Year (2008) and ran concurrently with the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS; Jacob et al. 2010) and the Aerosol, Radiation and Cloud Processes Affecting Arctic Climate (ARCPAC; Brock et al. 2011) campaigns organized by NASA and NOAA, respectively.

ISDAC aimed to study the influence of aerosols on cloud properties and the associated radiative forcing in the Arctic. The National Research Council of Canada Convair-580 collected data using over 40 cloud and aerosol instruments during more than 100 flight hours anchored over the ARM North Slope of Alaska site near Barrow. Coordinated flights were carried out with NASA's B-200 and NOAA's WP-3 aircraft.

To date, ISDAC aircraft data have been used in 19 publications (see Table 4). An overview of ISDAC that also included surface-based efforts has been presented by McFarquhar et al. (2011a). The more polluted conditions expected and found during ISDAC in April 2008 nicely contrasted the relatively pristine conditions observed during MPACE carried out in October 2004. As a result, numerous authors chose to contrast results from both campaigns in their work (Fan et al. 2011; Larson et al. 2011; Liu et al. 2011; Jackson et al. 2012; McFarquhar et al. 2013).

After initial test flights on the G-1, the single-particle mass spectrometer SPLAT II saw its first airborne deployment aboard the CV-580 during ISDAC, simultaneously measuring particle number concentrations, density, asphericity, and individual particle size and quantitative composition at a time resolution of 60 seconds (Zelenyuk et al. 2010; Vaden et al. 2011a,b). Shantz et al. (2012) studied the spatial and temporal variability of aerosol particles in ISDAC and put them in context with earlier campaigns carried out in the Arctic spring. Earle et al. (2011) used ISDAC data to investigate factors influencing the microphysics and radiative properties of liquid-dominated Arctic clouds, whereas Jouan et al. (2012) characterized the properties of observed ice clouds. Jackson et al. (2012) used data from MPACE and ISDAC to test three aerosol indirect effects hypothesized to act in mixed-phase clouds: the riming indirect effect, the glaciation indirect effect, and the thermodynamic indirect effect. McFarquhar et al. (2013) tested how the shapes of small particles varied between liquid-and ice-dominated mixed-phase clouds.

Numerous studies used ISDAC data to constrain cloud-resolving models for testing of hypotheses related to processes in mixed-phase cloud layers (Avramov et al. 2011; Botta et al. 2011; Fan et al. 2011; Ovchinnikov et al. 2011; Solomon et al. 2011). Larson et al. (2011), Liu et al. (2011), and Mitchell et al. (2011a,b) developed and tested various parameterizations for global climate models pertaining to mixed-phase and ice clouds using data from ISDAC and other campaigns.

CalWater

Precipitation in the western United States is significantly modulated by atmospheric rivers (i.e., narrow bands of enhanced water vapor; see, e.g., Dettinger et al. 2011) and perhaps by aerosols (e.g., Rosenfeld et al. 2008). Funded by the California Energy Commission, the G-1 aircraft was deployed to Sacramento to support the 2011 intensive observing period of the CalWater field campaign. The G-1 was equipped with state-of-the-art instruments for sampling cloud and aerosol properties including many of the AAF instruments acquired with Recovery Act funds (see instrumentation section). From 1 February to 7 March 2011 the G-1 flew 28 research flights (70 hours) over 33 days covering geographic regions in Northern California including the marine boundary layer and Coastal Mountains north of San Francisco, aerosol surveys in the central valley and over urban Sacramento, and orographic convective clouds over the Sierra Nevada and the foothills.

Data collected aboard the G-1 and from the surface during CalWater led Creamean et al. (2013, p. 1572) to conclude “that Saharan and Asian dust and biological aerosols probably serve as ice nuclei and play an important role in orographic precipitation processes over the western United States.” Rosenfeld et al. (2014, p. 112) found that “cloud condensation and ice nuclei were the limiting factors that controlled warm rain and ice processes, respectively, while the unpolluted clouds in the same air mass produced precipitation quite efficiently.” G-1 measurements during CalWater also provided new evidence that supercooled drizzle or rain can persist in clouds as cold as −21°C (Rosenfeld et al. 2013). Fan et al. (2014, p. 81) constrained WRF model simulations with aircraft data to demonstrate “the importance of the interactions between local pollution, dust, and environmental conditions for assessing aerosol effects on cold-season precipitation in California.” G-1 CalWater data were also used in the study by Cazorla et al. (2013) mentioned in the CARES section. Numerous other publications are in preparation.

Carbon cycle gases.

ACME

Airborne carbon measurements at SGP started in 2002 when a flask sampler was added to the Cessna 172 used for the IAP campaign discussed earlier. A pair of flasks (2 L each) was collected at a given altitude per flight, either near the middle of the planetary boundary layer (~600 m) or in the free troposphere (~3,000 m), and then analyzed in Boulder by the NOAA Carbon Cycle Greenhouse Gases group for CO2, CH4, CO, H2, N2O, and SF6 and by the University of Colorado Institute of Arctic and Alpine Research for many volatile organic compounds (VOCs) such as acetylene (C2H2) and propane (C3H8). In 2005 the instruments were migrated to the more capable Cessna 206 and in 2006, and the 2-flask sampler was upgraded to a 12-flask sampler allowing sampling at 12 heights up to ~5,300 m. In 2007, a continuous CO2 analyzer was added making these the only ongoing, continuous CO2 profile observations in the United States.

ACME was formally established in 2008 after the IAP campaign had ended. In 2010, a second continuous CO2 analyzer was added to assess the quality of the measurements collected. This new approach brought the ACME campaign one step closer to extending the World Meteorological Organization comparability goal of ~0.1 ppm from laboratory and ground-based settings to airborne observations (see Biraud et al. 2013).

A review of the 10-yr record of carbon cycle gases above SGP has been presented by Biraud et al. (2013). The measurements have been used to validate ground- and satellite-based column measurements of CO2 (Wunch et al. 2010, 2011; Kulawik et al. 2010, 2013; Kuai et al. 2013; Inoue et al. 2013, Miyamoto et al. 2013), airborne lidar measurements of CO2 profiles (Abshire et al. 2010), an Earth system model (Keppel-Aleks et al. 2013), and to estimate CH4 sources (Bergamaschi et al. 2013; Miller et al. 2013). The ACME campaign is still continuing and the current payload of the Cessna 206 is shown in Table 5.

Table 5.

Measurements made currently aboard the DOE Cessna 206 in the ongoing ACME campaign.

Measurements made currently aboard the DOE Cessna 206 in the ongoing ACME campaign.
Measurements made currently aboard the DOE Cessna 206 in the ongoing ACME campaign.

Current and planned future campaigns

The Biomass Burn Observation Project (BBOP) aims to quantify the time evolution of microphysical, morphological, chemical, hygroscopic, and optical properties of aerosols generated by biomass burning. The G-1 aircraft sampled biomass burning plumes first by operating from its home location in Pasco, Washington, from July to September 2013 and then by deploying to Memphis, Tennessee, in October 2013.

The Green Ocean Amazon Aerial Campaign (GoAmazon) aims to study how aerosol and cloud life cycles, including cloud–aerosol–precipitation interactions, are influenced by pollutant outflow from a tropical megacity. The ARM Mobile Facility will be located downwind of the city of Manaus, Brazil (3°6′47″S, 60°1′31″W), for a 2-yr period starting in January 2014. The G-1 will deploy to Manaus twice in 2014 to sample the Manaus plume and surrounding areas.

The ARM Cloud Aerosol Precipitation Experiment (ACAPEX) will take place in California from January to March 2015 in conjunction with the NOAA CalWater 2 campaign. The ARM Mobile Facility will be deployed on a research vessel offshore and the G-1 aircraft will probe the clouds that form over the ocean and their transformations upon landfall as well as the orographic effects over the coastal range and the Sierra Nevada.

SUMMARY.

Since 2006 AAF has carried out nine aircraft campaigns enabling research on aerosols, clouds, aerosol–cloud interactions, and trace gases.

During this period AAF has produced numerous scientific and logistical “firsts” including the 8-yr in situ aerosol optical properties record (IAP), the 11-yr carbon cycle gas profile record (ACME), two 6-month airborne cloud sampling campaigns (RACORO and SPartICus), and the first deployment ever of an airborne spectrometer capable of automated sun-tracking and sky-scanning measurements (4STAR).

While an imperfect measure of scientific impact, it is nonetheless worthy to point out that, to date, over 70 peer-reviewed journal publications have used airborne datasets from AAF campaigns (see Table 4). This number undoubtedly will continue to grow as datasets continue to be used in publications many years after the conclusion of a campaign. An example is the MPACE campaign, which concluded in 2004 and has led to publications as recently as 2012. We found the number of publications resulting from a campaign tends to be a function of several variables such as quality of the dataset, advocacy for it, availability of follow-on funding, and manifestation or absence of the desired meteorological conditions during the campaign.

AAF carries out campaigns with very different characteristics. Occasionally an AAF campaign involves only a single aircraft but many campaigns include multiple aircraft and the participation of multiple agencies. In terms of duration, campaigns range from 1-month intensive observation periods to 6-month routine-flying campaigns and even include a small-aircraft routine-flying campaign that is now in its 13th year. This ARM small-aircraft routine-measurements effort at SGP has proven to be very cost effective for the following reasons: a highly automated payload that can be operated by the single pilot on board, a constant and simple flight plan, a simple payload with only incremental changes over the years, and only episodic on-site presence of scientific staff.

The AAF has matured into a facility with extensive in-house capability. At this time AAF has over 50 state-of-the-art instruments at its disposal, which typically is further augmented by leading-edge guest instrumentation based on science requirements. The G-1 has become the dedicated aircraft for AAF and has undergone extensive upgrades to carry out a broad array of campaigns. Contracted aircraft will still be used, particularly for cirrus research where a higher operating ceiling is required.

Investigator teams can apply for the use of the AAF through the annual ARM facility call for proposals. The lead time required between the acceptance of the proposal and start of AAF operations ranges from 1 year for simple domestic campaigns to 3 years for complex international campaigns. More information on this competitive process can be found online (at www.arm.gov) and in Mather and Voyles (2013).

ACKNOWLEDGMENTS

The ARM Aerial Facility is an integral part of the Department of Energy (DOE) Office of Science Atmospheric Radiation Measurement (ARM) Program. Funding for the campaigns described here has been provided by ARM, with contributions from the DOE Atmospheric Systems Research Program, NASA, and the Canadian National Research Council. The CalWater campaign has been funded by the California Energy Commission. It is the dedication of lead scientists, instrument scientists, engineers, and technicians from many agencies, institutions, and research companies; pilots; ground crew; fixed base operators; contracting officers; administrative personnel; program managers; and many others that make the AAF successful

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