W. Richard Leaitch

Effect of organics of low solubility on the growth rate of cloud droplets

[1]xa0Organic aerosols represent an important fraction of the fine particle aerosol, yet little is known about the role that these particles play in the indirect effect of aerosols on climate. The growth rates of organic acid particles due to the condensation of water were measured in a cloud condensation nucleus chamber. Delays in the cloud activation of organic acid particles were observed relative to ammonium sulfate, (NH4)2SO4. The inclusion of particle dissolution with time according to its water solubility in a kinetic model of condensational growth of droplets was able to reasonably reproduce the observed delays, indicating that the delays in the growth of the organic acid particles were mainly due to their lower solubilities. Applying the results in an adiabatic simulation of cloud droplet nucleation, the number of cloud droplets nucleated on particles with solubility equivalent to adipic acid were reduced relative to those nucleated on (NH4)2SO4 by up to 85%. The relative solubility of organic species must be considered when simulating the indirect effect of organic aerosol particles.

An Intensive Study of the Size and Composition of Submicron Atmospheric Aerosols at a Rural Site in Ontario, Canada

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GC-FID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400–500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.

The physical and chemical evolution of aerosols in smelter and power plant plumes: an airborne study

National and international concern about the health effects and continued use of Pb, Cd, As and Hg as well as other metals has defined a need for improved estimates of the long-term risks to ecosystems and human health from metals released from mining, metallurgical and energy production activities. A research aircraft was used to determine the microphysical and chemical properties of airborne particulate metal emissions from the Nanticoke coal-fired power-generating station located on the north shore of Lake Erie, Ontario, and the Horne copper smelter at Rouyn-Noranda, Quebec. These properties are critical to the determination of the deposition rates of the metals emitted, and hence the potential for these species to have impacts on local or distant ecosystems. An overview of the measurements made during the study is given. The size distributions of particles emitted from the stacks and observed within 5u2009km of the point of emission are briefly described. After dilution by ambient air, the concentration of particles smaller than 0.135u2009μm in diameter in the plumes is tens of thousands per cubic centimetre, far exceeding the concentrations found in ambient air. However, in the size range 0.135 to 3u2009μm diameter the plumes generally contribute about one to four times more particles than present in ambient air.

Limitations of using an equilibrium approximation in an aerosol activation parameterization

[1]xa0Because of its complex nature, attempts have been made to parameterize the process of cloud droplet formation and growth. The parameterization developed by Abdul-Razzak et al. [1998] and Abdul-Razzak and Ghan [2000], henceforth referred to as the ARG parameterization, is based on the adiabatic ascent of an air parcel, assuming that each particle is in equilibrium with its environment. Some of the limitations of the equilibrium assumption are evaluated here through direct comparison of a kinetic parcel model with the ARG parameterization. Conditions are described in which the supersaturation and the cloud droplet number concentration are underpredicted by the parameterization. The underprediction happens because of an overestimation of the condensation rate of water onto the larger particles. The effect is significant for conditions of lower updraft velocities (V 500 cm−3). The parameterization behaves well for higher updraft velocities and lower aerosol number concentrations. The impact of this effect on the activation of sulfate aerosols in the presence of sea salt aerosol is also investigated. Using the ARG parameterization to diagnose Nd in the presence of sea salt aerosol leads to a greater reduction in the number of activated sulfate particles in lower updraft conditions than does using the kinetic parcel model. The relative contribution of sulfate particles to total Nd is also significantly lower for the ARG parameterization than it is for the parcel model in low to moderate updrafts. The error in the first indirect radiative forcing introduced by the equilibrium assumption is estimated to be 6 to 15 W m−2 for typical marine clouds, depending on the updraft velocity.

Modelling Regional Air Quality in the Canadian Arctic: Simulation of an Arctic Summer Field Campaign

Model simulations of an Arctic summer field campaign were carried out. The model results were compared with observational data from both ground-based monitoring and in situ measurements on-board multiple mobile platforms. The model was able to well capture regional sources and transport affecting the Arctic air quality. It is shown that the study area was impacted by North American (NA) regional biomass burning emissions. The model-observation comparison also corroborates previous findings on possible roles of marine-biogenic sources in aerosol production in the Arctic MBL during summertime.