The Atmospheric Radiation Measurement User Facility (ARM User Facility or ARM), a multi-laboratory U.S. Department of Energy Office of Science user facility, provides 30-plus years of atmospheric measurements, including data sets from all seven continents and five oceans, to advance the understanding of the Earth’s atmosphere.

The ARM User Facility consists of three instrumented fixed-location atmospheric observatories, as well as mobile and aerial observatories. Continuous data from these observatories, as well as data obtained through intensive field research campaigns and value-added products, are available to scientists online through the ARM Data Center.

Mission

ARM seeks to provide the atmospheric research community with strategically located atmospheric observatories designed to improve the understanding and representation of cloud, aerosol, and precipitation processes in earth system models in support of the U.S. Department of Energy’s science, energy, and national security missions.

History

ARM was established in 1989 by the U.S. Department of Energy to develop several highly instrumented ground stations. During the early years of the program, ARM focused on establishing field research sites, developing and procuring instruments, and developing techniques for both atmospheric retrievals and model evaluation. To obtain the most useful climate data, three main sites were chosen that represented a broad range of weather conditions.

Atmospheric Observatories

Each ARM observatory operates a similar set of instruments for measuring clouds, aerosols, precipitation, the surface energy balance, and fundamental meteorological parameters such as temperature, humidity, and wind. To address the need for improved characterization and model parameterizations of atmospheric processes, ARM observatories are deployed to collect data in a wide variety of meteorological regimes worldwide.

Fixed-location Observatories

The Southern Great Plains (SGP) atmospheric observatory, which has operated since 1992, was the first field measurement site established by ARM. As of December 2025, the SGP observatory consisted of instrument clusters arrayed across approximately 2,900 square miles in north-central Oklahoma, supported by a central facility near the town of Lamont. More than 1,800 journal articles using SGP data have been published, the most of any ARM observatory.

The North Slope of Alaska (NSA) atmospheric observatory provides data about cloud and radiative processes at high latitudes. This observatory includes a central facility at Utqiaġvik (formerly known as Barrow), which has operated since 1997. From 2013 to 2021, an ARM Mobile Facility operated at Oliktok Point, about 160 miles east of Utqiaġvik.

The Eastern North Atlantic (ENA) atmospheric observatory has operated since 2013 on Graciosa Island in the Azores archipelago. The ENA observatory is in a remote marine area that experiences a wide range of cloud types, with marine stratocumulus being the most frequently occurring type. In addition, the ENA area receives air masses from several different regions, including North America, the Arctic, and northern Europe.

From 1996 to 2014, ARM operated the Tropical Western Pacific (TWP), atmospheric observatory to obtain data from within the Tropical Warm Pool. There, the warmest sea surface temperatures on the planet and widespread convective clouds play a large role in the interannual variability observed in the global climate system. In fiscal year 2012, ARM was directed to develop new sites in Alaska and the Azores, which led to the eventual wind-down and conclusion of TWP operations. TWP data, which are still available from ARM, have contributed to understanding of tropical cloud systems and meteorology, evaluation and development of models, and validation and improvement of remote sensing from satellite instruments.

Mobile Observatories

To explore research questions beyond those addressed by ARM’s fixed atmospheric observatories, scientists can propose a field campaign to use one of three ARM Mobile Facilities to collect atmospheric data from under-sampled regions.

Each ARM Mobile Facility is designed to operate in any environment for campaigns typically lasting about a year. A mobile facility consists of portable shelters, a baseline suite of instruments, and communications and data systems.

Aerial Observatories

In addition to its ground-based data, ARM provides airborne measurements required to answer research questions proposed by scientists. ARM aerial measurements are usually coupled with ground-based data to enhance the value of both types of measurements. Airborne observations provide validation of ground- or space-based remote sensing data, information that cannot be obtained from ground- or satellite-based instruments, and spatial context for surface-based measurements.

ARM currently collects aerial data using a midsize (Group 3) unmanned aircraft system and tethered balloons. In 2019, ARM obtained a Bombardier Challenger 850 jet, which is being modified to host research instrumentation, to replace its retired Grumman Gulfstream I research aircraft.

Before ARM began managing operations of the Gulfstream I in 2009, it collaborated with outside agencies and institutions such as NASA and the Naval Postgraduate School’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies to obtain research data over ARM sites.

Instruments

ARM instruments measure key properties of aerosols, clouds, precipitation, radiation, and other atmospheric and meteorological variables.

Major ARM Instrument Classes and Measurements

Aerosols

ARM provides scientists with aerosol data to help them better understand aerosol processes, aerosol-cloud interactions, and aerosol impacts on the Earth’s energy balance for the purpose of evaluating and improving earth system models. ARM deploys and operates Aerosol Observing Systems to collect in situ aerosol measurements at Earth’s surface. Aerosol properties of interest include chemical composition, number concentration, scattering and absorption of energy, size distribution, and vertical profiles.

The following list describes some selected ARM instruments that provide data on aerosol properties:

  • Aerodynamic particle sizers – measure aerodynamic diameters of submicron and supermicron aerosol particles, typically in the range of 0.5–20 micrometers
  • Aerosol chemical speciation monitors – measure chemical composition and mass concentrations of non-refractory organic and inorganic submicron particles
  • Cloud condensation nuclei particle counters – measure number concentration of aerosol particles that can activate into cloud droplets at a given supersaturation
  • Condensation particle counters – provide number concentration of submicron particles; depending on instrument type, can count particles with sizes from 10 to 3,000 nanometers (fine condensation particle counter) or from 3 to 3,000 nanometers (ultrafine condensation particle counter)
  • High-spectral-resolution and Raman lidars – provide vertical profiles of aerosols and clouds
  • Micropulse lidars – used for aerosol and cloud detection
  • Nephelometers – measure light scattering by aerosols at three wavelengths
  • Optical particle counters – provide number size distribution for particles with optical diameters from 0.25 to 35 micrometers
  • Particle soot absorption photometers – measure light absorption by aerosols at three wavelengths
  • Scanning mobility particle sizers – measure mobility diameters of aerosol particles; depending on model, can measure particles with diameters from 10 to 500 nanometers (regular scanning mobility particle sizer), from approximately 10 to 800 nanometers (extended-range scanning mobility particle sizer), or from 2 to 150 nanometers (nano scanning mobility particle sizer)
  • Single-particle soot photometers – measure black carbon mass and number of individual aerosol particles; ARM operates regular single-particle soot photometer and extended-range version, which is more compact, covers a wider size range, and outputs in geophysical units
  • Ultra-high-sensitivity aerosol spectrometers (UHSAS) – measure optical diameters of aerosol particles from 60 to 1,000 nanometers

In addition, ARM operates systems to measure the atmospheric concentration of gases such as carbon monoxide, ozone, and sulfur dioxide.

Atmospheric State and Surface Fluxes

ARM collects measurements describing the physical and thermodynamic conditions of the Earth’s atmosphere. Such measurements characterize the atmosphere’s thermodynamic structure, processes that occur in the planetary boundary layer, and meteorological conditions at or near the Earth’s surface.

The following ARM instruments provide information on the atmospheric state and surface fluxes:

  • Atmospheric emitted radiance interferometers – provide profiles of tropospheric temperature and water vapor
  • Eddy correlation flux measurement systems – measure turbulent fluxes of carbon dioxide, heat, and momentum at the surface; system includes sonic anemometer, which provides fast measurements of the local three-dimensional turbulence field in combination with other instruments to obtain fluxes of the listed quantities
  • Ground infrared thermometers – measure the equivalent blackbody brightness temperature of what is in their field of view
  • Microwave radiometers – used to determine liquid water path and precipitable water vapor
  • Radar wind profilers – provide wind profile data, including wind speed and direction at various heights
  • Radiosondes – attached to weather balloons to measure profiles of humidity, temperature, pressure, and wind speed and direction
  • Scanning Doppler lidars – measure profiles of horizontal and vertical winds
  • Surface energy balance systems – estimate the total surface energy balance using net radiometer and soil sensor measurements
  • Surface meteorological instrumentation – measures barometric pressure, relative humidity, temperature, and wind speed and direction at the surface

Cloud Properties

Clouds play a critical role in the Earth’s energy balance as they help modulate the flow of energy from the sun to the Earth and back out to space.

ARM measures cloud properties using the following instruments:

  • Ceilometers – measure cloud base height, vertical visibility, and potential aerosol backscatter signals
  • Cloud radars – provide information on cloud dynamics, microphysics, and structure Scanning cloud radars – record details on the four-dimensional structure and evolution of clouds Vertically pointing (zenith) cloud radars – provide vertical profiles of clouds at millimeter wavelengths
  • High-spectral-resolution and Raman lidars – provide vertical profiles of clouds and aerosols
  • Micropulse lidars – used for cloud and aerosol detection
  • Microwave radiometers – used to determine liquid water path and precipitable water vapor
  • Sky imagers – measure cloud fraction

Precipitation

ARM collects precipitation data to help researchers better understand the water cycle and how water moves from the Earth’s atmosphere to the surface.

Using the following instruments, ARM measures when, where, and how much precipitation is falling:

  • Bulk precipitation gauges – measure the amount of liquid or solid precipitation over a specific time period Types of gagues: Tipping bucket rain gauges – collect and send precipitation into a container that tips after a specific amount of liquid enters to record how much rain has fallen Weighing bucket precipitation gauges – measure the weight of accumulated liquid or solid precipitation collected in a bucket
  • Disdrometers – measure the drop size distribution and velocity of falling hydrometeors
  • Multi-angle snowflake cameras – capture high-resolution images of hydrometeors in free fall
  • Scanning precipitation radars – allow for spatial characterization of precipitation
  • Solid particles mass flux sensors – measure the horizontal flux of blowing snow
  • Sonic ranging sounders – determine snow depth

Radiation

To improve understanding of the Earth’s energy balance, ARM measures the amount of energy from the sun (solar, or shortwave, radiation) reaching the surface and the amount of outgoing energy (infrared, or longwave, radiation) from the Earth. The planet’s energy balance is affected by interactions of incoming and outgoing radiation with the surface, atmosphere, aerosols, and clouds.

ARM collects radiation measurements with the following instruments:

  • Atmospheric emitted radiance interferometers – provide measurements of downwelling infrared radiance from the atmosphere
  • Microwave radiometers – measure microwave emissions of water vapor and liquid water molecules in the atmosphere at several discreet wavelengths in the 22–197 GHz range; data typically used to derive quantities of water vapor and liquid water
  • Multifilter radiometers – measure irradiance reflected off the surface
  • Multifilter rotating shadowband radiometers – measure components of solar irradiance in narrow wavelength bands; data typically used to derive aerosol optical properties
  • Pyranometers – measure broadband (0.3 to 3 micrometers) incoming and outgoing shortwave radiation
  • Pyrgeometers – provide measurements of broadband (3.5 to 50 micrometers) incoming and outgoing longwave radiation
  • Pyrheliometers – measure broadband (0.3 to 3 micrometers) direct beam solar irradiance
  • Sun photometers – measure direct solar irradiance and sky radiance in narrow wavelength bands at the Earth’s surface; data typically used to derive aerosol and cloud optical properties

Soil Properties

Soil measurements are important for understanding the exchange of energy and water between the Earth’s surface and atmosphere, as well as land-atmosphere interactions.

ARM instruments used for soil properties include:

  • Soil temperature and moisture profile systems – provide vertical profiles of plant water availability, soil conductivity, soil moisture, soil permittivity, and soil temperature
  • Surface energy balance systems – measure soil heat flow, moisture, and temperature

Organization

Staff at the following nine U.S. Department of Energy national laboratories work together to manage and operate ARM:

ARM’s Infrastructure Management Board manages components of the user facility on a day-to-day basis, receiving oversight from ARM’s program manager at the U.S. Department of Energy. The Infrastructure Management Board, chaired by the ARM director at Pacific Northwest National Laboratory, also includes ARM’s associate directors for operations and research, chief data and computing officer, instrument operations manager, aerial facility manager, and observatory site managers.

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