In the early 1950s, the USA and the USSR were engaged in an arms race of nuclear technology for fission and fusion weapons.  The USSR conducted open-air tests at remote locations in the Arctic, producing radioactive dust which circulated in the northern atmosphere.  The USA developed air samplers that collected the dust onto paper tapes that were then sent to the Radiation Laboratories in California.  Analysis of the various radionuclides (by decay energy and half-life) helped to identify the nuclear reactions and allow the Americans to assess the degree of Russian progress.

It was soon noticed that when these “dust collectors” were operated in urban areas, the spots had a gray or black coloration.  This was designated as the “Coefficient Of Haze” but (at the time) was not linked to any specific aerosol constituent.  “COH” monitoring was performed at many urban locations, especially in California, where “smog” was a growing problem. However, the emphasis (and financial support) at that time was entirely focused on the atmospheric reactions that convert vehicle emission vapors into ‘haze’ (light scattering aerosols) and ozone.  Those reactions could not produce “black” material … but no one asked about the nature or origins of the “gray stuff” on the paper tapes.

Until 1977.

This research “sensor head” was built at the Lawrence Berkeley Laboratory by Dr. Tony Hansen for his research projects.  It has three parallel sampling channels, collecting three small spots on a 47-mm. diameter filter from three sample streams.  An external pump with critical orifices created the air flow; the filter was illuminated by an incandescent bulb; the 4^th portion of the filter served as the optical reference; and the signals were fed to an A/D converter connected to a Commodore-64 computer.

The research objectives were to study the incorporation of combustion-derived BC aerosols into natural fogs, and in a laboratory cloud chamber, by measuring the “initial”, “total”, and “interstitial” aerosol components.  The results showed that fresh BC aerosols are actually hygroscopic, and therefore capable of influencing cloud optical and nucleation properties.

This is one of the earliest Aethalometers, made by Dr. Tony Hansen by hand in 1987 at the start of the business of “Magee Scientific” in his home workshop.

The instrument collects the sample on a 47-mm. diameter quartz fiber filter and illuminates it with light from a 12-volt incandescent bulb.  The Sensing and Reference photodetectors provide voltage signals to terminals on the rear panel.  The airflow had to be produced by an external pump with a mechanical control valve.

The user had to connect the ‘S’ and ‘R’ voltage signals to a computer, digitize them, and then process the data with a simple algorithm written in BASIC.  There was no airflow sensor: the sample flow rate was a parameter input to the algorithm. The filter had to be changed by hand when the loading reached a maximum.

Despite their primitive appearance, several of these “earliest Aethalometers” were used at remote locations in the Arctic for over 20 years.

This instrument – together with its ‘Pumping Unit’ – was also made “by hand” in Berkeley.  Its analytical chamber accepted a 47-mm diameter quartz filter and illuminated it with an incandescent lamp bulb.  The digital voltmeters displayed the Sensing and Reference signal voltages and the sample air flow rate measured by an internal mass flow sensor.  The control knobs allowed the user to set the ‘zero’ (lamp off) and ‘span’ (lamp on) voltages, although this was not actually necessary for the algorithm.  The chassis contained a commercial analog-digital converter with RS232 communications and two digital I/O control lines.  These were used to sequence a multiplexer to select the signal to be digitized and to control the lamp and the pump (on-off).  A COM plug on the rear panel is connected to an external computer, running a DOS program written in BASIC.

The ‘Pumping Unit’ was a diaphragm pump in a ventilated chassis, with a mechanical throttle valve, flow, and vacuum indicator.  The pump was controlled by a solid-state relay connected by a cable to the main Aethalometer chassis.  The system could be programmed to start and stop at predetermined times.

These instruments were typically shipped together with monochrome DOS-text “laptop” (transportable) computers.  Data was written to a floppy disk in a very simple and intuitive file structure, including measurements (BC_.csv) and a plain-text commentary log “message file” (MF_.txt).  This provided the user with a competitive self-contained system for the real-time measurement of aerosol Black Carbon.

This Aethalometer was the prototype ‘AE-14’ for an instrument collecting its sample on a roll of quartz fiber tape (hence the designator “T”).  It was intended for use by non-scientific operators, namely, air-quality monitoring agencies.  This was the first development using specially-made filter tape, in which the (very brittle) quartz fiber was supported on a cellulose reinforcement under-layer.  Otherwise, the quartz material could not be handled mechanically.  The optical chamber was deliberately designed to be made from standard plastic materials, and the lifting mechanism was as simple as possible.  The tape-advance pinch rollers used a cam action to grip and release the tape, in order to control the length of tape that was pulled forwards.  The electronics were contained in the lower compartment.  This prototype was the basis for the ‘AE-16’ designed by Matjaž Zalar of Optotek, the OEM company in Slovenia that took over the manufacturing of Aethalometers in 1996.

This Aethalometer was designed according to specifications (and financing) provided by the sales representative company ‘GIV’ in Germany, who believed it was necessary to make an instrument to a unique design for a German Network – which never materialized.  The design included a touch-screen (unusual, expensive, and non-functional in 1996); and a German embedded computer using a 80186 processor which was soon discontinued.  Only three of these were made before GIV went bankrupt.  This entire project was a textbook example of the inadvisability of permitting marketers to define technical specifications and the inadvisability of investing large sums of money to create a product before verifying that the market for it actually exists.

This was the ‘classic’ Aethalometer of the period 1996 – 2012.  It was designed by Matjaž Zalar of Optotek, based on the AE-14 prototype conceived by Tony Hansen.  It used large-capacity rolls of filter tape and an internal pump with a mechanical flow control valve: since it needed no routine attention, it was sold widely for environmental monitoring.  The photodetector electronics and control circuits were built on one board with a generic (interchangeable) digital interface, and the display screen and keypad used standard communications.  In this way, it could use a standard industrial single-board computer.  It recorded data to a built-in floppy disk drive, a great innovation and convenience at the time: this meant that the instrument required no supporting systems at all and could be used stand-alone from a single electrical outlet.  The earliest models used an incandescent lamp for illumination.  In 1998, the optical source was changed to LED’s emitting at 880 nm, which subsequently created the de-facto operational definition of Black Carbon.  In that same year, we created a variant in which the optical illumination was provided by tiny Hg-vapor discharge tubes: this proved the concept of increased optical absorption at short wavelengths by biomass smoke, specifically Tobacco Smoke, an important topic in the USA at that time.  In 1999 we found LED’s emitting in the UV, and created a dual-wavelength Aethalometer (denoted AE-21) in which the illumination was switched between 370 and 880 nm sequentially each timebase period.  Later that year, we added 5 more ‘colors’ of LED to create the 7-wavelength Aethalometer (denoted AE-31).  In 1999, BC emission from biomass burning was not known to be important, and the concept of the Ångström Exponent of optical absorption was brand-new and of unrecognized value.

This unit also used the ‘Extended Range’ enlarged oval collection spot.  This option was offered to reduce the rate of accumulation in polluted urban areas and thereby reduce the consumption of filter tape.  The ‘High Sensitivity’ option used an inlet chamber with a small circular collection spot.

The AE-45 Aethalometer “module” was designed by; and produced exclusively for; the ‘Rupprecht & Patashnick’ company in the USA, manufacturers of the very successful ‘TEOM’ PM2.5 analyzers.  They wished to offer a module that could be attached into the air flow stream of the TEOM and provide a simple, single-number measurement of Black Carbon. The design was based upon the existing and successful AE-42 ‘Portable’ Aethalometer, but unfortunately R&P demanded so much simplification that it was impossible for the user to know if the AE-45 ‘module’ was actually working correctly or not.

This model was the successor to the ‘classic’ Aethalometer, with identical optical analysis but improved supporting systems.  It used a display screen and keypad integrated into the door panel; replaced the floppy disk drive with a compact flash memory card; and used a speed-controlled pump with a brushless DC motor instead of the previous AC-motor pump.  The pump improvement permitted closed-loop control of the sample flow rate by software, while the flash memory card extended the data storage life from weeks to years.  These models were extremely reliable and capable of continued operation under very adverse conditions and mistreatment.

The AE-43 prototype was intended to be a ‘generational update’ to the established AE-42, with a touch screen panel and operational features specified by engineers.  It was believed that some customers would like a ‘survey’ instrument which they could carry: but the existence of this market niche was never verified and this model was never developed.  This could now be viewed as an example of “Engineering Push” rather than “Market Pull”.

The Optical Transmissometer performs a ‘static’ measurement of optical absorption on a sample already collected on a filter.  The scientific principle of the relationship between the attenuation of transmitted light and the surface density of Black Carbon was first investigated by Tony Hansen, Lara Gundel, Hal Rosen, and others at the Lawrence Berkeley National Laboratory starting in 1977.  In 1979, Tony Hansen realized that if this measurement was performed continuously while the sample was simultaneously in the process of being collected, then the rate of change of attenuation could be interpreted as a rate of accumulation of BC.  Knowing the airflow rate, the BC concentration could be calculated: and the principle of the Aethalometer was born.

The OT-21 Optical Transmissometer is a modern, microprocessor-controlled instrument that provides a simple measure of the Optical Attenuation (‘ATN’) of an aerosol deposit on a filter.  The filter containing the aerosol sample is inserted in a sliding tray, which also carries a blank ‘reference’ filter.  The OT-21 analyzes at the two wavelengths of 370 nm and 880 nm, identical to those used in Aethalometers to speciate Black Carbon.

The Model AE33 Aethalometer was introduced in 2013 to overcome the major problem associated with the measurement of optical transmission through filters: namely, a non-linearity “Loading Effect” whose magnitude is unpredictable and appears to depend on the freshness, coating, and origins of the aerosol.  This effect leads to a perturbation of the relationship between “Attenuation” and “Black Carbon content” at higher loadings, resulting in discontinuity of data before and after a filter tape spot advance.  The AE33 collects two spots in parallel at different flow rates.  Comparison of the data in the two channels allows the “Loading Effect” to be eliminated; its (situational) parameter to be determined; and the ‘correct’ (zero-loading) result to be calculated.

The AE33 analyzes at seven wavelengths on a fundamental timebase of 1 second.  Air flow is provided by a closed-loop-stabilized pump, with brushless variable-speed motor.  The optical performance may be validated by a ‘Neutral Density Optics Kit’, a set of traceable standard photometric glass elements.

Many accessories may be interfaced to the AE33, and it can be connected to networks for remote operation, data acquisition, and instrument management.

The instrument is extremely rugged and reliable.  As of 2020, more than 1,000 units have been shipped to all seven continents of the world.

The DRI Model 2015 Multiwavelength Thermal/Optical Carbon Analyzer enhances the widely-used DRI Model 2001 system (from Dessert Research Institute) for quantifying organic carbon (OC), elemental carbon (EC, also termed Black Carbon [BC]), and temperature-separated carbon fractions on aerosol filter deposits (Chow et al., 1993). The Model 2015 retains OC and EC consistency with previous measurements while reducing costs of supplies and maintenance compared with the Model 2001. It replaces the 633 nm optical monitoring that accounts for OC charring with reflected (R) and transmitted (T) intensities at wavelengths of 405, 445, 532, 635, 780, 808, and 980 nm. The additional optical information can be used to estimate multiwavelength light absorption of the sampled particles, infer the concentration of brown carbon (BrC) in each sample, and further complement the use of carbon fractions in source apportionment studies (Chen et al., 2015; Chow et al., 2015). Model 2015 software includes temperature programs for commonly-used protocols such as IMPROVE_A (Chow et al., 2007; 2011), EUSAAR (Cavalli et al., 2010), and NIOSH (Birch and Cary, 1996; Chow et al., 2001), and it can be programmed to emulate any other protocol. The simultaneous measurement of both R and T at all wavelengths throughout each analysis allows for reproducing any other thermal/optical method and holds the potential for better characterizing additional properties of the carbonaceous aerosol.

The TCA08 Total Carbon Analyzer instrument uses a thermal method for total carbon (TC) determination. The instrument collects a sample of atmospheric aerosols on a 47-mm diameter quartz fiber filter enclosed in a small stainless-steel chamber at a controlled sampling flow rate. The sampling time is 1 h (default) but maybe pre-set from 20 minutes to 24 hours. The instrument has two parallel sampling and analysis channels, with airflow controlled by ball valves. While one channel is collecting its sample for the next time-base period, the other channel is analyzing the sample collected during the previous period.  At the end of the period, the valves switch over to provide continuous operation and continuous data acquisition.