The Black Carbon Photometer™ (BCP™) is a rugged, portable, highly accurate and economical instrument for measuring atmospheric black carbon particulates in polluted urban environments.  The BCP uses the simple and direct technique of long-path photometry to quantify—in real time—the total extinction and mass concentrations of particulates in the aerosol (gas + particle) phase.  This direct approach avoids the artifacts and other issues associated with current commercially available instruments, such as the most common current technique of aethalometry that uses filters to preconcentrate samples for analysis.

The detection cell of the Black Carbon Photometer has a folded tubular design, a patented innovation that enables it to be rapidly flushed (necessary for fast switching between sample and reference measurements).  Consequently, the BCP has high sensitivity, needs no preconcentration on a filter, and gives fast measurements of black carbon.  The absorption cell is folded to give a long (2.1-meter) path length in a compact space, resulting in an instrument that is lightweight (~19 lb/8.6 kg) and sized for use in a standard rack mounting system.  We described the folded tubular design, such as the one used in the Black Carbon Photometer, in a peer-reviewed publication in Atmospheric Measurement Techniques (Birks et al., AMT, 11, 2821-2835, 2018).

The Black Carbon Photometer is designed for the direct measurement of total extinction (absorbance + scattering) due to particulate matter at two wavelengths, 405 nm and 880 nm.  The 880-nm channel can be used to compute black carbon mass concentration.  Extinction at the shorter 405 nm wavelength can be related empirically to PM2.5 in a given location.

The Black Carbon Photometer is useful for researchers studying the role of black carbon in air quality and climate, and for air quality managers interested in estimating community exposure risks. ​

 

The figure below is a simplified schematic diagram of the Black Carbon Photometer. Sample air is continuously drawn through the instrument and its optical detection cell by an air pump at a flow rate of ~1.0-1.6 L/min. The reference valve alternately bypasses or sends the sample air through a PTFE particulate filter (labeled Particulate Filter #1 below) that removes all particulates in the sample, thus allowing the alternating measurement of a light intensity in the absence of particulates (Io) and presence of particulates (I). The Beer-Lambert Law is then used to calculate the particulate extinction (Bext) from I and Io:
Bext = 1/L ln(Io/I)
Here, L is the path length (2.1 m).

Particulate extinction is the sum of two factors:  aerosol absorption and scattering.  At the longer wavelength of the Black Carbon Photometer (880 nm), absorption is a much larger contributor to extinction.  In polluted urban air, the response of the instrument at 880 nm is primarily due to black carbon absorption.   At the shorter wavelength of the instrument (405 nm), scattering dominates and the instrument response is more indicative of total aerosol concentration.