Source attribution can be made easier with the LI-7825 CO₂ Isotope/NH₃ Trace Gas Analyzer

Dave Lowry, a stable-isotope geochemist at Royal Holloway University of London (RHUL) in England, started his career studying the mineralization of deposits in the UK. His early work mostly involved the breakdown of carbonates by phosphoric acid.

In the late 1990s, he started work in the atmospheric sciences to assist with EU efforts to characterize methane emissions. At the time, measurements were made using a mass spectrometer. More recently, he has worked to develop techniques to expand the measurements to include δ¹³C. Dr. Lowry and his colleagues have collected many CO₂ records over the years, but the records are not as informative as they could be because researchers do not yet understand the sources and source signatures of these emissions.

Source signatures – a measurable indicator of the source of a gas plume – can be made by isotopic analysis. This presents clear research opportunities, which can be filled by the LI-7825 CO₂ Isotope/NH₃ Trace Gas Analyzer.

Dr. Lowry tested a prototype of the LI-7825 to address these questions. Here we describe his tests, the results, and the new research opportunities that are made available by widespread δ¹³C measurements.

The Old Oak Tree

When air sampling lines were first installed for offices of Royal Holloway University of London, the old oak tree nearby was of little concern. At the time, the researchers were interested solely in methane. Carbon dioxide gas was not a concern. Later, CO₂ gas analyzers were connected to the same sampling line, and that is when things got interesting. One warm spring day, when the oak tree buds burst open, they observed a sudden, dramatic decrease in the CO₂ concentrations.

Despite the unexpected magnitude of the concentration changes, the measurements were not in vain. The observations led to an experiment in which researchers attempted to measure the effects of the tree on the isotopic signature. Diurnal cycles of the CO₂ concentrations were evident. He also observed a strong effect of photosynthetic CO₂ uptake and a negative shift in δ¹³C, indicating that the ¹³C is being depleted in the sampled air.

The Forest

Testing the LI-7825 at a forest research site, Dr. Lowry assessed data from an overnight period to identify CO₂ source signatures. He observed significant differences between the evening peak before sunset and the wider, but smaller overnight peak. Optimizing the averaging period to 10 seconds brought out the most interesting features of the data, revealing the evening peak of δ¹³C of -31‰ and the overnight peak of -19‰. Although the reason for this difference, and thus the source, are not yet clear, vehicle exhaust is the most likely culprit. The location is relatively remote for the south of England, but there may be an influence of the evening rush hour – and this may be the subject of upcoming research.

The Mobile Survey

Deploying the instruments on their mobile platform in northwest England, the team surveyed a site that has known sources of CO₂ and CH₄ from nearby dairy farms and landfills.

The platform, which measured a single air stream, featured both the LI-7825 for CO₂ and δ¹³C with the LI-7810 for methane. When measuring a plume downwind of the landfill site, the team could identify clear source signatures in the CO₂ isotope measurements.

The mobile platform worked well near the landfill, but data were not as clear from the dairy farm because the cattle were indoors. The team expects a stronger source signature when livestock are outdoors, however.

Instrument Comparisons

In addition to the field and mobile surveys, Dr. Lowry and his team were interested in assessing measurements of known standards. At the National Physical Laboratory in nearby Teddington, West London, they measured a gas standard that is depleted of ¹³C, with a known value of -41.9‰. The LI-7825 offsets were <0.1‰ following a three-point calibration, which is “as good as can be expected,” according to Lowry.

Having measured known standards, the team moved on to instrument comparisons, in which they compared measurements from the LI-7825 with those from a Picarro G2201i gas analyzer. Sampling was from a roof-mounted air intake at the Imperial College, London. Data showed periodic spikes of higher-than-average CO₂ concentrations associated with ¹³C depletion. Periodic 30-second spikes occurred at intervals of 42 to 43 minutes when the wind originated from the south-southeast to south-southwest. After some deduction, Dr. Lowry determined that the data indicate emissions from a nearby boiler, which is being activated on regular intervals.

View Data

After his evaluation, Dr. Lowry points out that the instrument performed as well as the stated specification for δ¹³C measurements. With regular calibration checks and zero adjustments, the instrument reports isotopic measurements with less noise than the comparison instrument.

The LI-7825 is designed to make CO₂ isotope measurements more widespread by combining low operating costs, high-precision measurements, and portability. With more widespread measurements of isotopes of carbon in CO₂, sources of CO₂ can be more easily identified. Source attribution is difficult but is easier with instruments like the LI-7825, and this information can help humans make better decisions about climate and CO₂ emissions.

Dr. Lowry is a stable-isotope geochemist at Royal Holloway, University of London (RHUL) in England.

Contact LI-COR to learn more about the LI-7825 CO₂ Isotope/NH₃ Trace Gas Analyzer.