The PAN-CIGAR chemical ionization mass spectrometer which measures up to 7 PAN species simultaneously and semi-continuously with a time resolution of ~2 seconds. The method is based on the detection of the acylperoxy radicals formed from thermal decomposition of the PAN species at the inlet by reacting them with iodide ions, which are formed by passing methyl iodide diluted in nitrogen through an α–particle source. The reaction of the peroxy acyl radicals with I- forms IO and the acyl ion, which is detected using a quadrupole mass spectrometer (Extrel) at a mass to charge ratio of 59 in the case of PAN. The method is very specific for PAN type compounds and the limit of detection is ~1 pptv/s or better for most PAN species. The instrument employs a realtime continuous calibration using isotopically labeled PAN produced in-situ by a photolytic calibration source.

1) Time of Flight Aerosol Mass Spectrometer (ToF-AMS)

Total and single particle characterization of volatile aerosol ionic and organic components (50-700nm). Uncertainty depends on species and concentration.

2) Single Particle Soot Photometer (SP2)

Single particle measure of BC (soot) mass in particles and determination of mixed particle size and non-BC coating using laser scattering and incandescence. 70-700nm. Single particle counting up to 10,000 per sec.

3) A size-resolved thermo-optic aerosol discriminator (30 s avg.):

Aerosol size distribution from 0.12 up to 7.0 μm, often where most aerosol mass, surface area and optical effects are dominant. Uses a modified Laser Optical Particle Counter (OPC) and computer controlled thermal conditioning system is used upstream (airstream dilution dried). Characterizes aerosol components volatile at 150, 300 and 400C and refractory aerosol at 400C (sea salt, dust and soot/flyash). (Clarke, 1991, Clarke et al., 2004). Uncertianty about 15%

4) Condensation Nuclei - heated and unheated (available at 1Hz)

Two butanol based condensation nuclei (CN) counter (TSI 3010) count all particles between 0.01-3.0 um. Total CN, refractory CN (those remaining at 300C after sulfate is removed) and volatile CN (by difference) are obtained as a continuous readout as a fundamental air mass indicator (Clarke et al. 1996). Uncertainty ~ 5%.

5) Aerodynamic Particle Sizer – (APS-TSI3320) – (<5min/scan)

To further characterize larger “dry” particles, including dust, an APS is operated which sizes particles aerodynamically from 0.8 to 20 μm into 50 channels. Uncertainty~10%.

6) Differential Mobility Analyzer with thermal conditioning – (<3 min/scan)

Volatility tandem thermal differential mobility analyzer (VTTDMA) with thermal analysis that provides size information (mass, surface area, number distributions) and their state of mixing over the 0.01 to 0.3μm size range (Clarke et al., 1998, 2007) for sampling times of about 1-3 minutes. Uncertainty ~10%

7) Nephelometer (10-7 m-1 detection for 60s avg., recorded every 1 sec.)

A 3 wavelength nephelometer (450, 550, 700nm) is used for total scattering and submicrometer scattering values using a Radiance Research single wavelength nephelometer (and thereby coarse dust scattering by difference).

8) Two Particle Soot Absorption Photometers (PSAP-Radiance Research; detection <0.1μg m-3 for 5 min. avg. )

The PSAP is used to quantify the spectral light absorption coefficient of the total and submicron aerosol (eg. soot, BC) at three wavelengths (450, 550, 660nm).

9) Humidity Dependent Light-Scattering (10-6 m-1 detection for 60s avg.; recorded every 1 s)

Two additional Radiance Research single-wavelength nephelometers are operated at two humidities (high/low) to establish the humidity dependence of light scattering, f(RH).

The DLH has been successfully flown during many previous field campaigns on several aircraft, most recently ACTIVATE (Falcon); FIREX-AQ, ATom, KORUS-AQ, and SEAC4RS (DC-8); POSIDON (WB-57); CARAFE (Sherpa); CAMP2Ex and DISCOVER-AQ (P-3); and ATTREX (Global Hawk). This sensor measures water vapor (H2O(v)) via absorption by one of three strong, isolated spectral lines near 1.4 μm and is comprised of a compact laser transceiver and a sheet of high grade retroflecting road sign material to form the optical path. Optical sampling geometry is aircraft-dependent, as each DLH instrument is custom-built to conform to aircraft geometric constraints. Using differential absorption detection techniques, H2O(v) is sensed along the 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 found in the literature and/or measured in the laboratory. Preliminary water vapor mixing ratio and derived relative humidities are provided in real-time to investigators.

The in‐situ diode laser spectrometer system, referred to by its historical name DACOM, includes three tunable diode lasers providing 4.7, 4.5, and 3.3 μm radiation for accessing CO, N2O, and CH4 absorption lines, respectively. The three laser beams are combined by the use of dichroic filters and are then directed through a small volume (0.3 liter) Herriott cell enclosing a 36 meter optical path. As the three coincident laser beams exit the absorption cell, they are spectrally isolated using dichroic filters and are then directed to individual detectors, one for each laser wavelength. Wavelength reference cells containing CO, CH4, and N2O are used to wavelength lock the operation of the three lasers to the appropriate absorption lines. Ambient air is continuously drawn through a Rosemount inlet probe and a permeable membrane dryer which removes water vapor before entering the Herriott cell and subsequently being exhausted via a vacuum pump to the aircraft cabin. To minimize potential spectral overlap from other atmospheric species, the Herriott cell is maintained at a reduced pressure of ~90 Torr. At 5 SLPM mass flow rate, the absorption cell volume is exchanged nominally twice per second. Frequent but short calibrations with well documented and stable reference gases are critical to achieving both high precision and accuracy. Calibration for all species is accomplished by periodically (~4 minutes) flowing calibration gas through this instrument. Measurement accuracy is closely tied to the accuracy of the reference gases obtained from NOAA/ESRL, Boulder, CO. Both CO and CH4 mixing ratios are provided in real-time to investigators aboard the DC‐8.

COBALT makes measurements using off-axis integrated output spectroscopy.

Developed by Droplet Measurement Technologies, the CFSTGC is based on a concept by Roberts and Nenes [2005]. The instrument counts the fraction of aerosol particles that become droplets when exposed to a given water vapor supersaturation (RH > 100%).

As with all CCN counters, a temperature gradient is applied to produce a supersaturation of water vapor. However, the mechanism for generating supersaturation is not the same for all CCN counters. For example, for continuous flow parallel plate diffusion chambers, the temperature gradient is perpendicular to the flow, and supersaturation is a result of the nonlinear dependence of vapor pressure upon temperature. The same mechanism applies for static diffusion cloud chambers, where there is no flow at all.

However, as the name implies, for the Continuous Flow Streamwise Thermal Gradient CCN Counter, the temperature gradient is in the streamwise direction (maintained by thermoelectric coolers). In this case, supersaturation results as a consequence of the greater rate of mass transfer over heat transfer.

With laminar flow, heat and water vapor are transferred to the centerline of the column from the walls only by diffusion.

Since molecular diffusivity is greater than thermal diffusivity, the distance downstream that a water molecule travels before reaching the centerline is less than the distance the heat travels downstream before reaching the centerline. If you pick a point at the centerline, the heat originated from a greater distance upstream than the water vapor.

There are four facts that are necessary to explain how supersaturation is generated within the CFSTGC:

1) Assuming that the inner surface of the column is saturated with water vapor at all points, since the temperature is greater at point B than at point A, the water vapor partial pressure is also greater at point B than at point A.

2) The actual partial pressure of water vapor at point C is equal to the partial pressure of water vapor at point B.

3) However, since the temperature at point C is the same as at point A, the equilibrium water vapor pressure at point C is equal to the water vapor partial pressure at point A.

4) The saturation ratio is the ratio between the actual partial pressure of water vapor and the equilibrium vapor pressure. This is equivalent to the partial pressure at point B divided by the partial pressure at point A, which is always greater than one. Thus supersaturation is generated through a dynamic equilibrium.

The Broadband Radiometers (BBR) consist of modified Kipp & Zonen CM-22 pyranometers (to measure solar irradiance) and CG-4 pyrgeometers (to measure IR irradiance) (see http://www.kippzonen.com/). The modifications to make these instruments more suitable for aircraft use include new instrument housings and amplification of the signal at the sensor. The instruments are run in current-loop mode to minimize the effects of noise in long signal cables. The housing is sealed and evacuated to prevent condensation or freezing inside the instrument. Each BBR has the following properties: Field-of-view: Hemispheric Temperature Range: -65C to +80C Estimated Accuracy: 3-5% Data Rate: 1Hz

The Charged-coupled device Actinic Flux Spectroradiometers (CAFS) instruments measure in situ down- and up-welling radiation and combine to provide 4 pi steradian actinic flux density spectra from 280 to 650 nm. The sampling resolution is ~0.8 nm with a full width at half maximum (FWHM) of 1.7 nm at 297 nm. From the measured flux, photolysis frequencies are calculated for ~40 important atmospheric trace gases including O3, NO2, HCHO, HONO and NO3 using a modified version of the NCAR Tropospheric Ultraviolet and Visible (TUV) radiative transfer model. The absolute spectral sensitivity of the instruments is determined in the laboratory with 1000 W NIST-traceable tungsten-halogen lamps with a wavelength dependent uncertainty of 3–5%. During deployments, spectral sensitivity is assessed with secondary calibration lamps while wavelength assignment is tracked with Hg line sources and comparisons to spectral features in the extraterrestrial flux. The optical collectors are characterized for angular and azimuthal response and the effective planar receptor distance. CAFS have an excellent legacy of performance on the NASA DC-8 and WB-57 platforms during atmospheric chemistry and satellite validation mission. These include AVE Houston 2004 and 2005, PAVE, CR-AVE, TC4, ARCTAS, DC3, SEAC4RS, KORUS-AQ, ATom and FIREX-AQ. For FIREX-AQ, upgraded electronics and cooling reduced noise and allowed for a decrease to 1 Hz acquisition.

CAR is a multi-wavelength scanning radiometer for determining albedo of clouds in the visible and near-infrared and measuring the angular distribution of scattered radiation and bidirectional reflectance of various surface types. It acquires imagery of cloud and Earth surface features.

For details, visit: https://car.gsfc.nasa.gov/

The single mass analyzer CIMS (S-CIMS) was developed for use on NASA’s ER-2 aircraft. Its first measurements were made in 2000 (SOLVE, see photo). Subsequently, it has flown on the NASA DC-8 aircraft for INTEX-NA, DICE, TC4, ARCTAS, ATom, KORUS, FIREX, as well as on the NCAR C-130 during MILAGRO/INTEX-B. HNO3 is measured by selective ion chemical ionization via the fluoride transfer reaction: CF3O^- + HNO3 → HF • NO3^- + CF2O In addition to its fast reaction rate with HNO3, CF3O^- can be used to measure additional acids and nitrates as well as SO2 [Amelynck et al., 2000; Crounse et al., 2006; Huey et al., 1996]. We have further identified CF3O^- chemistry as useful for the measurement of less acidic species via clustering reactions [Crounse et al., 2006; Paulot et al., 2009a; Paulot et al., 2009b; St. Clair et al., 2010]: CF3O^- + HX → CF3O^- • HX where, e.g., HX = HCN, H2O2, CH3OOH, CH3C(O)OOH (PAA) The mass analyzer of the S-CIMS instrument was first upgraded from a quadrupole to a unit-mass resolution time-of-flight (ToF) analyzer. In 2023, the mass filter was again upgraded to an 1m flight path (~5000 deltaM/M).  The ToF admits the sample ion beam to the ion extractor, where a pulse of high voltage orthogonally deflects and accelerates the ions into the reflectron, which in turn redirects the ions toward the multichannel plate detector. Ions in the ToF follow a V-shaped from the extractor to detector, separating by mass as the smaller ions are accelerated to greater velocities by the high voltage pulse. The detector collects the ions as a function of time following each extractor pulse. The rapid-scan collection of the ToF guarantees a high temporal resolution (1 Hz or faster) and simultaneous data products from the S-CIMS instrument for all mass channels [Drewnick et al., 2005]. We have flown a tandem CIMS (TCIMS) instrument in addition to the SCIMS since INTEX-B (2006). The T-CIMS provides parent-daughter mass analysis, enabling measurement of compounds precluded from quantification by the S-CIMS due to mass interferences (e.g. MHP) or the presence of isobaric compounds (e.g. isoprene oxidation products) [Paulot et al., 2009b; St. Clair et al., 2010]. Calibrations of both CIMS instruments are performed in flight using isotopically-labeled reagents evolved from a gas cylinders or from a thermally-stabilized permeation tube oven [Washenfelder et al., 2003]. By using an isotopically labeled standard, the product ion signals are distinct from the natural analyte and calibration can be performed at any time without adversely affecting the ambient measurement.