Black Carbon Photometer™
Direct Measurement of Black Carbon without Filter Preconcentration
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 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.
Theory of Operation
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).
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.
Specifications
| Principle of Measurement | Direct extinction at 405 nm and 880 nm |
| Measurement Outputs | Extinction (Mm-1) at 405 and 880 nm Mass concentration (µg m-3) at 880 nm |
| Linear Dynamic Range | 0-10,000 Mm-1 (~0-1300 µg m-3 at 880 nm) |
| Resolution | 0.1 Mm-1 (0.1 µg m-3) |
| Precision (10-s measurement, 1σ rms noise) |
405 nm: 4.1 Mm-1; 0.7 Mm-1 with adaptive filter1 880 nm: 2.8 Mm-1; 0.5 Mm-1 with adaptive filter1 (0.35 µg m-3; 0.07 µg m-3 with adaptive filter1,2) 880 nm, 1-hr average: 0.018 µg m-3 |
| Accuracy | Greater of 2 Mm-1 or 2% of reading |
| Limit of Detection (2σ) | 405 nm: <1.6 Mm-1 (with adaptive filter1) 880 nm: <1.1 Mm-1 (with adaptive filter1) 880 nm, 1-hr average: 0.036 µg m-3 |
| Flow Rate | 1.3 L min-1 (Minimum: 1.0 L min-1; Maximum 1.6 L min-1) |
| Measurement Frequency | 0.1 Hz (once every 10 seconds) |
| Averaging Times | 10 s, 1 min, 5 min, 1 hr |
| Response Time, 100% of Step Change | 20 s for 10-s averaging 30 s with adaptive filter |
| Internal Data Logger Capacity | 10 Gbytes, >3.2 years of 10-s data |
| USB for Portable Data Logging | Yes |
| Pressure Units | mbar |
| Temperature Units | °C |
| T and P Corrected | User-Selected |
| Operating Temperature Range | 5 to 45 °C |
| Operating Pressure | 600-1000 mbar |
| Power Requirements | 11-14 V dc or 120/240 V ac, 1.3 A at 12 V, ~16 watt average during operation |
| Size | Rackmount: 17" w x 14.5" d x 5.5" h (43 x 37 x 14 cm) |
| Weight | ~19.2 lb (8.7 kg) |
| User Interface | Touchscreen or Serial |
| Data Outputs | RS232; 0-2.5 V Analog Outputs for 405 and 880 nm extinctions; USB Drive |
| Data Transfer Baud Rate | 2400 |
| Output Ranges | User-defined in scaling factor menu |
| Long Life Pump | Yes: 15,000 hours |
| Flow Meter | Yes |
| Built-In Auto Zeroing Capability | Yes |
1: An adaptive filter may be selected from the serial menu for signal averaging. Specifications above are for default parameters: Minimum Absolute Difference = 15 Mm-1, Percent Difference = 5%, Short Filter = 4 pts (40 s), Long Filter = 18 pts (3 min). Adaptive filter parameters may be adjusted by the user.
2: Using a default mass extinction coefficient of 7.77 m2g-1. This value can be adjusted by the user.
2: Using a default mass extinction coefficient of 7.77 m2g-1. This value can be adjusted by the user.
Features
- Direct measurement of extinction by particulates at two wavelengths (880 nm and 405 nm) enables real-time measurements of particulate matter, including black carbon, and PM2.5
- Direct approach eliminates the need to preconcentrate samples on filters, thereby avoiding associated artifacts
- Low power consumption (~16 watt on average after warmup)
- Lightweight (~19.2 lb, 8.7 kg), standard rack-mount size
- Long-life pump (~15,000 hours)
- Touchscreen
- Internal data logger capacity: 10 Gbytes
- USB port for portable data logging
- Both serial and user-scaled analog voltage outputs
- Convenient user interface to microprocessor, via either the touchscreen or serial, including calibration parameters (gain and zero) and mass conversion coefficients
- Selectable measurement time of 10 s or data averaging times of 1 min, 5 min and 1 hr
- Built-in zeroing capability
- Requires little technical maintenance and is suitable for continuous operation under field conditions (capable of battery-powered operation)
- Economical when compared with other commercially available instruments
System Includes
- Black Carbon Photometer
- 2.5A Power Adapter (2.5 amp, 100-240 VAC to 12 VDC) with select power cord
- Serial Port Cable (9PinF-9PinF)
- Operation Manual on USB Stick
- Calibration Data and NIST-Traceable Calibration Certificate
- Instrument Birth Certificate
- One-Year Warranty