Charles F. Clement

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;

Gas-to-particle conversion in the atmosphere: II. Analytical models of nucleation bursts

Simple models are developed to describe the formation of particles from condensable vapours in different atmospheric circumstances. The models are designed for use in large scale global transport models, where sub-grid descriptions are required for such phenomena. We solve the evolution equation for the density of a condensable vapour. When the concentration of existing aerosol is low, nucleation can occur, but only in intermittent, isolated bursts. In the absence of an initial aerosol, two analytical expressions are obtained for the number of particles produced in such bursts, valid for high and low rates of vapour production, respectively. These results compare favourably with calculations made using a detailed numerical code, using the homogeneous nucleation of sulphuric acid/water droplets as an illustration. Then we consider barrierless nucleation, where clusters are always stable against evaporation, which is relevant to the production of ammonium sulphate particles in the atmosphere. We go on to consider conditions where existing aerosol can affect the production of particles, and also consider slower bursts where the time dependence of the vapour production rate, and not condensation on the nucleated aerosol, cuts off nucleation.

Gas-to-particle conversion in the atmosphere: I. Evidence from empirical atmospheric aerosols

Abstract Condensable vapours such as sulphuric acid form aerosol in the atmosphere by the competing mechanisms of condensation on existing aerosol and the nucleation of new aerosol. Observational and theoretical evidence for the relative magnitudes of the competing processes is reviewed, and a number of general conclusions are made. Condensation is sensitive to the sticking probability of sulphuric acid molecules on aerosol particles, but there is now good evidence that it should be close to unity. In this case, equilibration timescales between acid vapour and the aerosol in most of the atmosphere are of the order of minutes or less, so that the acid concentration on such timescales given simply by the production rate times the equilibration time. When the acid concentration exceeds a threshold, nucleation will occur. The atmospheric aerosol therefore follows a history of initial formation in a nucleation burst followed by growth and coagulation with final removal by precipitation. This leads to the inverse correlation between aerosol number concentration and mass concentration found by Clarke (1992. Journal of Atmospheric Chemistry 14, 479–488) in the free troposphere. Binary homogeneous nucleation of sulphuric acid/water droplets, for which various simplified rates are compared, may dominate in such regions, but other mechanisms are possible elsewhere. A detailed analysis is performed of the number concentrations, removal rates, and masses of the components of the different types of global aerosols proposed empirically by Jaenicke (1993. Tropospheric Aerosols, Aerosol-Cloud-Climate Interaction. Academic Press, New York). There is a striking correlation between number concentrations in the nucleation and accumulation modes; and the giant aerosol mode, which if it is present dominates the mass, has little effect on the gas-to-particle conversion process. The mass of the atmospheric aerosol is therefore uncorrelated with the magnitude of molecular aerosol removal by condensation.

Observations and models of particle nucleation near cloud outflows

Aerosol has been detected in considerable concentrations in the outflow of an anvil cloud during a major storm over the midwestern United States. We make rough calculations to try to interpret the measurements using simple models of particle nucleation and coagulation. We find it difficult to account for the observations, particularly in understanding the origin of the observed number density.