ATom: L2 In Situ Atmospheric Water Vapor from the Diode Laser Hygrometer (DLH)

This dataset provides the concentrations of water measured by the Diode Laser Hygrometer (DLH) during airborne campaigns conducted by NASA's Atmospheric Tomography (ATom) mission from 2016 - 2018. The DLH measures the water vapor (H2O(v)) in the atmosphere by wavelength modulated differential absorption spectroscopy of an isolated rovibrational line. The measurements include water vapor mixing ratio in parts-per-trillion-by-volume (pptv) and relative humidity in percent.

The DLH is an open path airborne tunable diode laser-based instrument which operates in the near-infrared spectral region at a wavelength of approximately 1.4 micrometers. The DLH laser is modulated at approximately 4 kHz and operates in a ‘line-locked’ mode; the signal is demodulated at twice the modulation frequency (2F detection) to provide good sensitivity and rapid time response. The returned laser power (DC) is also measured. The DLH is calibrated in the laboratory at various combinations of pressure and water vapor density. From the calibration data and a multiparameter spectral model, a set of coefficients is developed, and these coefficients are used to convert the measured 2F/DC ratio, along with local temperature and pressure (which are measured by separate instruments onboard the aircraft), to water vapor mixing ratio.

This dataset includes 96 files in comma-delimited text (ICARTT) format, with two files per flight.

This dataset provides the concentrations of water measured by the Diode Laser Hygrometer (DLH) during airborne campaigns conducted by NASA's Atmospheric Tomography (ATom) mission from 2016 - 2018. The DLH measures the water vapor (H2O(v)) in the atmosphere by wavelength modulated differential absorption spectroscopy of an isolated rovibrational line. The measurements include water vapor mixing ratio in parts-per-trillion-by-volume (pptv) and relative humidity in percent.

The DLH is an open path airborne tunable diode laser-based instrument which operates in the near-infrared spectral region at a wavelength of approximately 1.4 micrometers. The DLH laser is modulated at approximately 4 kHz and operates in a ‘line-locked’ mode; the signal is demodulated at twice the modulation frequency (2F detection) to provide good sensitivity and rapid time response. The returned laser power (DC) is also measured. The DLH is calibrated in the laboratory at various combinations of pressure and water vapor density. From the calibration data and a multiparameter spectral model, a set of coefficients is developed, and these coefficients are used to convert the measured 2F/DC ratio, along with local temperature and pressure (which are measured by separate instruments aboard the aircraft), to water vapor mixing ratio.

Project: Atmospheric Tomography Mission (ATom)

The Atmospheric Tomography Mission (ATom) was a NASA Earth Venture Suborbital-2 mission. It studied the impact of human-produced air pollution on greenhouse gases and on chemically reactive gases in the atmosphere. ATom deployed an extensive gas and aerosol payload on the NASA DC-8 aircraft for systematic, global-scale sampling of the atmosphere, profiling continuously from 0.2 to 12 km altitude. Flights occurred in each of four seasons over a 4-year period.

Related Data:

ATom: Merged Atmospheric Chemistry, Trace Gases, and Aerosols. Data from all ATom instruments and all four flight campaigns, including aircraft location and navigation data, merged to several different time bases: https://doi.org/10.3334/ORNLDAAC/1581  

ATom Flight Track and Navigational Data. Flight path (location and altitude) data for each of the four campaigns provided in KML and csv format: https://doi.org/10.3334/ORNLDAAC/1613

Spatial Coverage: Global. Flights circumnavigate the globe, primarily over the oceans

Spatial Resolution: Point measurements

Temporal Coverage: Periodic flights occurred during each campaign

Table 1. Flight campaign schedule

Deployment	Date Range
ATom-1	July 29 - August 23, 2016
ATom-2	January 26 - February 21, 2017
ATom-3	September 28 - October 28, 2017
ATom-4	April 24 - May 21, 2018

Temporal Resolution: 1 second

Data File Information

This dataset includes 96 files in comma-delimited text (ICARTT) format, with two files per flight date for all four ATom flight campaigns. Data files conform to the ICARTT File Format Standards V1.1.

Files are provided at two sampling resolutions: 1 Hz and 20 Hz

DLH-H20_DC8_YYYYMMDD_R#.ict  --  water vapor mixing ratio and relative humidity averaged to 1 second temporal resolution.

DLH-H20-20Hz_DC8_YYYYMMDD_R#.ict -- water vapor mixing ratio at 20 Hz temporal resolution.

where YYYYMMDD is the start date (in UTC time) of the flight and R# is the file version or revision number. There are 48 files (one for each flight date) of each type.

Data Variables

Missing data are indicated by -9999.000.

Table 2. Variables in the data files DLH-H2O_DC8_YYYYMMDD_R#.ict.

Name	Units	Description
Time_UTC	seconds	seconds since midnight UTC
H2O_DLH	ppmv	water vapor mixing ratio
RHi_DLH	percent	relative humidities with respect to ice
RHw_DLH	percent	relative humidities with respect to liquid

 

Table 3. Variables in the data files DLH-H2O-20Hz_DC8_YYYYMMDD_R#.ict.

Name	Units	Description
Time_UTC	seconds	seconds since midnight UTC
H2O_DLH	ppmv	water vapor mixing ratio

ATom builds the scientific foundation for mitigation of short-lived climate forcers, in particular methane (CH4), tropospheric ozone (O3), and Black Carbon aerosols (BC).

ATom Science Questions

Tier 1

• What are chemical processes that control the short-lived climate forcing agents CH4, O3, and BC in the atmosphere? How is the chemical reactivity of the atmosphere on a global scale affected by anthropogenic emissions? How can we improve chemistry-climate modeling of these processes?

Tier 2

• Over large, remote regions, what are the distributions of BC and other aerosols important as short-lived climate forcers? What are the sources of new particles? How rapidly do aerosols grow to CCN-active sizes? How well are these processes represented in models?
• What type of variability and spatial gradients occur over remote ocean regions for greenhouse gases (GHGs) and ozone depleting substances (ODSs)? How do the variations among air parcels help identify anthropogenic influences on photochemical reactivity, validate satellite data for these gases, and refine knowledge of sources and sinks?

Significance

ATom delivers unique data and analysis to address the Science Mission Directorate objectives of acquiring “datasets that identify and characterize important phenomena in the changing Earth system” and “measurements that address weaknesses in current Earth system models leading to improvement in modeling capabilities.” ATom will provide unprecedented challenges to the CCMs used as policy tools for climate change assessments, with comprehensive data on atmospheric chemical reactivity at global scales, and will work closely with modeling teams to translate ATom data to better, more reliable CCMs. ATom provides extraordinary validation data for remote sensing.

The water vapor volumetric mixing ratio in ppmv was measured using the formula: p(H2O) / p(ambient) * 10^6. Relative humidities are calculated from mixing ratio, project static pressure, and project static temperature. Uncertainty for H2O(v) is 5% and for the relativity humidity it is 15%.

Project Overview

ATom makes global-scale measurements of the chemistry of the atmosphere using the NASA DC-8 aircraft. Flights span the Pacific and Atlantic Oceans, nearly pole-to-pole, in continuous profiling mode, covering remote regions that receive long-range inputs of pollution from expanding industrial economies. The payload has proven instruments for in situ measurements of reactive and long-lived gases, diagnostic chemical tracers, and aerosol size, number, and composition, plus spectrally resolved solar radiation and meteorological parameters.

Combining distributions of aerosols and reactive gases with long-lived GHGs and ODSs enables disentangling of the processes that regulate atmospheric chemistry: emissions, transport, cloud processes, and chemical transformations. ATom analyzes measurements using customized modeling tools to derive daily averaged chemical rates for key atmospheric processes and to critically evaluate Chemistry-Climate Models (CCMs). ATom also differentiates between hypotheses for the formation and growth of aerosols over the remote oceans.

Diode Laser Hygrometer​

Instrument	Full Name	Contact Person	Type	Measurements	Data Variables
DLH	Diode Laser Hygrometer	Glenn S. Diskin	laser absorption	water vapor	H2O

The DLH measures water vapor (H2O(v)) via absorption by one of three strong, isolated lines in the (101) combination band near 1.4 μm and is comprised of a compact laser transceiver mounted to a DC-­8 window plate and a sheet of high grade retroflecting road sign material applied to an outboard DC-8 engine housing to complete the optical path. Using differential absorption detection techniques, H2O(v) is sensed along the 28.5m external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters that are measured in the laboratory. Preliminary water vapor mixing ratio and derived relative humidities are provided in real-time to investigators aboard the DC-8. More information is provided in Glenn et al. (2002).

Glenn S. Diskin, James R. Podolske, Glen William Sachse, Thomas A. Slate, "Open-path airborne tunable diode laser hygrometer," Proc. SPIE 4817, Diode Lasers and Applications in Atmospheric Sensing, (23 September 2002); https://doi.org/10.1117/12.453736