Assessing Air Quality Using USDA Shadow-band

Radiometers

James SlusserUSDA UV-B Monitoring and Research Program

Natural Resource Ecology LaboratoryColorado State University

Purposes of USDA UVB Monitoring and Research Program

• Furnishes basic information necessary to support research on potential damaging effects of UVB on agriculture, forests, and livestock

• Provides UV climatology data to agricultural and scientific researchers

• Supplies ground truth for satellite retrievals and model development

• Establishes long-term record of UVB to assess trends

Quebec Fires, 7 July 2002

SeaWIFS image

7 July 2002

showing USDA sites

USDA UV-B Radiation Monitoring Network

Climatological Sites

Operational

Selected

Incident Solar Radiation (I0) = 1

Stratosphere Ozone Layer

Clouds

fS - Backscattering to space

fA - Absorption in atmosphere

fG - Absorption at ground

fG

fS + fA + fG = 1

Surface Albedo

fS

fA

fG

(absorption <320nm)

(UVB - 290-320 nm)

fS

AerosolsfSAir

PollutionfA

FATE OF SOLAR RADIATION

Clouds

fG

Surface Absorption-Reflection

fS

fA

fG

Ozone Layer(absorption <320nm)

fS

AerosolsfS

AirPollution

fA

Troposphere

Stratosphere

Incident Radiation Incident Radiation

FATE OF SOLAR RADIATION(UVB - 290-320 nm)

Three measures of aerosol and smoke loading:

Aerosol optical depth (AOD) quantifies the attenuation of the direct Solar beam. The AOD is proportional to total column loading of particles of the same size and related to visibility.

Angstrom exponent (alpha) gives the wavelength dependence of the AOD. Large particles (such as cloud drops or ice crystals) are spectrally neutral (alpha close to 0). The very smallest particles , molecules, have an inverse (wavelength) to the fourth dependence (alpha equal to 4). Most aerosols have alpha between 1 and 3.

Single scattering albedo (SSA) is the ratio of scattering to scattering plus absorption. For non absorbing (sulfate and water) particles SSA equals 1.0. Aerosols with black carbon have SSA between 0.7 and 0.9.

USDA INSTRUMENTATION

Effects of thick urban haze on irradiances

Model no aerosols

Beltsville, MD

July 17, 1999

Non-absorbing aerosols

NASA TOMS Aerosol Index for May 15, 1998

Biomass burning in Mexico

Effects of thick smoke on 368 nm UV irradiances

Baton Rouge, LA May 15, 1998

Absorbing aerosols

Begin Beltsville Slides

Beltsville, MD

Note saturation after 12:00

Begin Colorado Slides

18 June 2002

Langley Calibration

Logistics• Basic cost around $12,500• Requires 110 VAC and phone line OR batteries

and memory card• Can be polled in real time as Web site• Calibration is established in situ during clear days• Wind and ancillary instruments can be logged as

well• USDA would be willing to assist with data

analysis on a per cost basis

Conclusions

1. Visible Shadow-band radiometers provide aerosol and cloud information critical to forest fire and air quality studies.

2. Low cost ($12.5K) and automated operation are attractive features.

3. This method provides fine temporal resolution useful for studying smoke and pollution plumes.

4. Ancillary information (e.g. T, RH, wind speed and direction) can be logged simultaneously.

5. USDA would be glad to work with State and Federal agencies on data analysis.