Our atmosphere is home to a range of carbon-containing chemicals called ‘volatile organic compounds’ (VOCs). Identifying these compounds in the air helps scientists understand the degree to which human and natural events influence Earth’s atmosphere.
While many of these compounds are naturally emitted from trees and vegetation, human activities such as transportation and industry release chemicals into the atmosphere, such as benzene which is toxic to humans and other animals. Often these chemicals are present in such small concentrations that they are not considered harmful. However, when combined with other chemicals that are present in the air, they can form new toxic chemicals. For example, when VOCs combine with nitrogen oxides from car exhaust and sunlight they can form ozone and harmful particulate matter, impacting air quality and causing health problems for the people who breathe it.
The chemistry of the atmosphere is important to measure, not only for the health of our communities but for the sustainability of our planet. As climate change is becoming an increasingly important subject, sophisticated measurements coupled to sophisticated modelling are needed to increase our understanding of natural and human-caused processes that contribute to the rapidly changing atmosphere of the planet we live on.
Knowledge about the chemical make-up of the Earth’s atmosphere has been limited in the past because of our difficulty in accessing more remote regions with the right equipment. To approach this problem, advanced aircraft-based measurement methods are now used to measure the chemistry of the troposphere and lower stratosphere – the region of Earth’s atmosphere that reaches up to 15 km in height.
Dr Eric Apel and his group specialise in the measurement of volatile organic compounds in the air, and are producing cutting-edge technologies to assist them. They aim to devise powerful methods for analysing the chemical state of Earth’s atmosphere, which benefits our understanding of the global environment.
Their Trace Organic Gas Analyzer (TOGA) is unique to previous instruments used in that it rapidly detects up to 100 chemical compounds in-situ and is used to measure the air chemistry from the ground all the way to the lower stratosphere.
Having been deployed recently on NSF’s and NASA’s airborne missions, Dr Apel’s TOGA has measured large regions of the global atmosphere, tracking emissions and chemical transformations from such wide ranging sources such as industry, forest fires and naturally emitted compounds from the biosphere. The tool is also contributing to our understanding of the global climate, by measuring specific greenhouse gases as well as VOCs that impact the rate at which other greenhouse gases are removed from the atmosphere.
During the on-going NASA Atmospheric Tomography (ATom) program, Dr Apel and his team joined other researchers in making comprehensive measurements of the troposphere spanning significant areas above the Pacific and Atlantic Oceans in the Northern and Southern Hemispheres several times in different seasons. The NASA DC-8 aircraft is essentially a ‘flying laboratory’, making measurements of important chemicals like ozone, nitrogen oxides, carbon dioxide, oxygen, methane, atmospheric particles, and other assorted chemical compounds and pollutants. One of the main goals of ATom is to determine how greenhouse gases and pollution traverse the atmosphere, and the way in which different parts of the air systems mix together and interact. Some of their early results showed unexpected regions of high ozone in the remote troposphere. Ozone is a major pollutant that is a known by-product of chemical processing in the atmosphere and is also a climate-relevant (greenhouse) gas. A preliminary analysis has connected the observations of high ozone to the burning of biomass (forest fire and burning vegetation), as indicated by increased levels of chemical tracers from fires directly measured by TOGA. TOGA measurements provide key information which allows the ATom science team to make connections between specific chemicals in the atmosphere, and their potential for influencing the chemical reactivity of the planet.
From their studies, Dr Apel and his colleagues suggest that the levels of hydrocarbon emissions in the Northern Hemisphere have been underestimated, and their methods have shed new light on the distribution of VOCs in the Southern Hemisphere – an area that has not been studied as much in the past because of its remoteness.
TOGA – the technology
What makes TOGA so state-of-the-art is its high speed of sampling and its ability to measure many compounds at a part per trillion or less, which is a much lower concentration than standard instrumentation can achieve. TOGA’s inlet pulls air through the system rapidly, separating out VOCs of interest and quantifying them, providing a quick response time. The technology can measure rapidly enough so that it is well suited for use on high-speed vehicles such as aircraft. TOGA works by drawing in air and freezing out the VOCs with cold nitrogen gas to “trap” them. The trapped species are then heated up and sent into a specialised custom-made gas chromatograph, and following that to be detected by a quadrupole mass spectrometer.
The gas chromatograph separates the VOCs that are in the air by allowing some compounds to move more quickly through the system than others, based on size and other physical properties like boiling point and solubility. With the individual compounds separated, the gases are directed into the mass spectrometer which breaks them apart and ionises them into molecular fragment ions. By measuring the mass to charge ratio of the ions, we can find out about the underlying molecular structure of the compounds allowing for a positive identification for each.
Molecular fragments are like broken shards of pottery that scientists piece back together like a puzzle. Since there are only a certain number of ways to combine the fragments, they can compare the fragments to a mass spectrometry library or a known chemical mixture to identify each compound.
These mechanisms combined with methods to calibrate the system provide the scientists with the atmospheric concentrations of the VOCs they wish to study. A wide range of different compounds can be identified and quantified, particularly those organic compounds containing ten carbons or less in their molecular structures; as well as sulphur and nitrogen containing gases.
TOGA-TOF – a recent innovation
Dr Eric Apel and his collaborators have recently pushed this technology further, by combining TOGA with a specialised mass spectrometer that is faster and produces an even higher amount of data with more molecular information than the quadrupole mass spectrometer previously used. This ‘TOGA-TOF’ new TOGA mass spectrometer tool works by using a ‘time-of-flight’ (TOF) method which identifies and quantifies all fragment ions simultaneously, an improvement over the quadrupole mass spectrometer, which can only measure a small number of ions at a time. The TOF mass spectrometer measures the entire air chemistry plume in real-time and when combined with the gas chromatograph the resulting TOGA-TOF can fingerprint and quantitate a much larger range of atmospheric VOCs than the previous version. The instrument has the potential to detect and discover previously unknown chemicals and to analyse the complex air chemistry of polluted air masses above, for example, cities, industrial zones, and forest fires.
After an ion is created by bombarding the analyte molecule with electrons, the ion is accelerated through an electric field. Its velocity depends on its mass to charge ratio. At a given charge, the heavier the ion, the longer it takes the ion to reach the detector. So, for example, if an unknown/unidentified species is introduced into the TOF, the species is ionised and then the time it takes to reach the detector over a known distance within the electric field is measured. When calibrated, this measurement determines the mass to charge ratio of the species and ultimately allows for the species to be identified.
The big picture
Research technologies developed by Dr Apel and his team can offer increasingly rapid, accurate and reliable ways of analysing the Earth’s atmosphere – even in areas that had previously been considered difficult to access. The high-quality VOC measurements can allow us to make connections between the chemical composition of the atmosphere and key events across the planet – which may be human-caused or natural. Using this technology, new knowledge about the environment in which we live is gained; whether we wish to consider climate change, pollution generated by oil and gas production, or the circulation of the Earth’s air masses. This work is sure to continue to help us understand the changing planet and our true impact upon it.
I was inspired by a need within the community to develop a technique to measure a wide range of volatile organic compounds (VOCs) aboard aircraft in near real time at high sensitivity, accuracy and precision. Prior to the development of TOGA, no such technique existed. The TOGA technique circumvents shortcomings of other techniques and has proven to excel in these measurements.
How can your tools be used to help us understand man’s impact on the planet?
TOGA and TOGA-TOF can measure emissions from a wide range of human activities. Some of these emissions contribute to poor air quality, some are climate-impacting greenhouse gases and others are toxic substances. All of these are important for understanding man’s impact on the planet.
What do you think has been your most significant discovery so far – using TOGA on an aircraft?
Pollution from cities, oil and gas operations, and other concentrated sources can have a large impact on the regional air quality downwind of the sources. Also that human influence can be seen in a number of ways using a sensitive technique like TOGA – even in the most remote regions of our planet.
Why is it particularly important to measure volatile organic compounds where people live?
Because these gases combine with sunlight and nitrogen oxides to generate pollutants that are harmful to humans. Some VOCs are toxic and directly affect peoples’ health.
How do you see your research developments making an impact in the future?
Future research programs will involve comprehensive measurements and modelling of fire emissions and their impact on air quality in the Western United States. We will use the TOGA – TOF and the NCAR C-130 for this study. Another study is planned for Australia where emissions from the temperate forests are not well understood. We will use this study to gain insight into an environment where low nitrogen oxide emissions interact with the natural ecosystem, a situation that is expected to develop in the southeast United States and Europe over the coming years as nitrogen oxide emissions are continually being reduced. This will help us to predict air quality in the future for these areas.
Dr Eric Apel and his group, which includes Dr Alan Hills and Dr Rebecca Hornbrook, study volatile organic compounds in the Earth’s atmosphere, to aid our understanding of the global environment. They implement sophisticated technologies to identify important compounds in the air, and seek to improve the speed and quality of these tools.
National Science Foundation (NSF) and National Aeronautics and Space Administration (NASA)
Colleagues at NOAA, NASA, EPA, and many Universities both within and outside of the US. Colleagues include Professor Elliot Atlas, University of Miami, Professor Rainer Volkamer, Dept. of Chemistry, University of Colorado, Boulder, Professor Donald Blake, University of California, Irvine, Dr. Tom Ryerson, NOAA, ESRL, Dr. Steven Montzka, NOAA, ESRL, Dr. James Crawford, NASA Langley Research Center, Professor Steven Wofsy, Harvard University, Dr. Daniel Riemer, Apel-Riemer Env., Inc. and former team member
Project Scientist IV, National Center for Atmospheric Research, VOC measurement group leader, PhD, Physical Chemistry, University of California, Irvine, Irvine, CA. Specialises in developing VOC measurement technologies, conducting VOC measurements, principally on aircraft platforms, and the interpretation of VOC measurements in the context of other measurements.
Dr Eric Apel
Atmospheric Chemistry Observations & Modelling Laboratory
UCAR, P.O. Box 3000, Boulder,
CO 80307-3000 USA
T: + 1 303 497 1452
W: https://staff.ucar.edu/users/apel and