Nucleation mode, atmospheric particles

The nucleation mode (ii ie < 0.1 pm) accounts for the majority of particles by number but because of their small size, these particles rarely account for more than a few percent of the total mass of atmospheric particles. These particles originate from condensation of supersaturated vapors from combustion processes and from the nucleation of atmospheric particles to form fresh particles (Seinfeld and Pandis, 1998 Horvath, 2000). 

Particles in nucleation mode are generally formed due to condensation of the vapour present in the exhaust gases and nucleation (gas-to-particle conversion) in the atmosphere after rapid cooling and dilution of exhaust emissions [31,32]. These particles originate mainly from unbumed fuel and lubricating oil consisting of sulphates, nitrates and organic compounds [33]. 

Kulmala M, Toivonen A, Ma kela JM, Laaksonen A (1998b) Analysis of the growth of nucleation mode particles observed in boreal forest. Tellus B 50 449-462 Kulrnala M, Piijola U, Ma kela JM (2000) Stable srrlpbadusters as a source of new atmospheric particles. Nature 404 66-69... 

Nuclei mode particles in this mode are formed by nucleation of atmospheric gases in a supersaturated atmosphere and their size is of the order of nanometres. 

The phenomena that influence particle sizes are shown in an idealized schematic in Figure 2.7, which depicts the typical distribution of surface area of an atmospheric aerosol. Particles can often be divided roughly into modes. The nucleation (or nuclei) mode comprises particles with diameters up to about 10 nm. The Aitken mode spans the size range from about lOnm to lOOnm (0.1 pm) diameter. These two modes account for the... 

A different picture of the ambient aerosol distribution is obtained if one focuses on the number of particles instead of their mass (Figure 8.10, upper panel). The particles with diameters larger than 0.1 pm, which contribute practically all the aerosol mass, are negligible in number compared to the particles smaller than 0.1 pm. Two modes usually dominate the aerosol number distribution in urban and rural areas the nucleation mode (particles smaller than 10 nm or so) and the Aitken nuclei (particles with diameters between 10 and 100 nm or so). The nucleation mode particles are usually fresh aerosols created in situ from the gas phase by nucleation. The nucleation mode may or may not be present depending on the atmospheric conditions. Most of the Aitken nuclei start their atmospheric life as primary particles, and secondary material condenses on them as they... 

The nucleation mode occurs in the atmosphere only sporadically, depending on favorable conditions for new particle formation (see under Sec. III.B) or on mixing processes (2). The Aitken mode and accumulation mode are usually always present in the boundary layer of the atmosphere. The Aitken mode... 

The ultrafine range is usually composed of emissions from local combustion sources or particles generated by atmospheric photochemical activity that leads to homogeneous nucleation. The principal mechanism of decay of the ultrafine range is attachment to particles in the accumulation mode by diffusion. Neglecting the Brownian motion of the coarse particles compared with the fine particles, the fractional rate of decay of particles in the ultrafine range is given by (Chapter 7)... 

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