David C. Rogers

African dust aerosols as atmospheric ice nuclei

[1]xa0Measurements of the ice nucleating ability of aerosol particles in air masses over Florida having sources from North Africa support the potential importance of dust aerosols for indirectly affecting cloud properties and climate. The concentrations of ice nuclei within dust layers at particle sizes below 1 μm exceeded 1 cm−3; the highest ever reported with our device at temperatures warmer than homogeneous freezing conditions. These measurements add to previous direct and indirect evidence of the ice nucleation efficiency of desert dust aerosols, but also confirm their contribution to ice nuclei populations at great distances from source regions.

Rain in shallow cumulus over the ocean: the RICO Campaign

Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow ...

Observation of playa salts as nuclei in orographic wave clouds

During the Ice in Clouds Experiment-Layer Clouds (ICE-L), dry lakebed, or playa, salts from the Great Basin region of the United States were observed as cloud nuclei in orographic wave clouds over Wyoming. Using a counterflow virtual impactor in series with a single-particle mass spectrometer, sodium-potassium-magnesium-calcium-chloride salts were identified as residues of cloud droplets. Importantly, these salts produced similar mass spectral signatures to playa salts with elevated cloud condensation nuclei (CCN) efficiencies close to sea salt. Using a suite of chemical characterization instrumentation, the playa salts were observed to be internally mixed with oxidized organics, presumably produced by cloud processing, as well as carbonate. These salt particles were enriched as residues of large droplets (>19 μm) compared to smaller droplets (>7 μm). In addition, a small fraction of silicate-containing playa salts were hypothesized to be important in the observed heterogeneous ice nucleation processes. While the high CCN activity of sea salt has been demonstrated to play an important role in cloud formation in marine environments, this study provides direct evidence of the importance of playa salts in cloud formation in continental North America has not been shown previously. Studies are needed to model and quantify the impact of playas on climate globally, particularly because of the abundance of playas and expected increases in the frequency and intensity of dust storms in the future due to climate and land use changes.

An overview of aircraft observations from the Pacific Dust Experiment campaign

[1]xa0Fourteen research flights were conducted in the Pacific Dust Experiment (PACDEX) during April and May 2007 to sample pollution and dust outbreaks from east Asia as they traveled across the northern Pacific Ocean into North America and interacted with maritime storms. Significant concentrations of black carbon (BC, consisting of soot and other light-absorbing particles measured with a soot photometer 2 instrument) and dust were observed both in the west and east Pacific Ocean from Asian plumes of dust and pollution. BC particles were observed through much of the troposphere, but the major finding is that the percentage of these particles compared with the total number of accumulation mode particles increased significantly (by a factor of 2–4) with increasing altitude, with peak values occurring between 5 and 10 km. Dust plumes had only a small impact on total cloud condensation nuclei at the sampling supersaturations but did exhibit high concentrations of ice nuclei (IN). IN concentrations in dust plumes exceeded typical tropospheric values by 4–20 times and were similar to previous studies in the Saharan aerosol layer when differences in the number concentrations of dust are accounted for. Enhanced IN concentrations were found in the upper troposphere off the coast of North America, providing a first direct validation of the transport of high-IN-containing dust layers near the tropopause entering the North American continent from distant sources. A source-specific chemical transport model was used to predict dust and other aerosols during PACDEX. The model was able to predict several features of the in situ observations, including the general altitudes where BC was found and a peak in the ratio of BC to sulfate between 5 and 10 km.

Ice Nucleation Studies of Mineral Dust Particles with a New Continuous Flow Diffusion Chamber

A new continuous flow diffusion chamber (CFDC) has been designed and constructed to study the ice nucleation efficiency of natural and anthropogenic aerosol particles over a range of temperatures and supersaturations. The CFDC system at Dalhousie University, Canada is based on the design of (Rogers et al. 1988, 1994) at Colorado State University, USA. A steady airflow (2.83 lpm) composed of sheath flows and an aerosol flow passes through the annular gap of the diffusion chamber. The walls of the chamber are ice-covered and are held at different temperatures. Aerosol particles are injected into the center of the gap near the location of maximum supersaturation. Particles greater than 5 μm in aerodynamic diameter are removed with impactors before entry to the chamber. Ice crystals are identified with an optical particle counter at the outlet of the chamber. In this article we report on the ice nucleation results of two mineral dust particles of potential atmospheric relevance, kaolinite and montmorillonite. Our results indicate that kaolinite and montmorillonite act as efficient ice nuclei in deposition/condensation nucleation mode. The onset relative humidity of both kaolinite and montmorillonite mineral dust particles were determined. The percentage of active ice nuclei is higher in montmorillonite compared to kaolinite at each temperature within the experimental conditions. The fraction of active ice nuclei increases with decreasing temperature and also with increasing relative humidity.

Observations of nighttime new particle formation in the troposphere

[1]xa0We present atmospheric observations which indicate efficient new particle formation during the nighttime in the troposphere under low condensation sinks, in contrast to the current prevailing assumption that aerosol nucleation takes place only during the daytime and typically from sulfuric acid. High concentrations of ultrafine particles with diameters from 4 to 9 nm (∼1000 cm−3) were measured from the three days of nighttime observations in the upper troposphere during the NSF/NCAR GV Progressive Science Missions. Long-term ground-based observations of charged and neutral clusters and aerosols made in Tumbarumba, Australia, also showed surprisingly high frequency of nighttime new particle formation (30%) with low condensation sinks. Nighttime nucleation can be significant for global aerosol load and cloud condensation nuclei productions and thus needs to be included in global climate models. Future studies are required to understand the nighttime nucleation mechanisms.

Examinations of ice formation processes in Florida cumuli using ice nuclei measurements of anvil ice crystal particle residues

[1] A continuous flow diffusion chamber (CFDC) was used to measure ice formation by cloud particle residuals during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment. These measurements were directed toward determining the relative contributions of homogeneous nucleation, heterogeneous nucleation, and secondary ice formation processes to the concentrations of ice crystals in anvil cirrus formed from convection. The CFDC sampled residual particles remaining after evaporation of cloud particles initially collected by a counterflow virtual impactor. This allowed, for the first time, determination of the ice nucleation ability of particles that included the presumed nuclei for cloud-ice formation. The approach proved successful for estimating concentrations of heterogeneous ice nuclei (IN) transported into anvil clouds, but experimental issues limited measurements of homogeneous freezing and, consequently, in determining the role of secondary ice formation. Results suggest agreement within a factor of 2–3 between CFDC heterogeneous IN concentrations and anvil ice crystal concentrations in the size range above 30 mm. IN concentrations also correlated with ice concentrations inferred from measurements by the FSSP (Forward Scattering Spectrometer Probe). However, measured IN concentrations were nearly two orders of magnitude lower than FSSP concentrations. This difference may have resulted from homogeneous freezing, secondary ice formation, or other unidentified ice formation processes that were not fully captured by the CFDC. The data suggest that heterogeneous nucleation played a smaller role than homogeneous nucleation in determining anvil ice crystal concentrations, except during periods of strong desert dust ingestion by cumuli. Nevertheless, heterogeneous nucleation may provide the source for larger ice crystals present in anvil regions.

Convective Distribution of Tropospheric Ozone and Tracers in the Central American ITCZ Region: Evidence from Observations During TC4

During the Tropical Composition, Clouds and Climate Coupling (TC4) experiment that occurred in July and August of 2007, extensive sampling of active convection in the ITCZ region near Central America was performed from multiple aircraft and satellite sensors. As part of a sampling strategy designed to study cloud processes, the NASA ER-2, WB-57 and DC-8 flew in stacked “racetrack patterns” in convective cells. On July 24, 2007, the ER-2 and DC-8 probed an actively developing storm and the DC-8 was hit by lightning. Case studies of this flight, and of convective outflow on August 5, 2007 reveal a significant anti-correlation between ozone and condensed cloud water content. With little variability in the boundary layer and a vertical gradient, low ozone in the upper troposphere indicates convective transport. Because of the large spatial and temporal variability in surface CO and other pollutants in this region, low ozone is a better convective indicator. Lower tropospheric tracers methyl hydrogen peroxide, total organic bromine and calcium substantiate the ozone results. OMI measurements of mean upper tropospheric ozone near convection show lower ozone in convective outflow. A mass balance estimation of the amount of convective turnover below the tropical tropopause transition layer (TTL) is 50%, with an altitude of maximum convective outflow located between 10 and 11 km, 4 km below the cirrus anvil tops. It appears that convective lofting in this region of the ITCZ is either a two-stage or a rapid mixing process, because undiluted boundary layer air is never sampled in the convective outflow.

Enhanced new particle formation observed in the northern midlatitude tropopause region

3960 cm � 3 , were measured during tropopause folds. Our observations show that stratospheric and tropospheric air exchange during tropopause folding events, with a large gradient of temperature and relative humidity, may have enhanced new particle formation. Our results are consistent with other modeling predictions showing that nucleation rates are increased with mixing of two air masses with different temperatures and relative humidities. In addition, new particle formation events were also associated with vertical motion that may also have brought higher concentrations of water vapor and aerosol precursors (that originate at the ground level) from lower altitudes to higher altitudes where temperatures and surface areas are lower. The average ultrafine particle concentrations for the regions that were not affected by tropopause folds were also high (>100 cm � 3 ), indicating that nucleation is active in the tropopause region, in general. Our results suggest that atmospheric dynamics, such as stratosphere and troposphere exchange and vertical motion, affect new particle formation in this region.

Biomass burning as a potential source for atmospheric ice nuclei: Western wildfires and prescribed burns

[1]xa0The sources, abundance and nature of atmospheric particles that serve as ice nuclei (IN) for cold cloud formation remain some of the most important, yet poorly-characterized, features of aerosol-cloud interactions that indirectly affect climate. Although a great deal of effort has focused on characterizing the ice nucleating ability of mineral dusts, less is known about carbonaceous particles. A primary source for carbonaceous IN is from biomass combustion. Here we report new measurements of IN activity at −30°C and above water saturation from biomass burning generated particles from prescribed burns and wildfires in the western US. These measurements suggest a range of IN activity, with variability largely dependent on the intensity of the fire. Although the fraction of particles generated during burns which serve as IN is quite small, the large numbers of particles generated in fires make biomass burning a potentially important source of IN to the atmosphere.