M ULTI -W AVELENGTH O PTICAL C ALIBRATION OF T HERMAL /O PTICAL ANALYZER AND POTENTIAL APPLICATIONS John G. Watson, Judith C. Chow, L.-W. Antony Chen, Xiaoliang Wang, Ben Sumlin Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada, USA

O BJECTIVES Describe how relative values of transmittance (T) and reflectance (R) can be related to primary calibration standards Demonstrate the utility of additional measurements for source apportionment Identify some of the other potential uses of additional multiwavelength data on may samples

M OTIVATION Single wavelength R and T have only been used to adjust for pyrolysis, normalized to initial R or T. It can also be normalized to final R or T to approximate b abs. The light source/detector combination yields different intensities within and between instruments. More accurate b abs for several wavelengths and detection of brown carbon requires consistency of light intensity measurements within and among instruments Absolute reflectance and transmittance (in %) may be used separately or together for calculating b abs on filters, using radiative transfer models (e.g., Beer’s Law, Kubelka-Munk Theory, or Monte Carlo Ray Tracing, etc.)

A PPROACH 405, 445, 532, 635, 780, 808, 980 nm Perform spectral (UV-VIS-IR) characterization of aerosol deposits throughout thermal analysis Achieve light absorption (b abs ) measurement and apportionment (e.g., into BC and BrC) Allow OC-EC split made by R and T at different wavelengths 633 nm ▪The DRI Model 2015 Carbon Analyzer

T HERE ARE SEVERAL POSSIBILITIES FOR OPTICAL STANDARDS Neutral density filters  commercially available  well characterized  need cutting to fit the sample holder  only for transmittance  do not mimic filter optical properties Diffusive reflectance standard  commercially available  only for reflectance  difficult to cut for fitting the sample holder Real-world samples  need to be characterized before deployment  need to cover a wide range of R and T  need to be abundant in quantity and with reasonable lifetimes

S PECTRALON ® D IFFUSIVE R EFLECTANCE P ANELS ARE IN COMMON USE TO STANDARDIZE UV-VIS SPECTROMETERS

F ILTER TRANSFER STANDARDS WITH VARIABLE DEPOSITS CAN BE STANDARDIZED AGAINST THESE P LATES Lambda 31 Integrating-Sphere Spectrometer Measure R and T in two positions Filter R and T are scaled to 0 and 100% R and T standards

P ROCEDURE TO QUANTIFY ABSOLUTE R AND T FOR REAL - WORLD S AMPLES (II) I. Prepare filters and standard II. Load sample onto cartridge III. Load samples or standard onto a holder IV. Measure 100% R standard V. Measure 0% R Standard (empty) VI. Measure filter sample R

P ROCEDURE TO QUANTIFY ABSOLUTE R AND T FOR REAL - WORLD S AMPLES (III) VII. Measure 100% T standard (empty) with closed R port. VIII. Measure 0% T standard (blocker) with closed R port. IX. Measure filter sample T SmolderingFlamingDiesel Road DustYellow Sand

T RANSFER STANDARDS CONSIST OF DIFFERENT LOADINGS OF AN AMBIENT OR LABORATORY - GENERATED AEROSOL #108#107#73#122#99#77#74#65 High-Vol samples acquired from the Fresno Supersite during 2003 Reflectance Transmittance 7- λ for carbon analysis 5/6 9/29 12/286/611/4 11/13 6/19 7/3

C ALIBRATION OF R AND T M EASUREMENTS BY C ARBON A NALYZER Reflectance Transmittance Optical sensing of carbon analyzer is calibrated with transfer standards traceable to absolute filter R and T measurements. The uncertainty in R and T measurements by carbon analyzer is estimated to be within ±10% and much lower for some wavelengths.

E XAMPLE T HERMOGRAMS Diesel Soot Sample Ambient Sample from IMPROVE Site ROMA1 (1)Raw Data (Laser Reflectance [LR] and Transmittance [LT] (2) After Calibration (Filter Reflectance [FR] and Transmittance [FT])

S PECTRAL A BSORPTION I NFERRED FROM I NITIAL AND F INAL F ILTER T RANSMITTANCE VARY BY SAMPLE TYPE  Spectral absorption averaged by sample type  Smoldering samples acquired in DRI combustion chamber for burning peat

S PECTRAL A BSORPTION I NFERRED FROM I NITIAL AND F INAL F ILTER R EFLECTANCE In general, reflectance has lower S/N ratios than transmittance R and T can be combined for better quantification of light absorption as indicated by Petzold et al. (2004)

D ECOUPLING BC AND B R C C ONTRIBUTIONS TO ATN Fresno Ambient Smoldering Biomass Burning Use the Absorption Angström Exponent (AAE) Model r

BC AND B R C C ONTRIBUTIONS TO L IGHT A BSORPTION (ATN_405 NM ) IN EACH SAMPLE * Assuming only BC absorbs at 980 nm and an AAE_BC of 1 to extrapolate BC absorption to 405 nm. * Samples sorted by BrC fraction (0 to 100%) in ATN_405 nm. BC dominatedBrC dominated BC dominatedBrC dominated BC dominatedBrC dominatedr

IMPROVE_A EC PREDICTS ATN_BC BUT OC CORRELATES WITH ATN_B R C ECR OCR ECT OCT Pure BrCr

P OTENTIAL FUTURE USES OF CALIBRATED MULTIWAVELENGTH R AND T ON THOUSANDS OF SAMPLES More accurate measures of radiative transfer relevant to visibility and climate Ground truth for remote measurements from space More accurate quantification of biomass burning and fugitive dust contributions Separation of adsorbed organic vapors from organic carbon in aerosol deposit

C ONCLUSIONS Reflectance and Transmittance can be traceable to primary standards and be made consistent among wavelengths and instruments The detailed absorption spectrum can be approximated by the seven wavelengths Brown carbon can be separated from black carbon