J. R. Spackman

The Detection Efficiency of the Single Particle Soot Photometer

A single particle soot photometer (SP2) uses an intense laser to heat individual aerosol particles of refractory black carbon (rBC) to vaporization, causing them to emit detectable amounts of thermal radiation that are used to quantify rBC mass. This approach is well suited for the detection of the majority of rBC mass loading in the ambient atmosphere, which occurs primarily in the accumulation mode (∼ 1–300 fg-rBC/particle). In addition to operator choices about instrument parameters, SP2 detection of rBC number and/or mass can be limited by the physical process inherent in the SP2 detection technique — namely at small rBC mass or low laser intensities, particles fail to heat to vaporization, a requirement for proper detection. In this study, the SP2s ability to correctly detect and count individual flame-generated soot particles was measured at different laser intensities for different rBC particle masses. The flame-generated soot aerosol used for testing was optionally prepared with coatings of organic and non-organic material and/or thermally denuded. These data are used to identify a minimum laser intensity for accurate detection at sea level of total rBC mass in the accumulation mode (300 nW/(220-nm PSL)), a minimum rBC mass (∼ 0.7-fg rBC-mass corresponding to 90 nm volume-equivalent diameter) for near-unity number detection efficiency with a typical operating laser intensity (450 nW/(220-nm PSL)), and a methodology using observed color temperature to recognize laser intensity insufficient for accurate rBC mass detection. Additionally, methods for measurement of laser intensity using either laboratory or ambient aerosol are presented.

Black carbon aerosol size in snow

The effect of anthropogenic black carbon (BC) aerosol on snow is of enduring interest due to its consequences for climate forcing. Until now, too little attention has been focused on BCs size in snow, an important parameter affecting BC light absorption in snow. Here we present first observations of this parameter, revealing that BC can be shifted to larger sizes in snow than are typically seen in the atmosphere, in part due to the processes associated with BC removal from the atmosphere. Mie theory analysis indicates a corresponding reduction in BC absorption in snow of 40%, making BC size in snow the dominant source of uncertainty in BCs absorption properties for calculations of BCs snow albedo climate forcing. The shift reduces estimated BC global mean snow forcing by 30%, and has scientific implications for our understanding of snow albedo and the processing of atmospheric BC aerosol in snowfall.

Validation of the Harvard Lyman-α in situ water vapor instrument: Implications for the mechanisms that control stratospheric water vapor

Building on previously published details of the laboratory calibrations of the Harvard Lyman-α photofragment fluorescence hygrometer (HWV) on the NASA ER-2 and WB-57 aircraft, we describe here the validation process for HWV, which includes laboratory calibrations and intercomparisons with other Harvard water vapor instruments at water vapor mixing ratios from 0 to 10 ppmv, followed by in-flight intercomparisons with the same Harvard hygrometers. The observed agreement exhibited in the laboratory and during intercomparisons helps corroborate the accuracy of HWV. In light of the validated accuracy of HWV, we present and evaluate a series of intercomparisons with satellite and balloon borne water vapor instruments made from the upper troposphere to the lower stratosphere in the tropics and midlatitudes. Whether on the NASA ER-2 or WB-57 aircraft, HWV has consistently measured about 1–1.5 ppmv higher than the balloon-borne NOAA/ESRL/GMD frost point hygrometer (CMDL), the NOAA Cryogenic Frost point Hygrometer (CFH), and the Microwave Limb Sounder (MLS) on the Aura satellite in regions of the atmosphere where water vapor is <10 ppmv. Comparisons in the tropics with the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite show large variable differences near the tropopause that converge to ~10% above 460 K, with HWV higher. Results we show from the Aqua Validation and Intercomparison Experiment (AquaVIT) at the AIDA chamber in Karlsruhe do not reflect the observed in-flight differences. We illustrate that the interpretation of the results of comparisons between modeled and measured representations of the seasonal cycle of water entering the lower tropical stratosphere is dictated by which data set is used.

Comparison of ER‐2 aircraft and POAM III, MLS, and SAGE II satellite measurements during SOLVE using traditional correlative analysis and trajectory hunting technique

We compared the version 5 Microwave Limb Sounder (MLS), version 3 Polar Ozone and Aerosol Measurement III (POAM III), version 6.0 Stratospheric Aerosol and Gas Experiment II (SAGE II), and NASA ER-2 aircraft measurements made in the Northern Hemisphere in January–February 2000 during the SAGE III Ozone Loss and Validation Experiment (SOLVE). This study addresses one of the key scientific objectives of the SOLVE campaign, namely, to validate multiplatform satellite measurements made in the polar stratosphere during winter. This intercomparison was performed by using a traditional correlative analysis (TCA) and a trajectory hunting technique (THT). TCA compares profiles colocated within a chosen spatial-temporal vicinity. Launching backward and forward trajectories from the points of measurement, the THT identifies air parcels sampled at least twice within a prescribed match criterion during the course of 5 days. We found that the ozone measurements made by these four instruments agree most of the time within ±10% in the stratosphere up to 1400 K (∼35 km). The water vapor measurements from POAM III and the ER-2 Harvard Lyman α hygrometer and Jet Propulsion Laboratory laser hygrometer agree to within ±0.5 ppmv (or about ±10%) in the lower stratosphere above 380 K. The MLS and ER-2 ClO measurements agree within their error bars for the TCA. The MLS and ER-2 nitric acid measurements near 17- to 20-km altitude agree within their uncertainties most of the time with a hint of a positive offset by MLS according to the TCA. We also applied the Atmospheric and Environmental Research, Inc. box model constrained by the ER-2 measurements for analysis of the ClO and HNO3 measurements using the THT. We found that: (1) the model values of ClO are smaller by about 0.3–0.4 (0.2) ppbv below (above) 400 K than those by MLS and (2) the HNO3 comparison shows a positive offset of MLS values by ∼1 and 1–2 ppbv below 400 K and near 450 K, respectively. Our study shows that, with some limitations (like HNO3 comparison under polar stratospheric cloud conditions), the THT is a more powerful tool for validation studies than the TCA, making conclusions of the comparison statistically more robust.

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

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 vapo...

A new photolysis laser-induced fluorescence instrument for the detection of H2O and HDO in the lower stratosphere

We present a new instrument, Hoxotope, for the in situ measurement of H(2)O and its heavy deuterium isotopologue (HDO) in the upper troposphere and lower stratosphere aboard the NASA WB-57. Sensitive measurements of deltaD are accomplished through the vacuum UV photolysis of water followed by laser-induced fluorescence detection of the resultant OH and OD photofragments. The photolysis laser-induced fluorescence technique can obtain S/N>20 for 1 ppbv HDO and S/N>30 for 5 ppmv H(2)O for 10 s data, providing the sensitivity required for deltaD measurements in the tropopause region. The technique responds rapidly to changing water concentrations due to its inherently small sampling volume, augmented by steps taken to minimize water uptake on instrument plumbing. Data from the summer 2005 Aura Validation Experiment Water Isotope Intercomparison Flights (AVE-WIIF) out of Houston, TX show agreement for H(2)O between Hoxotope and the Harvard water vapor instrument and for HDO between Hoxotope and the Harvard ICOS water isotope instrument, to within stated instrument uncertainties. The successful intercomparison validates Hoxotope as a credible source of deltaD data in the upper troposphere and lower stratosphere.