Mixing ratios:
SPiFFY measurements of isocyanic acid (HNCO) mixing ratios were made using an Aerodyne High Resolution - Time of Flight - Chemical Ionization Mass Spectrometer with acetate reagent ion (hereafter CIMS). The instrument was operated at 15kHz extraction frequency for the pulser extraction plate in the time-of-flight region. Mass resolution was >3,500 (m/deltam). Average total ion counts were >800,000. Averaging time for data acquisition was 0.2 s (5 Hz frequency). Data are all reported in parts per trillion by volume (pptv). The lower limit of detection was 0.006 pptv calculated using counting statistics and three times the signal-to-noise ratio. An estimated upper limit of detection was 100,000 pptv.

Fluxes:
HNCO fluxes are the 30-minute averages of the products of instanteneous deviations from the mean for mixing ratio (C') and vertical wind speed (w') (F = <w'C'>). Wind speed was measured with a Gill Wavemaster Pro sonic anemometer, which was installed normal to ground surface at 30 m above ground level on the Chemistry walk-up tower at MEFO. Wind speed data were transmitted via RS-422 to the same data acquisition device used by the CIMS and averaged to 0.2 s time resolution. Flux uncertainties were calculated according to Finkelstein and Sims (2001). Average flux uncertainty is +/- 30%. Shared flux data are quality filtered according to the techniques described in Fulgham et al. (2019).

REFERENCES:
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Finkelstein and Sims, Journal of Geophysical Research, Atmospheres, 106,D4,3503-3509, 2001.  https://doi.org/10.1029/2000JD900731

Fulgham, S.R., Brophy, P., Link, M., Ortega, J., Pollack, I., Farmer, D.K., Flux Measurements over a Colorado Pine Forest Demonstrate a Persistent Source of Organic Acids, ACS Earth Space Chem. 3, 9, 2017-2032, 2019, 3, 9, 2017-2032, 2019, https://doi.org/10.1021/acsearthspacechem.9b00182

