PM Formation in the Atmosphere Primary and Secondary PM Sulfate Formation in the Atmosphere SO4 Formation in Clouds Season SO2-SO4 Transformation rate Residence Time of Sulfur and Organics Internal and External Mixtures of Particles Resource Links Contact: Rudolf Husar,

Primary and Secondary PM Primary PM is released into the atmosphere directly from the source ( e.g. flyash from coal or soot from diesel exhaust). Secondary PM is formed within the atmosphere from precursor gases, such as SO2, NOx and organics through gas-phase photochemical reactions or through liquid phase reactions in clouds and fog droplets. Most of the PM2.5 in the rural atmosphere is secondary. In urban areas under poorly ventilated winter conditions, primary emissions are also important.

The condensation of H 2 SO 4 molecules results in the accumulation and growth of particles in the size range - hence the name ‘accumulation-mode’ particles. Sulfate Formation in the Atmosphere Sulfates constitute about half of the PM2.5 in the Eastern US. Virtually all the ambient sulfate (99%) is secondary, formed within the atmosphere from SO2. About half of the SO2 oxidation to sulfate occurs in the gas phase through photochemical oxidation in the daytime. NOx and hydrocarbon emissions tend to enhance the photochemical oxidation rate.

Only a small fraction of the cloud droplets rain out, most droplets evaporate at night and leave a sulfate residue or ‘convective debris’. Most elevated layers above the mixing layer are pancake-like cloud residues. Such cloud ‘processing’ is responsible for internally mixing aerosol particles from many different sources. It is also believed that such ‘wet’ processes are significant in the formation of the organic fraction of PM2.5. SO4 Formation in Clouds At least half of the SO2 oxidation is taking place in cloud droplets as air molecules pass through convective clouds at least once every summer day. Within clouds, the soluble pollutant gases such as SO2, get scavenged by the water droplets and rapidly oxidize to sulfate.

Season SO 2 -SO 4 Transformation rate The SO 2 to SO 4 transformation rates are summer peaked due to enhanced summer time photochemical oxidation and SO 2 oxidation in clouds Transformation rates derived from the CAPITA Monte Carlo Model, Schichtel and Husar,

Residence Time of Sulfur and Organics. SO2 is depleted mostly by dry deposition (2-3%/hr), and also by conversion to SO4 (1%/hr). This gives SO2 an atmospheric residence time of only days. It takes about a day to form the sulfate aerosol. Once formed, SO4 is removed mostly by wet deposition at a rate of 1-2 %/hr yielding a residence time of 3-5 days. Overall, sulfur as SO2 and SO4 is removed at a rate of 2-3%/hr, which corresponds to a residence time of 2-4 days. These processes have at least a factor of two seasonal and geographic variation. It is believed that the organics in PM2.5 have a similar conversion rate, removal rate and atmospheric residence time.

Electron micrograph of a PM2.5 droplet residue. Evidently, the droplet contained a solid particle, possibly soot. Internal and External Mixtures of Particles During their atmospheric residence of 3-5 days, the atmospheric processes trend to mix PM2.5 particles into external and internal mixtures. In an external mixture, particles from different sources remain separate i.e not attached or each other. In an internal mixture, individual particles are mixed (aggregated), from particles of different types (e.g. a soot particle inside a sulfate droplet) as illustrated by the electron micrograph below. The main cause of internal mixing is cloud scavenging and subsequent evaporation.

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