The said instrument can not give quantitative estimate of Br C.

More details you can have from the following paper

Srinivas, B. and M.M. Sarin. Brown carbon in atmospheric outflow from the Indo-Gangetic Plain: Mass absorption efficiency and temporal variability, Atm. Environ. 89, 835-843; doi.org/10.1016/j.atmosenv.2014.03.030, 2014

The determination of Brown Carbon (BrC) is in itself problematic, when the desired quantity is mass concentration. In sharp contrast with Black Carbon (BC), the mass absorption cross-section (MAC) of BrC is highly variable. The determination of the mass concentration can therefore be made relative to a  known model substance for BrC (such as humic acid salts), or using an assumed MAC, based on chemical analysis.

To use the OT21 and convert the measurement of attenuation into the mass concentration, a loading compensation algorithm needs to be applied. There was an excellent presentation at the last European Aerosol Conference (Milano, Sept 2015) by Fuller et al. The following two references give a good background on the loading effect.

Drinovec, L., Močnik, G., Zotter, P., Prévôt, A. S. H., Ruckstuhl, C., Coz, E., Rupakheti, M., Sciare, J., Müller, T., Wiedensohler, A., and Hansen, A. D. A.: The "dual-spot" Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation, Atmos. Meas. Tech., 8, 1965-1979, doi:10.5194/amt-8-1965-2015, 2015.

http://www.atmos-meas-tech.net/8/1965/2015/

Aki Virkkula, Timo Mäkelä, Risto Hillamo, Tarja Yli-Tuomi, Anne Hirsikko, Kaarle Hämeri, Ismo K Koponen, A simple procedure for correcting loading effects of aethalometer data. J Air Waste Manag Assoc 2007 Oct;57(10):1214-22

http://www.gaw-wdca.org/Portals/63/media/Virkkula_et_al_J_Air_Waste_Man_Assoc_2007.pdf

Thanks Grisa for the detailed clarification. It is more a research problem than an operational question

Assuming that brown carbon (BrC),  representing only humic like substances (HULS), is emitted from biomass burning, it is possible to quantify its mass concentration relative to a standard prepared from salts of humic acid/fulvic acid. However, there are recent reports suggesting BrC from fossil-fuel combustion, with different spectral properties and mass absorption efficiency. Thus, quantitative determination of BrC is a matter of ongoing debate. You may like to go thru following article:

Srinivas, B. and M.M. Sarin. Brown carbon in atmospheric outflow from the Indo-Gangetic Plain: Mass absorption efficiency and temporal variability, Atm. Environ. 89, 835-843; doi.org/10.1016/j.atmosenv.2014.03.030, 2014

Thanks everyone for your feedback. Your suggestions were very helpful. :)

The emissions profile of the biomass combustion is highly variable, depending on the source appliances and how they are used. You can have smoldering combustion and get tar and no soot, or have flaming combustion and get only black carbon from flame quenching. Likely brown carbon is somewhere in between.

We have had excellent results measuring particulate mass using the Condar portable dilution tunnel, when sampling directly from the stack of the appliance. Very good correlation with EPA Method 5G-3. It is a diluted sample, therefore a good representation of what ends up in the atmosphere since the semi-volatiles have been given a chance to condense.