1. Development of a QCL-based spectrometer for measurements of biogenic volatile organic compounds
Jacob T. Stewart
Department of Chemistry, Connecticut College
ISMS 2016

2. Atmospheric chemistry is driven by trace species
Biogenic
Volatile
Organic
Compounds play a key role
Trace gases drive atmospheric chemistry
Image from: by Paul Young and Oliver Wild

3. What are some common BVOCs?
isoprene
α-pinene
β-pinene
myrcene

4. BVOCs lead to ozone formation
OH, NO3, O3
“Processed” BVOCs
oxidation
BVOCs
Oxidation process produces NO2 from reaction of peroxy radicals with NO
NO2 + hν → NO + O(3P)
O(3P) + O2 → O3

5. BVOCs lead to aerosol formation
OH, NO3, O3
“Processed” BVOCs
oxidation
BVOCs
Oxidized BVOCs are less volatile and condense to form secondary organic aerosol (SOA)
Aerosols impact climate

6. IR laser spectroscopy for measuring BVOCs
Rapid measurements
Relatively inexpensive
Can obtain rotationally-resolved spectra
Portable instruments and remote sensing
Schiller et al., Appl. Phys. B, 92, , 2008

7. Spectroscopy in the mid-IR
Atmospheric “window” allows BVOCs to be studied by IR spectroscopy
Target strong fundamental transitions in mid-IR
Quantum cascade lasers provide access

8. External-cavity quantum cascade laser
Daylight Solutions mode-hop-free cw QCL
Tuning range cm-1 (mode hop free cm-1)
Linewidth ~10 MHz over 1 s integration
Room temperature operation (using a water chiller)
methyl vinyl ketone
isoprene
ethylene

9. Isoprene is our first target
Estimated to be 50% of total BVOC emission globally*
No reported high-resolution IR spectrum
Also of interest in human breath analysis (related to cholesterol formation pathway)
isoprene
*Guenther et al., Geosci. Model Dev., 5, , 2012.

10. Multipass direct absorption spectroscopy
Our initial setup is utilizing multipass direct absorption spectroscopy
Astigmatic Herriott cell (76 m path length) on loan from Dave Lewis

11. Instrumental setup Multipass cell and detector QCL and water chiller
Beamsplitter and etalon

12. Rotationally-resolved spectroscopy of isoprene
First high-resolution IR spectrum of isoprene

13. Rotationally-resolved spectroscopy of isoprene
First high-resolution IR spectrum of isoprene
Matches previously reported low resolution spectrum
Limited frequency accuracy (imprecise motor for sweeping grating in laser)
PNNL data from Brauer et al., Atmos. Meas. Tech., 7, , 2014.

14. Rotationally-resolved spectroscopy of isoprene
First high-resolution IR spectrum of isoprene
Matches previously reported low resolution spectrum
Limited frequency accuracy (imprecise motor for sweeping grating in laser)
PNNL data from Brauer et al., Atmos. Meas. Tech., 7, , 2014.

15. Preliminary concentration data
Measurements of isoprene in nitrogen at 100 torr
Average of 100 scans for each concentration
Limit of detection is about 0.3 ppm using direct absorption

16. Future work Wavelength modulation spectroscopy to improve sensitivity
Implementing more precise scanning using built-in piezo control
Begin measurements of other BVOCs such as methyl vinyl ketone, an oxidation product of isoprene

17. Conclusions
BVOCs play an important role in atmospheric chemistry, leading to formation of ozone and aerosols
We have developed an IR laser spectrometer for measuring BVOCs with high resolution and high sensitivity
To our knowledge, we have recorded the first high- resolution IR spectrum of isoprene

18. Acknowledgements Dave Lewis Patrick Williams Julia Packman Minh Tran
Rakhshi Qureshi
Michael Iranpour