Where Did This Particle Come From? Sources of Particle Number and Mass for Human Exposure Estimates
Atmospheric chemistry dominates the size distribution and composition of most fine particles inhaled by humans. However, it is important to distinguish between secondary particles—new particles formed in the atmosphere—and secondary mass—molecules formed in the atmosphere that condense to existing particles. In many ways the life stories of particles viewed from the perspectives of particle number concentrations and particle mass concentrations are distinct. Individual particle cores can often be said to have an individual source, while the mass on individual particles comes from myriad sources. This, plus the aforementioned chemical processing in the atmosphere, must be kept in mind when considering the health effects of fine particles.