Volatile organic compounds (VOCs) range in their reaction rates with atmospheric oxidants by several orders of magnitude. Therefore, studying their atmospheric concentrations across seasons and years requires isomer resolution to fully understand their impact on oxidant budgets and secondary organic aerosol formation. An automated gas chromatograph/flame ionization detector (GC-FID) was developed for hourly sampling and analysis of C 5 –C 15 hydrocarbons at remote locations. Samples are collected on an air-cooled multibed adsorbent trap for preconcentration of hydrocarbons in the target volatility range, specifically designed to minimize dead volume and enable rapid heating and sample flushing. Instrument control uses custom electronics designed to allow flexible autonomous operation at moderate cost, with automated data transfer and processing. The instrument has been deployed for over two years with samples collected mid-canopy from the Virginia Forest Laboratory located in the Pace research forest in central Virginia. We present here the design of the instrument itself, control electronics, and calibration and data analysis approaches to facilitate the development of similar systems by the atmospheric chemistry community. Detection limits of all species are in the range of a few to tens of ppt and the instrument is suitable for detection of a wide range of biogenic, lightly oxygenated, and anthropogenic (predominantly hydrocarbon) compounds. Data from calibrations are examined to provide understanding of instrument stability and quantify uncertainty. In this work, we present challenges and recommendations for future deployments, as well as suggested adaptions to decrease required maintenance and increase instrument up-time. The presented design is particularly suitable for long-term and remote deployment campaigns where access, maintenance, and transport of materials are difficult.
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Evaluation of ambient ammonia measurements from a research aircraft using a closed-path QC-TILDAS operated with active continuous passivation
Abstract. A closed-path quantum-cascade tunable infrared laserdirect absorption spectrometer (QC-TILDAS) was outfitted with an inertialinlet for filter-less separation of particles and several custom-designedcomponents including an aircraft inlet, a vibration isolation mountingplate, and a system for optionally adding active continuous passivation forgas-phase measurements of ammonia (NH3) from a research aircraft. Theinstrument was then deployed on the NSF/NCAR C-130 aircraft during researchflights and test flights associated with the Western wildfire Experiment forCloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) field campaign.The instrument was configured to measure large, rapid gradients in gas-phaseNH3, over a range of altitudes, in smoke (e.g., ash and particles), inthe boundary layer (e.g., during turbulence and turns), in clouds, and in ahot aircraft cabin (e.g., average aircraft cabin temperatures expected toexceed 30 ∘C during summer deployments). Important designgoals were to minimize motion sensitivity, maintain a reasonable detectionlimit, and minimize NH3 “stickiness” on sampling surfaces to maintainfast time response in flight. The observations indicate that adding ahigh-frequency vibration to the laser objective in the QC-TILDAS andmounting the QC-TILDAS on a custom-designed vibration isolation plate weresuccessful in minimizing motion sensitivity of the instrument during flight.Allan variance analyses indicate that the in-flight precision of theinstrument is 60 ppt at 1 Hz corresponding to a 3σ detection limitof 180 ppt. Zero signals span ±200, or 400 pptv total, withcabin pressure and temperature and altitude in flight. The option for activecontinuous passivation of the sample flow path with1H,1H-perfluorooctylamine, a strong perfluorinated base, preventedadsorption of both water and basic species to instrument sampling surfaces.Characterization of the time response in flight and on the ground showedthat adding passivant to a “clean” instrument system had little impact onthe time response. In contrast, passivant addition greatly improved the timeresponse when sampling surfaces became contaminated prior to a test flight.The observations further show that passivant addition can be used tomaintain a rapid response for in situ NH3 measurements over the duration of anairborne field campaign (e.g., ∼2 months) since passivantaddition also helps to prevent future buildup of water and basic species oninstrument sampling surfaces. Therefore, we recommend the use of activecontinuous passivation with closed-path NH3 instruments when rapid(>1 Hz) collection of NH3 is important for the scientificobjective of a field campaign (e.g., sampling from aircraft or anothermobile research platform). Passivant addition can be useful for maintainingoptimum operation and data collection in NH3-rich and humid environments orwhen contamination of sampling surfaces is likely, yet frequent cleaning isnot possible. Passivant addition may not be necessary for fast operation,even in polluted environments, if sampling surfaces can be cleaned when thetime response has degraded.
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- NSF-PAR ID:
- 10136088
- Date Published:
- Journal Name:
- Atmospheric Measurement Techniques
- Volume:
- 12
- Issue:
- 7
- ISSN:
- 1867-8548
- Page Range / eLocation ID:
- 3717 to 3742
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/amt-12-3717-2019
