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WP-3D Science Background

Map showing the area of operation for ARCPAC. The WP-3D aircraft will be based in Fairbanks, Alaska. The typical out-and-return range of the aircraft is shown by the red circle.

NOAA will undertake a airborne field experiment, the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) in Alaska in late March and April of 2008 to address the four major areas of non-greenhouse-gas atmospheric climate processes in the Arctic. A NOAA WP-3D aircraft will be used for this experiment and will be based at Fairbanks, Alaska. This experiment will be coordinated with the POLARCAT activity of the IPY, with the NOAA baseline climate research station at Barrow, Alaska, and with the intensive operations period executed at the DOE-sponsored Atmospheric Radiation Measurement site adjacent to NOAA's Barrow site.

Specific scientific questions to be addressed are:

Q1: What are the chemical, optical, and microphysical characteristics of aerosols in the Arctic in springtime?

• What is the solar extinction and absorption of the aerosol, and how do these properties vary with relative humidity?
• What is the mass concentration and size distribution of soot?
• To what extent are soot particles coated with other materials, and do such coatings influence the radiative and cloud-nucleating properties of the soot particles?
• What is the contribution of organic material to the optical and chemical properties to the aerosol?
• How do aerosol concentrations, composition, optical properties, and cloud nucleating properties above the surface relate to values measured at the surface?
• What is the radiative forcing and resulting atmospheric heating rates due to the aerosol, and how do these values compare with those derived from spaceborne lidar, surface lidar, and surface aerosol measurements?
• How do the composition and hygroscopic properties of aerosols relate to chemical processing estimated from trace gases?

Q2: What are the source types (industrial, urban, biomass/biofuel, dust, sea-salt) of the aerosol components, and the absorbing components in particular?

• What are the correlations between aerosol components and trace gases?
• How does the composition of the aerosol and trace gases compare to that expected from transport and emission models such as FLEXPART?
• Does the vertical distribution of aerosol properties reflect differences in source region, transport, and removal?
• What are the major sources that contribute to atmospheric and surface soot during the critical springtime warming period?

Q3: What are the microphysical and optical characteristics of optically thin clouds in the lower Arctic troposphere in springtime, and do pollution particles affect these cloud properties?

• What is the number density of CCN present in aerosol layers and in clean air, and is there closure between the predicted CCN, from the observed aerosol composition and size distribution?
• How does the number concentration of CCN, as a function of water supersaturation, vary as a function of altitude?
• Is the cloud droplet number concentration in liquid clouds consistent with that predicted from the observed CCN and cloud cooling rate?
• What is the relationship between measured IN concentrations and cloud ice number concentrations and size? Payload no longer includes IN counter
• What are the measured solar reflectance and transmission, the IR radiance, and the effective radius of Arctic clouds, and how do these values vary with CCN and IN concentration? Payload no longer includes IN counter
• How do directly measured and derived cloud properties compare with remotely measured and derived parameters at the DOE ARM site?

Q4: What are the concentration of particles that serve as ice nuclei (IN) in background and polluted air? Payload no longer includes IN counter

• What is the number density of IN present in aerosol layers and in clean air?
• What are the geographic sources of the IN in the Arctic?

Q5: Is soot present in particles that serve as IN and CCN? Payload no longer includes IN counter

• Is soot efficiently scavenged by cloud droplet nucleation, ice crystals, and snowfall?
• What role do coatings on soot particles play in nucleation scavenging and removal of soot?

Q6: What halogen chemistry is occurring during Arctic spring?

• What is the distribution of gas phase chlorine and bromine compounds, especially ClNO2?
• What is the vertical distribution of sea-salt aerosol and what chemical processing has it undergone?
• What is the relative importance of the sources of O3 in the Arctic and subArctic lower troposphere in springtime (production vs. stratospheric vs. long-range transport)?

The six science questions lead to specific measurement requirements:

R1) The stratified nature of the Arctic lower stratosphere requires airborne and remote-sensing measurements so that the properties and processes occurring in and near radiatively important haze layers and stratiform clouds can be investigated.

R2) Because of the vertically stratified and spatially non-uniform distribution of Arctic haze, fast-response in situ gas- and aerosol-phase instruments are required.

R3) The climatic importance of aerosol optical properties and soot number and mass require accurate and fast-response measurements of these parameters, along with measurements of the variation in optical properties with relative humidity.

R4) Because of the strong potential climate interaction between aerosols and cloud microphysical and radiative properties, detailed cloud microphysical and visible and infrared radiation measurements are needed. Modeling is essential to interpret the aerosol, cloud and radiation observations and extrapolate them to climate-relevant scales.

R5) Improving understanding of halogen photochemistry in the Arctic requires accurate measurement of gas phase halogen species and their vertical distribution, as well as measurements of ozone and photolytic fluxes.

R6) Transport, chemistry, and climate models are needed to relate the observed aerosol and gas-phase characteristics to sources and transport mechanisms and to evaluate their importance.

R7) Because ground sites are essential for developing climatologies and for understanding the temporal changes in atmospheric processes in the Arctic, short term airborne studies should be made at locations and times that can be linked to the surface sites.

Based on scientific questions Q1-Q6 and the measurement requirements R1-R7 that logically follow, NOAA will operate a WP-3D aircraft in the Alaskan Arctic in spring of 2008 as part of the International Polar Year (IPY). NOAA's Earth System Research Laboratory (ESRL) and extramural colleagues have developed a powerful set of precise and accurate gas- and particle-phase instruments for airborne investigations of air quality and climate-relevant chemical and microphysical processes ranging in scale from tens of meters to intercontinental distances. In particular, NOAA has developed new, sensitive instruments for determining aerosol optical properties, including a cavity ringdown method for directly measuring aerosol extinction at multiple wavelengths and its variation with relative humidity.

ESRL scientists have also substantially modified, evaluated, tested, and operated on aircraft a recently developed commercial instrument that measures the number and mass of individual soot particles and that can determine the amount of condensed coating on them. NOAA has also developed an unique instrument for measuring the composition of single aerosol particles and the residue from evaporated cloud particles, and has optimized a commercial aerosol mass spectrometer for airborne non-refractory aerosol composition measurements. In addition, the NOAA Aircraft Operations Center has recently purchased a set of state-of-the-art cloud probes for measuring the number, size, and shape of cloud and precipitation particles. With the addition of well-tested gas-phase and radiometric measurements, this payload is ideal for addressing the climate-relevant scientific questions outlined above (Table 1).

The diverse objectives of the ARCPAC project cannot be met without the experimental and scientific talents of non-NOAAcolleagues. In particular, measurements of IN, CCN, bulk aerosol composition, solar spectral irradiance and infrared irradiance, VOCs, and transport and chemical-transport modeling require equipment and expertise from researchers from universities, other governmental laboratories, and international research organizations.

Table 1. Priority instruments for the WP-3D aircraft during ARCPAC.