Darrel Baumgardner

Particle size distributions in Arctic polar stratospheric clouds, growth and freezing of sulfuric acid droplets, and implications for cloud formation

Particle size and volume measurements obtained with the forward scattering spectrometer probe (FSSP), model 300 during January and February 1989 in the Airborne Arctic Stratospheric Experiment are presented and used to study processes important in the formation and growth of polar stratospheric cloud (PSC) particles. Comparisons of the observations with expected sulfuric acid droplet deliquescence suggest that in the Arctic a major fraction of the sulfuric acid droplets remain liquid until temperatures at least as low as 193 K. Arguments are presented to suggest that homogeneous freezing of the sulfuric acid droplets might occur near 190 K and might play a role in the formation of PSCs. The first suggestion of nitric acid trihydrate (NAT) particles appears near saturation ratios of HNO3 with respect to NAT of 1 (about 195 K) as an enhancement, of the large particles on the tail of the sulfuric acid droplet size distribution. The major increases in number and volume indicative of the main body of the NAT cloud are not seen in these Arctic investigations until 191 to 192 K, which corresponds to an apparent saturation ratio of HNO3 with respect to NAT of about 10, unlike the Antarctic where clouds were encountered at saturation ratios near 1. A decrease in the number of particles was observed in regions in which the airmass was denitrified, i.e. NOy, the sum of all reactive nitrogen species, was reduced. This was especially true for the larger particles on the upper tail of the sulfate size distribution. The loss of these largest particles supports the idea that denitrification may be the result of the preferential nucleation and growth of NAT on only the largest sulfate particles, which then sediment out of the airmass.

A study of new particle formation and growth involving biogenic and trace gas species measured during ACE 1

Measurements are presented of ambient nanoparticle distributions (2.7 to 10 nm diameter) in regions of high biogenic emissions encountered during the First Aerosol Characterization Experiment (ACE 1), November 15 to December 14, 1995. Large numbers of newly formed nanoparticles were observed directly downwind of penguin colonies on Macquarie Island (54.5thinsp{degree}S, 159.0thinsp{degree}W). In these regions, nanoparticle concentrations were also correlated with sulfuric acid (H{sub 2}SO{sub 4(g)}) gas concentrations. The measurements show that biogenic species, possibly ammonia (NH{sub 3}), either by itself or with H{sub 2}SO{sub 4}, nucleated to form new particles at rates much higher than bimolecular H{sub 2}SO{sub 4}/H{sub 2}O nucleation. Nanoparticle distributions evolved as air was advected away from the island showing clear evidence of growth of the newly formed particles. Observed growth rates were in the range of 2 to 5 nmthinsph{sup {minus}1} and were about a factor of 4 to 17 times higher than the growth by condensing H{sub 2}SO{sub 4(g)} and associated water. The cause for fast growth of the newly formed particles is unknown. {copyright} 1998 American Geophysical Union

Evaluation of the Forward Scattering Spectrometer Probe. Part I: Electronic and Optical Studies

Abstract Laboratory studies of the Forward Scattering Spectrometer Probe (FSSP), were conducted to better understand the operation, to determine limitations and to define the measurement accuracy of the instrument for airborne cloud physics research. The studies included electronic cheeks of the instrument sensitivity to simulated particles of different sizes, airspeeds and arrival rates; measurement of important aspects of the optical configuration; and intercomparisons of six different FSSPs in a small wind tunnel with a droplet spray. The tests demonstrated measurement differences between various probes in several areas as well as areas in which there was reasonable agreement. Part of the differences can be attributed to different feature and design specifications of different probes as changes were made to improve the FSSP. Areas in which care needs to be taken in the calibration and processing of data from the FSSP are identified.

Evaluation of the Forward Scattering Spectrometer Probe. Part II: Corrections for Coincidence and Dead-Time Losses

Abstract Cloud particle concentrations measured by the Forward Scattering Spectrometer Probe (FSSP) can be underestimated when particles are either coincident or pass through the sensing area of the probe during the electronic dead-time. In the absence of any corrections, the differences between actual and measured concentrations can typically exceed 15% when aircraft mounted probes measure droplet concentrations > 500 cm−3. The sources of counting losses are described and correctional procedures derived and demonstrated.

Interpretation of measurements made by the forward scattering spectrometer probe (FSSP‐300) during the Airborne Arctic Stratospheric Expedition

An improved forward scattering spectrometer probe, the FSSP-300, was developed for the Airborne Arctic Stratospheric Expedition. The 300 measures particles in the size range 0.3 μm to 20 μm and has a greater sensitivity and faster time response than its predecessor, the FSSP-100X. An intensive characterization of this probes operating characteristics has been made and its limitations evaluated. Measurements from this probe are affected by Mie scattering ambiguities and index of refraction uncertainties, nonuniform laser intensity, uncertainties in sample volume definition, and time response roll-off. Correction algorithms have been developed to account for some of the probe limitations. After applying these corrections, the uncertainties in number and mass concentration are on the order of 25% and 60%, respectively.

Chemical loss of ozone in the arctic polar vortex in the winter of 1991-1992.

In situ measurements of chlorine monoxide, bromine monoxide, and ozone are extrapolated globally, with the use of meteorological tracers, to infer the loss rates for ozone in the Arctic lower stratosphere during the Airborne Arctic Stratospheric Expedition II (AASE II) in the winter of 1991-1992. The analysis indicates removal of 15 to 20 percent of ambient ozone because of elevated concentrations of chlorine monoxide and bromine monoxide. Observations during AASE II define rates of removal of chlorine monoxide attributable to reaction with nitrogen dioxide (produced by photolysis of nitric acid) and to production of hydrochloric acid. Ozone loss ceased in March as concentrations of chlorine monoxide declined. Ozone losses could approach 50 percent if regeneration of nitrogen dioxide were inhibited by irreversible removal of nitrogen oxides (denitrification), as presently observed in the Antarctic, or without denitrification if inorganic chlorine concentrations were to double.

An Analysis and Comparison of Five Water Droplet Measuring Instruments

Abstract Five different instruments have been used routinely by the University of Wyoming for aircraft measurements of water droplets. A study has been conducted in which the measurement accuracies of these instruments are analyzed and evaluated. This study includes an analysis of the instruments’ operating characteristics, inherent limitations and calibration errors. Data from these instruments are compared and discussed with an emphasis on the effect of measurement uncertainties. The instruments studied were the Axially Scattering Spectrometer Probe (ASSP), the Forward Scattering Spectrometer Probe (FSSP), the cloud gun droplet impactor, the J-W liquid-water probe, and the CSIRO liquid-water probe.

Airspeed Corrections for Optical Array Probe Sample Volumes

The Particle Measuring System’s optical array probes have a sample volume that depends upon the diameter of the particle measured. The sample volume also depends upon the velocity of particles that pass through the probe because of the electronic response time of these instruments. This note discusses an algorithm that has been derived to calculate sample volume as a function of size and velocity, and demonstrates the need for such an algorithm by comparison of measurements from several types of optical array probes and a forward-scattering spectrometer probe. These comparisons show that the optical array probes greatly underestimate droplet concentrations of particles less than 100 mm in diameter at typical aircraft research speeds unless sample volumes are adjusted for electronic response time limitations.

Analysis of the physical state of one Arctic polar stratospheric cloud based on observations

During the Arctic Airborne Stratospheric Expe- dition, simultaneous measurements of aerosol size distribution and NOy (HNO 3 + NO + NO 2 + 2.N20 5) were made along ER-2 flight paths. The flow characteristics of the NOy instrument allow us to derive the condensed NOy amount (assumed to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24 th flight indicates that this condensed HNO 3 amount does not agree well with the aerosol volume if the observed PSCs are composed of solid nitric acid trihydrate (NAT), as is generally assumed. However, the composition agrees well with that predicted for liquid H2SO4/I-INO3/I-I2 ¸ solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variations in temperature and humidity. The weight percentages of H2SO 4, HNO 3, and H20 derived from the measurements all correspond to those predicted for ternary, liquid solutions.

Near‐field measurements on contrail properties from fuels with different sulfur content

Microphysical properties of jet exhaust aerosol and contrails were studied in the near field of the emitting aircraft for different fuel sulfur contents. Measurements were performed behind two different aircraft (ATTAS test aircraft of type VFW 614 and Airbus A310-300) using fuels with sulfur contents of 6 ppm and 2700 ppm, respectively. At closest approach (plume age ‹ 1 s), the total number concentrations exceeded the measuring range of the condensation particle counter, i.e., N › 10 5 cm -3 . The concentration of the dry accumulation mode aerosol, i.e., predominantly soot particles, was not affected by the fuel sulfur content. At a plume age of 10 s, an increase in total number concentration (D p › 0.01 µm) by a factor of 3.5 in the high sulfur case compared to the low sulfur case was observed. The ultrafine condensation nuclei fraction (0.007 µm ‹ D p ‹ 0.018 µm) contributed at maximum 70% to the total aerosol in the plume while this fraction was much less outside the plume. The high fuel sulfur content also caused an increase in the typical number concentrations of contrail particles by about one third with respect to low sulfur fuel, while the effective diameter of the size distribution was lowered at a fuel sulfur independent ice water content. The major differences in accumulation mode aerosol and microphysical contrail properties between the used aircraft were an increased number concentration of both the accumulation mode aerosol and the contrail particles in the Airbus A310-300 plume relative to the ATTAS plume. Part of the difference in contrail particles may be caused by different ambient conditions, but the major differences are assumed to be caused by different engine and wake properties.