scholarly journals The airborne mass spectrometer AIMS – Part 1: AIMS-H<sub>2</sub>O for UTLS water vapor measurements

2016 ◽  
Vol 9 (3) ◽  
pp. 939-953 ◽  
Author(s):  
Stefan Kaufmann ◽  
Christiane Voigt ◽  
Tina Jurkat ◽  
Troy Thornberry ◽  
David W. Fahey ◽  
...  
Keyword(s):  
Water Vapor ◽  
Mass Spectrometer ◽  
Ambient Air ◽  
Reaction Scheme ◽  
Gas Discharge ◽  
Ion Source ◽  
Tunable Diode Laser ◽  
Cloud Formation ◽  
Calibration Source ◽  
Mixing Ratios

Abstract. In the upper troposphere and lower stratosphere (UTLS), the accurate quantification of low water vapor concentrations has presented a significant measurement challenge. The instrumental uncertainties are passed on to estimates of H2O transport, cloud formation and the role of H2O in the UTLS energy budget and resulting effects on surface temperatures. To address the uncertainty in UTLS H2O determination, the airborne mass spectrometer AIMS-H2O, with in-flight calibration, has been developed for fast and accurate airborne water vapor measurements. We present a new setup to measure water vapor by direct ionization of ambient air. Air is sampled via a backward facing inlet that includes a bypass flow to assure short residence times (< 0.2 s) in the inlet line, which allows the instrument to achieve a time resolution of  ∼ 4 Hz, limited by the sampling frequency of the mass spectrometer. From the main inlet flow, a smaller flow is extracted into the novel pressure-controlled gas discharge ion source of the mass spectrometer. The air is directed through the gas discharge region where ion–molecule reactions lead to the production of hydronium ion clusters, H3O+(H2O)n (n = 0, 1, 2), in a complex reaction scheme similar to the reactions in the D-region of the ionosphere. These ions are counted to quantify the ambient water vapor mixing ratio. The instrument is calibrated during flight using a new calibration source based on the catalytic reaction of H2 and O2 on a Pt surface to generate a calibration standard with well-defined and stable H2O mixing ratios. In order to increase data quality over a range of mixing ratios, two data evaluation methods are presented for lower and higher H2O mixing ratios respectively, using either only the H3O+(H2O) ions or the ratio of all water vapor dependent ions to the total ion current. Altogether, a range of water vapor mixing ratios from 1 to 500 parts per million by volume (ppmv) can be covered with an accuracy between 7 and 15 %. AIMS-H2O was deployed on two DLR research aircraft, the Falcon during CONCERT (CONtrail and Cirrus ExpeRimenT) in 2011, and HALO during ML-CIRRUS (Mid-Latitude CIRRUS) in 2014. The comparison of AIMS-H2O with the SHARC tunable diode laser hygrometer during ML-CIRRUS shows a correlation near to 1 in the range between 10 and 500 ppmv for the entire campaign.

scholarly journals The airborne mass spectrometer AIMS – Part 1: AIMS-H<sub>2</sub>O for UTLS water vapor measurements

2015 ◽  
Vol 8 (12) ◽  
pp. 13525-13565 ◽  
Author(s):  
S. Kaufmann ◽  
C. Voigt ◽  
T. Jurkat ◽  
T. Thornberry ◽  
D. W. Fahey ◽  
...  
Keyword(s):  
Water Vapor ◽  
Mass Spectrometer ◽  
Ambient Air ◽  
Reaction Scheme ◽  
Gas Discharge ◽  
Ion Source ◽  
Tunable Diode Laser ◽  
Cloud Formation ◽  
Calibration Source ◽  
Mixing Ratios

Abstract. In the upper troposphere and lower stratosphere (UTLS), the accurate quantification of low water vapor concentrations has presented a significant measurement challenge. The instrumental uncertainties are passed on to estimates of H2O transport, cloud formation and the H2O role in the UTLS energy budget and resulting effects on surface temperatures. To address the uncertainty in UTLS H2O determination, the airborne mass spectrometer AIMS-H2O, with in-flight calibration, has been developed for fast and accurate airborne water vapor measurements. We present the new setup to measure water vapor by direct ionization of ambient air. Air is sampled via a backward facing inlet that includes a bypass flow to assure short residence times (< 0.2 s) in the inlet line, which allows the instrument to achieve a time resolution of ∼ 4 Hz. From the main inlet flow, a smaller flow is extracted into the novel pressure-controlled gas discharge ion source of the mass spectrometer. The air is directed through the gas discharge region where water molecules react to form hydronium ion clusters, H3O+(H2O)n (n= 0, 1, 2), in a complex reaction scheme similar to the reactions in the D-region of the ionosphere. These ions are counted to quantify the ambient water vapor mixing ratio. The instrument is calibrated during flight using a new calibration source based on the catalytic reaction of H2 and O2 on a Pt surface to generate a calibration standard with well defined and stable H2O mixing ratios. In order to increase data quality over a range of mixing ratios, two data evaluation methods are presented for lower and higher H2O mixing ratios respectively, using either only the H3O+(H2O) ions or the ratio of all water vapor dependent ions to the total ion current. Altogether, a range of water vapor mixing ratios from 1 to 500 ppmv (mole ratio, 10−6 mol mol−1) can be covered with an accuracy between 7 and 15 %. AIMS-H2O was deployed on two DLR research aircraft, the Falcon during CONCERT (Contrail and Cirrus Experiment) in 2011, and HALO during ML-CIRRUS (Mid-Latitude Cirrus) in 2014. The comparison of AIMS-H2O with the SHARC tunable diode laser hygrometer during ML-CIRRUS shows a very good overall agreement between both instruments for the entire campaign.

scholarly journals Intercomparison of midlatitude tropospheric and lower-stratospheric water vapor measurements and comparison to ECMWF humidity data

2018 ◽  
Vol 18 (22) ◽  
pp. 16729-16745 ◽  
Author(s):  
Stefan Kaufmann ◽  
Christiane Voigt ◽  
Romy Heller ◽  
Tina Jurkat-Witschas ◽  
Martina Krämer ◽  
...  
Keyword(s):  
Water Vapor ◽  
Climate Impact ◽  
Cloud Formation ◽  
Mean Values ◽  
Stratospheric Water Vapor ◽  
Mixing Ratios ◽  
Quality Of Water ◽  
Research Aircraft ◽  
German Aerospace

Abstract. Accurate measurement of water vapor in the climate-sensitive region near the tropopause is very challenging. Unexplained systematic discrepancies between measurements at low water vapor mixing ratios made by different instruments on airborne platforms have limited our ability to adequately address a number of relevant scientific questions on the humidity distribution, cloud formation and climate impact in that region. Therefore, during the past decade, the scientific community has undertaken substantial efforts to understand these discrepancies and improve the quality of water vapor measurements. This study presents a comprehensive intercomparison of airborne state-of-the-art in situ hygrometers deployed on board the DLR (German Aerospace Center) research aircraft HALO (High Altitude and LOng Range Research Aircraft) during the Midlatitude CIRRUS (ML-CIRRUS) campaign conducted in 2014 over central Europe. The instrument intercomparison shows that the hygrometer measurements agree within their combined accuracy (±10 % to 15 %, depending on the humidity regime); total mean values agree within 2.5 %. However, systematic differences on the order of 10 % and up to a maximum of 15 % are found for mixing ratios below 10 parts per million (ppm) H2O. A comparison of relative humidity within cirrus clouds does not indicate a systematic instrument bias in either water vapor or temperature measurements in the upper troposphere. Furthermore, in situ measurements are compared to model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) which are interpolated along the ML-CIRRUS flight tracks. We find a mean agreement within ±10 % throughout the troposphere and a significant wet bias in the model on the order of 100 % to 150 % in the stratosphere close to the tropopause. Consistent with previous studies, this analysis indicates that the model deficit is mainly caused by too weak of a humidity gradient at the tropopause.

scholarly journals Intercomparison of mid-latitude tropospheric and lower stratospheric water vapor measurements and comparison to ECMWF humidity data

2018 ◽  
Author(s):  
Stefan Kaufmann ◽  
Christiane Voigt ◽  
Romy Heller ◽  
Tina Jurkat-Witschas ◽  
Martina Krämer ◽  
...  
Keyword(s):  
Water Vapor ◽  
Climate Impact ◽  
Cloud Formation ◽  
Mean Values ◽  
Stratospheric Water Vapor ◽  
Mixing Ratios ◽  
Quality Of Water ◽  
Research Aircraft ◽  
German Aerospace

Abstract. Accurate measurement of water vapor in the climate sensitive region near the tropopause turned out to be very challenging. Unexplained systematic discrepancies between measurements at low water vapor mixing ratios made by different instruments on airborne platforms have limited our ability to adequately address a number relevant scientific questions on the humidity distribution, cloud formation and climate impact in that region. Therefore, during the past decade, the scientific community has undertaken substantial efforts to understand these discrepancies and improve the quality of water vapor measurements. This study presents a comprehensive intercomparison of airborne state-of-the-art in situ hygrometers deployed onboard the DLR (German Aerospace Center) research aircraft HALO during the Mid-Latitude CIRRUS (ML-CIRRUS) campaign conducted in 2014 over central Europe. The instrument intercomparison shows that the hygrometer measurements agree within their combined accuracy (±10 to 15 %, depending on the humidity regime), total mean values even agree within 2.5 %. However, systematic differences on the order of 10 % and up to a maximum of 15 % are found for mixing ratios below 10 parts per million (ppm) H2O. A comparison of relative humidity within cirrus clouds does not indicate a systematic instrument bias in either water vapor or temperature measurements in the upper troposphere. Furthermore, in situ measurements are compared to model data from the European Centre for Medium-Range Weather Forecasts (ECMWF) which are interpolated along the ML-CIRRUS flight tracks. We find a mean agreement within ±10 % throughout the troposphere and a significant wet bias in the model on the order of 100 % to 150 % in the stratosphere close to the tropopause. Consistent with previous studies, this analysis indicates that the model deficit is mainly caused by a blurred humidity gradient at tropopause altitudes.

scholarly journals Measurement of low-ppm mixing ratios of water vapor in the upper troposphere and lower stratosphere using chemical ionization mass spectrometry

2013 ◽  
Vol 6 (1) ◽  
pp. 381-422 ◽  
Author(s):  
T. D. Thornberry ◽  
A. W. Rollins ◽  
R. S. Gao ◽  
L. A. Watts ◽  
S. J. Ciciora ◽  
...  
Keyword(s):  
Water Vapor ◽  
Chemical Ionization ◽  
Lower Stratosphere ◽  
Signal To Noise Ratio ◽  
Ion Source ◽  
Ionization Mass ◽  
Measurement Sensitivity ◽  
Particle Radiation ◽  
Upper Troposphere ◽  
Mixing Ratios

Abstract. A chemical ionization mass spectrometer (CIMS) instrument has been developed for the fast, precise, and accurate measurement of water vapor (H2O) at low mixing ratios in the upper troposphere and lower stratosphere (UT/LS). A low-pressure flow of sample air passes through an ionization volume containing an α-particle radiation source, resulting in a cascade of ion-molecule reactions that produce hydronium ions (H3O+) from ambient H2O. The production of H3O+ ions from ambient H2O depends on pressure and flow through the ion source, which were tightly controlled in order to maintain the measurement sensitivity independent of changes in the airborne sampling environment. The instrument was calibrated every 45 min in flight by introducing a series of H2O mixing ratios between 0.5 and 153 parts per million (ppm) generated by Pt-catalyzed oxidation of H2 standards while overflowing the inlet with dry synthetic air. The CIMS H2O instrument was deployed in an unpressurized payload area aboard the NASA WB-57 high altitude research aircraft during the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) mission in March and April 2011. The instrument performed successfully during seven flights, measuring H2O mixing ratios below 5 ppm in the lower stratosphere at altitudes up to 17.7 km, and as low as 3.5 ppm near the tropopause. Data were acquired at 10 Hz and reported as 1-s averages. In-flight calibrations demonstrated a typical sensitivity of 2000 Hz ppm−1 at 3 ppm with a signal to noise ratio (2σ, 1-s) greater than 32. The total measurement uncertainty was 9 to 11%, derived from the uncertainty in the in situ calibrations.

scholarly journals Portable Ozone Calibration Source Independent of Changes in Temperature, Pressure and Humidity for Research and Regulatory Applications

2018 ◽  
Author(s):  
John W. Birks ◽  
Craig J. Williford ◽  
Peter C. Andersen ◽  
Andrew A. Turnipseed ◽  
Stanley Strunk ◽  
...  
Keyword(s):  
Pulse Width Modulation ◽  
Uv Light ◽  
Ambient Pressure ◽  
Volumetric Flow Rate ◽  
Ambient Air ◽  
Mixing Ratio ◽  
Mercury Lamp ◽  
Calibration Source ◽  
Figures Of Merit ◽  
Mixing Ratios

Abstract. A highly portable ozone (O3) calibration source that can serve as a U.S. EPA Level 4 transfer standard for the calibration of ozone analyzers is described and evaluated with respect to analytical figures of merit and effects of ambient pressure and humidity. Reproducible mixing ratios of ozone are produced by the photolysis of oxygen in O3-scrubbed ambient air by UV light at 184.9 nm light from a low pressure mercury lamp. By maintaining a constant volumetric flow rate (thus constant residence time within the photolysis chamber), the mixing ratio produced is independent of both pressure and temperature and can be varied by varying the lamp intensity. Pulse width modulation of the lamp with feedback from a photodiode monitoring the 253.7-nm emission line is used to maintain target ozone mixing ratios in the range 30–1,000 ppb. In order to provide a constant ratio of intensities at 253.7 and 184.9 nm, the photolysis chamber containing the lamp is regulated at a temperature of 40 °C. The resulting O3 calibrator has a response time for step changes in output ozone mixing ratio of

scholarly journals Design and performance of a Nafion dryer for continuous operation at CO<sub>2</sub> and CH<sub>4</sub> air monitoring sites

2013 ◽  
Vol 6 (5) ◽  
pp. 1217-1226 ◽  
Author(s):  
L. R. Welp ◽  
R. F. Keeling ◽  
R. F. Weiss ◽  
W. Paplawsky ◽  
S. Heckman
Keyword(s):  
Water Vapor ◽  
Ambient Air ◽  
Gas Mixing ◽  
Inlet System ◽  
Simple Method ◽  
Mixing Ratios ◽  
Peak Broadening ◽  
Order Of Magnitude ◽  
And Performance ◽  
The Impact

Abstract. In preparation for routine deployment in a network of greenhouse gas monitoring stations, we have designed and tested a simple method for drying ambient air to near or below 0.2% (2000 ppm) mole fraction H2O using a Nafion dryer. The inlet system was designed for use with cavity ring-down spectrometer (CRDS) analyzers such as the Picarro model G2301 that measure H2O in addition to their principal analytes, in this case CO2 and CH4. These analyzers report dry-gas mixing ratios without drying the sample by measuring H2O mixing ratio at the same frequency as the main analytes, and then correcting for the dilution and peak broadening effects of H2O on the mixing ratios of the other analytes measured in moist air. However, it is difficult to accurately validate the water vapor correction in the field. By substantially lowering the amount of H2O in the sample, uncertainties in the applied water vapor corrections can be reduced by an order of magnitude or more, thus eliminating the need to determine instrument-specific water vapor correction coefficients and to verify the stability over time. Our Nafion drying inlet system takes advantage of the extra capacity of the analyzer pump to redirect 30% of the dry gas exiting the Nafion to the outer shell side of the dryer and has no consumables. We tested the Nafion dryer against a cryotrap (−97 °C) method for removing H2O and found that in wet-air tests, the Nafion reduces the CO2 dry-gas mixing ratios of the sample gas by as much as 0.1 ± 0.01 ppm due to leakage across the membrane. The effect on CH4 was smaller and varied within ± 0.2 ppb, with an approximate uncertainty of 0.1 ppb. The Nafion-induced CO2 bias is partially offset by sending the dry reference gases through the Nafion dryer as well. The residual bias due to the impact of moisture differences between sample and reference gas on the permeation through the Nafion was approximately −0.05 ppm for CO2 and varied within ± 0.2 ppb for CH4. The uncertainty of this partial drying method is within the WMO compatibility guidelines for the Northern Hemisphere, 0.1 ppm for CO2 and 2 ppb for CH4, and is comparable to experimentally determining water vapor corrections for each instrument but less subject to concerns of possible drift in these corrections.

scholarly journals Portable ozone calibration source independent of changes in temperature, pressure and humidity for research and regulatory applications

2018 ◽  
Vol 11 (8) ◽  
pp. 4797-4807 ◽  
Author(s):  
John W. Birks ◽  
Craig J. Williford ◽  
Peter C. Andersen ◽  
Andrew A. Turnipseed ◽  
Stanley Strunk ◽  
...  
Keyword(s):  
Humidity Sensor ◽  
Pulse Width Modulation ◽  
Uv Light ◽  
Ambient Pressure ◽  
Volumetric Flow Rate ◽  
Ambient Air ◽  
Mixing Ratio ◽  
Calibration Source ◽  
Figures Of Merit ◽  
Mixing Ratios

Abstract. A highly portable ozone (O3) calibration source that can serve as a U.S. EPA level 4 transfer standard for the calibration of ozone analyzers is described and evaluated with respect to analytical figures of merit and effects of ambient pressure and humidity. Reproducible mixing ratios of ozone are produced by the photolysis of oxygen in O3-scrubbed ambient air by UV light at 184.9 nm light from a low-pressure mercury lamp. By maintaining a constant volumetric flow rate (thus constant residence time within the photolysis chamber), the mixing ratio produced is independent of both pressure and temperature and can be varied by varying the lamp intensity. Pulse width modulation of the lamp with feedback from a photodiode monitoring the 253.7 nm emission line is used to maintain target ozone mixing ratios in the range 30–1000 ppb. In order to provide a constant ratio of intensities at 253.7 and 184.9 nm, the photolysis chamber containing the lamp is regulated at a temperature of 40 ∘C. The resulting O3 calibrator has a response time for step changes in output ozone mixing ratio of < 30 s and precision (σp) of 0.4 % of the output mixing ratio for 10 s measurements (e.g., σp=±0.4 ppb for 100 ppb of O3). Ambient humidity was found to affect the output mixing ratio of ozone primarily by dilution of the oxygen precursor. This potential humidity interference could be up to a few percent in extreme cases but is effectively removed by varying the lamp intensity to compensate for the reduced oxygen concentration based on feedback from a humidity sensor.

scholarly journals Measurements of the Total Water Content of Cirrus Clouds. Part I: Instrument Details and Calibration

2006 ◽  
Vol 23 (11) ◽  
pp. 1397-1409 ◽  
Author(s):  
E. M. Weinstock ◽  
J. B. Smith ◽  
D. Sayres ◽  
J. R. Spackman ◽  
J. V. Pittman ◽  
...  
Keyword(s):  
Water Vapor ◽  
Water Content ◽  
Solid Phase ◽  
Ambient Air ◽  
Cirrus Clouds ◽  
Specific Humidity ◽  
Total Water Content ◽  
Total Water ◽  
Measurement Capability ◽  
Mixing Ratios

Abstract This paper describes an instrument designed to measure the sum of gas phase and solid phase water, or total water, in cirrus clouds, and to be mounted in a pallet in the underbelly of the NASA WB-57 research aircraft. The ice water content of cirrus is determined by subtracting water vapor measured simultaneously by the Harvard water vapor instrument on the aircraft. The total water instrument uses an isokinetic inlet to maintain ambient particle concentrations as air enters the instrument duct, a 600-W heater mounted directly in the flow to evaporate the ice particles, and a Lyman-α photofragment fluorescence technique for detection of the total water content of the ambient air. Isokinetic flow is achieved with an actively controlled roots pump by referencing aircraft pressure, temperature, and true airspeed, together with instrument flow velocity, temperature, and pressure. Laboratory calibrations that utilize a water vapor addition system that adds air with a specific humidity tied to the vapor pressure of water at room temperature and crosschecked by axial and radial absorption of Lyman-α radiation at the detection axis are described in detail. The design provides for in-flight validation of the laboratory calibration by intercomparison with total water measured by radial absorption at the detection axis. Additionally, intercomparisons in clear air with the Harvard water vapor instrument are carried out. Based on performance of the Harvard water vapor instrument, this instrument has the detection capability of making accurate measurements of total water with mixing ratios in the mid- to upper troposphere of up to 2500 ppmv and mixing ratios in the lower stratosphere of about 5 ppmv, corresponding to almost three orders of magnitude in measurement capability.

scholarly journals Using in situ GC-MS for analysis of C<sub>2</sub>–C<sub>7</sub> volatile organic acids in ambient air of a boreal forest site

2017 ◽  
Vol 10 (1) ◽  
pp. 281-289 ◽  
Author(s):  
Heidi Hellén ◽  
Simon Schallhart ◽  
Arnaud P. Praplan ◽  
Tuukka Petäjä ◽  
Hannele Hakola
Keyword(s):  
Organic Acids ◽  
Boreal Forest ◽  
Mass Spectrometer ◽  
Detection Limits ◽  
Ambient Air ◽  
Cold Trap ◽  
Mixing Ratios ◽  
Volatile Organic ◽  
Volatile Organic Acids

Abstract. An in situ method for studying gas-phase C2–C7 monocarboxylic volatile organic acids (VOAs) in ambient air was developed and evaluated. Samples were collected directly into the cold trap of the thermal desorption unit (TD) and analysed in situ using a gas chromatograph (GC) coupled to a mass spectrometer (MS). A polyethylene glycol column was used for separating the acids. The method was validated in the laboratory and tested on the ambient air of a boreal forest in June 2015. Recoveries of VOAs from fluorinated ethylene propylene (FEP) and heated stainless steel inlets ranged from 83 to 123 %. Different VOAs were fully desorbed from the cold trap and well separated in the chromatograms. Detection limits varied between 1 and 130 pptv and total uncertainty of the method at mean ambient mixing ratios was between 16 and 76 %. All straight chain VOAs except heptanoic acid in the ambient air measurements were found with mixing ratios above the detection limits. The highest mixing ratios were found for acetic acid and the highest relative variations for hexanoic acid. In addition, mixing ratios of acetic and propanoic acids measured by the novel GC-MS method were compared with proton-mass-transfer time-of-flight mass spectrometer (PTR-TOFMS) data. Both instruments showed similar variations, but differences in the mixing ratio levels were significant.

scholarly journals Measurement of low-ppm mixing ratios of water vapor in the upper troposphere and lower stratosphere using chemical ionization mass spectrometry

2013 ◽  
Vol 6 (6) ◽  
pp. 1461-1475 ◽  
Author(s):  
T. D. Thornberry ◽  
A. W. Rollins ◽  
R. S. Gao ◽  
L. A. Watts ◽  
S. J. Ciciora ◽  
...  
Keyword(s):  
Water Vapor ◽  
Chemical Ionization ◽  
Lower Stratosphere ◽  
Signal To Noise Ratio ◽  
Ion Source ◽  
Ionization Mass ◽  
Measurement Sensitivity ◽  
Particle Radiation ◽  
Upper Troposphere ◽  
Mixing Ratios

Abstract. A chemical ionization mass spectrometer (CIMS) instrument has been developed for the fast, precise, and accurate measurement of water vapor (H2O) at low mixing ratios in the upper troposphere and lower stratosphere (UT/LS). A low-pressure flow of sample air passes through an ionization volume containing an α-particle radiation source, resulting in a cascade of ion-molecule reactions that produce hydronium ions (H3O+) from ambient H2O. The production of H3O+ ions from ambient H2O depends on pressure and flow through the ion source, which were tightly controlled in order to maintain the measurement sensitivity independent of changes in the airborne sampling environment. The instrument was calibrated every 45 min in flight by introducing a series of H2O mixing ratios between 0.5 and 153 parts per million (ppm, 10−6 mol mol−1) generated by Pt-catalyzed oxidation of H2 standards while overflowing the inlet with dry synthetic air. The CIMS H2O instrument was deployed in an unpressurized payload area aboard the NASA WB-57F high-altitude research aircraft during the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) mission in March and April 2011. The instrument performed successfully during seven flights, measuring H2O mixing ratios below 5 ppm in the lower stratosphere at altitudes up to 17.7 km, and as low as 3.5 ppm near the tropopause. Data were acquired at 10 Hz and reported as 1 s averages. In-flight calibrations demonstrated a typical sensitivity of 2000 Hz ppm−1 at 3 ppm with a signal to noise ratio (2 σ, 1 s) greater than 32. The total measurement uncertainty was 9 to 11%, derived from the uncertainty in the in situ calibrations.

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