B. W. Gandrud

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.

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.

Ice nucleation processes in upper tropospheric wave‐clouds observed during SUCCESS

We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical simulations. Observations of high supersaturations with respect to ice (>50%) near the leading edge of a very cold wave cloud (T <−60°C) are approximately consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4 cm−3) are consistent with the number densities calculated in our model. In the warmer wave-cloud (T ≃−37°C) relatively large ice number densities were observed (20–40 cm−3). Our model calculations suggest that these large number densities are probably caused by activation of sulfate aerosols into liquid droplets followed by subsequent homogeneous freezing. If moderate numbers of effective heterogeneous freezing nuclei (0.5–1 cm−3) had been present in either of these clouds, then the number densities of ice crystals and the peak relative humidities should have been lower than the observed values.

Prevalence of ice-supersaturated regions in the upper troposphere: Implications for optically thin ice cloud formation

In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from −40°C to −70°C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5–20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5 × 10-3 to 2 × 10-2 cm-3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m-2. If 0.02–0.1 cm-3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere.

Deep convection as a source of new particles in the midlatitude upper troposphere

downwind of the cirrus anvil, with maximum concentrations of 45,000 per standard cm 3 . Volatility and electron microscope measurements indicated that most of the particles were likely to be small sulfate particles. The enhancement extended over at least a 600-km region. Multivariate statistical analysis revealed that high CN concentrations were associated with surface tracers, as well as convective elements. Convection apparently brings gas-phase particle precursors from the surface to the storm outflow region, where particle nucleation is favored by the extremely low temperatures. Simple calculations showed that deep convective systems may contribute to a substantial portion of the background aerosol in the upper troposphere at midlatitudes. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry;

The Arctic polar stratospheric cloud aerosol: Aircraft measurements of reactive nitrogen, total water, and particles

The reactive nitrogen (NOy), total water, and particle components of the polar stratospheric cloud (PSC) aerosol in the Arctic are studied using in situ aircraft measurements in the lower stratosphere. The results are compared to findings from the Antarctic derived using similar measurements and interpretive techniques. The Arctic data show that particle volume well above background values is present at temperatures above the frostpoint, confirming the result from the Antarctic that the observed PSCs are not water ice particles. NOy measurements inside a PSC are enhanced above ambient values consistent with anisokinetic sampling of particles containing NOy. Furthermore, relative changes in the measured particle volume along a flight track are well correlated with changes in the amount of NOy estimated to be in the particle phase. With the exception of data from one flight, assuming that the composition of the PSC particles is nitric acid trihydrate (NAT), the HNO3 content of the measured particle volume is consistent with the amount of HNO3 predicted to be available for condensation. To this extent, the Arctic observations are consistent with NAT as the composition of PSC particles, in agreement with the Antarctic findings. In the Arctic data over long segments of several flights, calculations show saturation with respect to NAT without significant PSC particle growth above background. PSCs in the Arctic are not observed in situ until the apparent saturation ratio of HNO3 with respect to NAT is greater than 10, in marked contrast to the Antarctic, where PSCs are observed in conditions of apparent HNO3 saturation of 1 and above. This difference cannot be resolved by known measurement uncertainties. Also, a discrepancy is noted in the comparison of the amount of condensed HNO3 derived from the particle distribution measurements with that derived from the NOy measurements, assuming a NAT composition for the particles. Although the relative variations in the derived quantities are similar, as in the Antarctic, the mean values consistently disagree by about a factor of 2. The differences suggest that there may be systematic errors in the data and/or physical assumptions used in the analysis. Several possibilities are discussed.

Environmental conditions required for contrail formation and persistence

The ambient temperatures and humidities required for contrail for- mation and persistence are determined from in situ measurements during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) exper- iment. Ambient temperatures and water vapor concentrations were measured with the meteorological measurement system, a laser hygrometer, and a cryogenic hygrometer (all onboard the DC-8). The threshold temperatures are compared with theoretical estimates based on simple models of plume evolution. Observed contrail onset temperatures for contrail formation are shown to be 0-2 K below the liquid-saturation threshold temperature, implying that saturation with respect to liquid water must be reached at some point in the plume evolution. Visible contrails observed during SUCCESS persisted longer than a few minutes only when substantial ambient supersaturations with respect to ice existed over large regions. On some occasions, contrails formed at relatively high temperatures (>_ -50oC) due to very high ambient supersaturations with respect to ice (of the order of 150%). These warm contrails usually formed in the presence of diffuse cirrus. Water vapor from sublimated ice crystals that entered the engine was probably necessary for contrail formation in some of these cases. At temperatures above about -50oC, contrails can only form if the ambient air is supersaturated with respect to ice, so these contrails should persist and grow.

Refractive indices of aerosols in the upper troposphere and lower stratosphere

A new instrument for simultaneously measuring aerosol diameter from 0.4–10 µm and the refractive index between 1.30–1.60 has recently been flown on the NASA ER-2 aircraft during a stratospheric measurement campaign. Average stratospheric refractive indices varied from 1.40 to 1.42 over a latitude range from 70°S to 50°N and from 1.34 to 1.46 over a vertical range from 4–20 km. The measured stratospheric refractive indices do not agree well with theoretical predictions and vertical profiles suggest the presence of non-spherical or absorbing particles in the altitude range of 7–9 km.

Evaluating the role of NAT, NAD, and liquid H2SO4/H2O/HNO3 solutions in Antarctic polar stratospheric cloud aerosol: Observations and implications

Airborne measurements of total reactive nitrogen (NOy) and polar stratospheric cloud (PSC) aerosol particles were made in the Antarctic (68°S) as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAES A) campaign in late July 1994. As found in both polar regions during previous studies, substantial PSC aerosol volume containing NOy was observed at temperatures above the frost point, confirming the presence of particles other than water ice. The composition of the aerosol particles is evaluated using equilibrium expressions for nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and the supercooled ternary solution (STS) composed of nitric acid (HNO3), sulfuric acid (H2SO4), and water (H2O). The equilibrium abundance of condensed HNO3 is calculated for each phase and compared to estimates made using observations of aerosol volume and NOy. The best agreement is found for STS composition, using criteria related to the onset and abundance of aerosol volume along the flight track. Throughout the PSC region, a comparison of the number of particles between 0.4 and 4.0 μm diameter with the number of available nuclei indicates that a significant fraction of the background aerosol number participates in PSC growth. Modeled STS size distributions at temperatures below 191 K compare favorably with measured size distributions of PSC aerosol. Calculations of the heterogeneous loss of chlorine nitrate (ClONO2) show that the reactivity of the observed PSC surface area is 30 to 300% greater with STS than with NAT composition for temperatures less than 195 K. The total volume of STS PSCs is shown to be more sensitive than NAT to increases in H2O, HNO3, and H2SO4 from supersonic aircraft fleet emissions. Using the current observations and perturbations predicted by the current aircraft assessments, an increase of 50 to 260% in STS aerosol volume is expected at the lowest observed temperatures (190 to 192 K), along with an extension of significant PSC activity to regions ∼0.7 K higher in temperature. These results improve our understanding of PSC aerosol formation in polar regions while strengthening the requirement to include STS aerosols in studies of polar ozone loss and the effects of aircraft emissions.

Uptake of NOy on wave-cloud ice particles

In a flight through a wave cloud during SUCCESS on 2 May 1996, simultaneous forward- and aft-facing NOy inlets were used to infer the amount of condensed-phase NOy present on ice particles that were up to a few minutes old. Condensed-phase amounts were 25–75 pptv, or 10–20% of gas-phase NOy. Given the rapid HNO3 uptake on ice observed in the laboratory, a model calculation implies that virtually all of the gas-phase HNO3 will be depleted in the first 1–2 minutes after the appearance of ice. Thus the NOy observations are consistent with the laboratory results only if the ambient HNO3/NOy ratio is 10–20%.