Characterization of Aerosol Physical, Optical and Chemical Properties During the Big Bend Regional

Aerosol and Visibility Observational Study (BRAVO)

Jenny Hand*

Eli Sherman*, Sonia Kreidenweis*, Jeff Collett, Jr.*, Taehyoung Lee*, Derek Day and Bill Malm

Colorado State University

*Atmospheric ScienceCIRA/National Park Service

Funding by National Park Service

OUTLINE

•Motivation for participating in BRAVO

•Chemical measurements and preliminary results

•Fine (PM2.5) and Coarse (PM10- PM2.5) species

•Size distribution measurements•Experimental set-up and instrument calibration

•Alignment method: retrieved refractive index and density

•Comparisons between chemical and physical properties

•Optical properties: column and point measurements•bsp (fine and coarse), aer, Ångstrom exponent

•Summary

BRAVO STUDY

July - October 1999

•Big Bend NP has some of the poorest visibility of any monitored Class 1 area in the western U.S.

•Seasonal trends•Sulfates: highest in summer•Organic carbon: highest in spring•Blowing soil: highest in July (Saharan dust episodes)(Gebhart et al., 2000)

•Recent work in Grand Canyon NP demonstrated that discrepancies of up to 50% or more exist between measured and reconstructed extinction (Malm and Day, 2000)

•Particle absorption or coarse scattering?

Aerosol Chemistry Measurements

•PM2.5 composition

•CSU: daily samples, on-site analyses of major ionic species and particle acidity

•IMPROVE: daily samples: major ionic species, plus soil, organic and elemental carbon

•PM10 composition

•IMPROVE: daily samples: major ionic species, plus soil, organic and elemental carbon Coarse composition (PM10 - PM2.5)

•Ionic species’ particle size distribution: MOUDI samples

•Aethalometer- black carbon

BRAVO PM2.5 Aerosol Acidity

BRAVO Soil Composition

Aerosol Size Distribution Measurements

•Dry size distributions were measured continuously ranging from 0.05< Dp< 20 µm

•Instruments:

•TSI Differential Mobility Analyzer (DMA): 0.05 < Dp < 0.87 µm (21 bins)•PMS Optical Particle Counter (OPC): 0.1 < Dp < 2 µm (8 bins)•TSI Aerodynamic Particle Sizer 3320 (APS): 0.5 < Dp < 20 µm (51 bins)

•Pre-, during-, and post-study calibration were performed using PSL, ammonium sulfate and oleic acid.

Instrument Calibration

•Empirical equations determined from instrument calibration relate real refractive index to OPC channel diameter (Dopt Dp)

•Channel collection efficiencies were determined

•Effective density (e) was related to APS channel diameter

(Dae Dp) by the following equation:

whereDp =Dae

CaeρeCp

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

12

ρe =ρp

χ

Examples of Aligned and Unaligned DMA and OPC Volume Distributions

Unaligned Aligned

Example of Combined Volume DistributionBRAVO 991008

BRAVO Volume Distributions

Comparisons between chemical and physical properties

•Refractive index and density: retrieved from alignment method and calculated from chemical composition

•Total (PM10) reconstructed mass and M = Vtot from size

distributions, assuming X=1.2

•MOUDI mass size distributions and volume distributions

•EC and aethalometer measurements

Accumulation Mode Parameters

Dgv g

Coarse Mode Parameters

Dgv g

Refractive Index and Density

•Real refractive index and effective density were retrieved from size distribution alignment method

•Values based on chemistry were calculated using a volume weighted method:

and

Species included:

(NH4)2SO4: m = 1.53, = 1.76 g cm-3

OC: m = 1.55, = 1.4 g cm-3

EC: m = 1.96 - 0.66i, = 2.0 g cm-3

NH4NO3: m = 1.554, = 1.725 g cm-3

Soil: SiO2, Al2O3, Fe2O3, CaO, TiO2 (IMPROVE)

m =ρ mi Xiρii

∑ ρ −1 =Xiρii

∑

Aerosol Refractive Index and Density

Total Mass Comparisons

•PM10 total mass concentration•M = Vtot, assuming X = 1.2

MOUDI Mass and Volume Distributions

Calculations of Light Scattering Coefficient (bsp)

•bsp was calculated using combined volume distributions and converged values of refractive index

•Qsp is the Mie scattering efficiency assuming spherical particles.

•bsp was calculated for the accumulation and coarse particle modes

bsp=32

Qsp

Dp

dVdlogDp

dlogDp∫

BRAVO scattering distribution

Comparisons of NPS and CSU Dry bsp

Dry Mass Scattering Efficiency

Accumulation Mode Coarse Mode

Calculation of Aerosol Optical Depth (aer)

•USDA UV-B radiation monitoring program has a fully instrumented site approximately 30 miles from BRAVO site in Big Bend National Park

•YES visible Multi-Filter Rotating Shadowband Radiometer measures irradiance with seven wavelength channels: 415, 500, 610, 665, 860, and 940 nm (Bigelow et al., 1998)

•Rayleigh and ozone optical depths were removed from column measurements of total optical depth

•Clouds and high sun angle measurements were removed

•Point measurements of aer were determined by assuming a well-mixed layer and estimates of boundary layer heights

Two days were chosen for comparison:

Aerosol Optical Depth at 500 nm

August 15, 1999 October 12, 1999

Ångstrom Wavelength Exponent ()•Calculated for both point and column measurements over the wavelength range from 415 nm - 860 nm (Eck et al., 1999 & Reid et al., 1999)

•Two days were chosen for comparison, demonstrating very different aerosol physical, chemical and optical properties

αc =−dlnτaerdlnλ

αp =−dlnbextdlnλ

Column: Point:

Ångstrom wavelength exponent

(415 - 860 nm)

August 15, 1999 October 12, 1999

Correlations between bsp and aer were found for several days:

Correlations between bext and aer were found for all months:

Correlations between CSU and UVB were found for all months:

Summary

•Sulfate was typically the major chemical species in the fine mode, although soil and OC were important during certain events

•Size distributions suggested that high coarse mode volume contributed significantly to total volume, especially during suspected Saharan dust events

•A new alignment method allowed for retrieving refractive index and effective density, in agreement with calculated values

•Calculated light scattering coefficients agreed well with measured values, and demonstrated periods when coarse scattering was important, often during suspected Saharan dust events

Summary, continued

•Time resolved sulfate measurements were observed to trend with light scattering coefficients, suggesting sulfate was the major contributor to visibility degradation during the study

•MOUDI mass distributions compared well with measured volume distributions

•Column and point measurements of aerosol optical depth were observed to be correlated for several days investigated

•Angstrom wavelength exponents agreed well between the two methods, and reflected the different aerosol types observed