What are nanoparticles?

Nanoparticles are tiny chunks of matter whose size is in the nanometer range - hence the name. For each nanoparticle to be on its own, they need to be suspended in a carrier gas, such as soot particles from combustion, cloud nuclei, dust and ash particles etc. Being so small, it takes them weeks to months to settle to the ground. Settling is overlapped by thermal motion, "diffusion" of the particles. Table 1 compares settling and diffusion velocities for a few sizes. Obviously, diffusion is much more important for nanoparticle motion than settling.

How are nanoparticles released into the atmosphere?

Nanoparticles can only be formed in chemical reactions, e.g. combustion, or by condensation. They cannot be produced by mechanical processes such as brushing or grinding, because the strong van-der-Waals forces will keep them attached to one or the other surface and prevent them from being released into the air.
When two particles collide those forces will keep them irreversibly together. This "hit and stick" process among particles is called agglomeration. As agglomeration proceeds, particle number is reduced, but the average diameter grows. Eventually an agglomerate grows to such size that it settles to the ground and is thereby removed from the atmosphere.

What are the major sources of nanoparticles?

There are natural and anthropogenic processes that release nanoparticles into the atmosphere. Natural sources include volcanic activity, natural fires, chemical reactions in the atmosphere, or sea spray leading to salt particles.
Human activity produces nanoparticles mostly by combustion of fossil fuels. Starting from wood fire, humans have developed industrial-scale nanoparticle sources with more or less continuous operation. Stationary sources, such as waste incinerators or power stations, have largely eliminated their nanoparticle emissions following environmental legislation. Today, one of the major sources of nanoparticles are on- and off-road vehicles, with the largest contribution of carbonaceous soot stemming from diesel engines.

Why should we care about nanoparticles?

Nanoparticles are all around us all the time, and they are in us, too, because we breathe them. Post-industrial "clean" urban air contains some 10'000 nanoparticles per ccm. With every breath we take, about one out of three particles is deposited in our lungs. We have not developed any biological mechanism to protect us against the tiny intruders because on an evolutionary time scale, nanoparticles in such high concentrations are a completely new phenomenon.
Once deposited in the human organism, solid insoluble nanoparticles cause a multitude of health effects. While the exact mechanisms by which they affect our health are still subject to intense research, it is clear what they result in: increased mortality and morbidity, cardiovascular disease, asthma, lung diseases any some more unspecific symptoms.
One of the largest ongoing nanoparticle inhalation experiments of mankind can be observed almost any time anywhere, although few of us are aware of it. Cigarette smoking, that is inhalation of high numbers of combustion generated nanoparticles, is known to kill people by the million.