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 Science  CIRA/National Park Service Funding by National Park Service

OUTLINE Motivation for participating in BRAVO Chemical measurements and preliminary results Fine (PM 2.5 ) and Coarse (PM 10 - PM 2.5 ) 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 b sp (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 PM 2.5 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 PM 10 composition IMPROVE: daily samples: major ionic species, plus soil, organic and elemental carbon  Coarse composition (PM 10 - PM 2.5 ) Ionic species’ particle size distribution: MOUDI samples Aethalometer- black carbon

BRAVO PM 2.5 Aerosol Acidity

BRAVO Soil Composition

Aerosol Size Distribution Measurements Dry size distributions were measured continuously ranging from 0.05< D p < 20 µm Instruments: TSI Differential Mobility Analyzer (DMA): 0.05 < D p < 0.87 µm (21 bins) PMS Optical Particle Counter (OPC): 0.1 < D p < 2 µm (8 bins) TSI Aerodynamic Particle Sizer 3320 (APS): 0.5 < D p < 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 (D opt  D p ) Channel collection efficiencies were determined Effective density (  e ) was related to APS channel diameter (D ae  D p ) by the following equation: where

Examples of Aligned and Unaligned DMA and OPC Volume Distributions Unaligned Aligned

Example of Combined Volume Distribution BRAVO

BRAVO Volume Distributions

Comparisons between chemical and physical properties Refractive index and density: retrieved from alignment method and calculated from chemical composition Total (PM 10 ) reconstructed mass and M =  V tot from size distributions, assuming X=1.2 MOUDI mass size distributions and volume distributions EC and aethalometer measurements

Accumulation Mode Parameters D gv gg

Coarse Mode Parameters D gv g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: (NH 4 ) 2 SO 4 : m = 1.53,  = 1.76 g cm -3 OC: m = 1.55,  = 1.4 g cm -3 EC: m = i,  = 2.0 g cm -3 NH 4 NO 3 : m = 1.554,  = g cm -3 Soil: SiO 2, Al 2 O 3, Fe 2 O 3, CaO, TiO 2 (IMPROVE)

Aerosol Refractive Index and Density

Total Mass Comparisons PM 10 total mass concentration M =  V tot, assuming X = 1.2

MOUDI Mass and Volume Distributions

Calculations of Light Scattering Coefficient (b sp ) b sp was calculated using combined volume distributions and converged values of refractive index Q sp is the Mie scattering efficiency assuming spherical particles. b sp was calculated for the accumulation and coarse particle modes

BRAVO scattering distribution

Comparisons of NPS and CSU Dry b sp

Dry Mass Scattering Efficiency Accumulation ModeCoarse 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 nm (Eck et al., 1999 & Reid et al., 1999) Two days were chosen for comparison, demonstrating very different aerosol physical, chemical and optical properties Column:Point:

Ångstrom wavelength exponent ( nm) August 15, 1999October 12, 1999

Correlations between b sp and  aer were found for several days:

Correlations between b ext 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