Agriculture Reference
In-Depth Information
9.5
BEHAVIOR OF NPS IN ATMOSPHERE
The atmospheric scientists describe NPs as ultrafine particles, a category
consisting of particles smaller than 10 nm (referred to as nucleation mode)
and those from 10 to 100 nm (referred to as Aitken mode). Particles in
this size range originate mainly from combustion and photochemical re-
actions. Natural and incidental NPs dominate the number distribution of
particles in the atmosphere in polluted urban, rural, remote continental and
marine atmosphere, typical concentration ranges are 10
5
to 4´10
6
, 2´10
3
to
10
4
, 50 to 10
4
and 100 to 400/cm
3
, respectively
(Pandis, 2006) NPs in the
atmosphere are subject to coagulation, surface coating through condensa-
tion of semi volatile compounds and heterogenous reaction with gaseous
pollutants. For NPs, coagulation is dominated by Brownian (thermal) mo-
tion that leads particles to collide with each other, grow in size, and shrink
in number. Coagulation rates are low when particles are of same size but
increases by orders of magnitude as the differences in size grows. The time
scale for coagulation is dependent on the particle size of interest and the
background particle size distribution. For example, in mono disperse pop-
ulation of 20 nm particles, the time scale for coagulation in urban and rural
setting are around 10 min and 10 h (Tiwari, 2010). Particles are removed
from atmosphere mainly by Brownian diffusion, gravitational settling
(dry deposition) and wet deposition in precipitation to the earth's surface.
Time scale for wet deposition is highly dependent on the frequency of
precipitation events. During a precipitation event, atmospheric NPs may
be removed via wet deposition mechanism in a matter of minutes to hours
(Tiwari, 2010). The time scale for removal by dry deposition in an urban
area in 20 h for 40-50 nm size particles (Berkowicz, 2004). The most
common types of naturally occurring particles in the atmosphere consists
of black carbon or soot, a combustion by product which is thought to very
harmful because it contains fullerenic compounds and carbon nanostruc-
tures (Vander et al., 2000).
9.6
BEHAVIOUR OF NPS IN AQUATIC ENVIRONMENT
Engineered and natural NPs, after entering the aqueous environment, will
interact with the ubiquitous natural aquatic colloids which affect the sta-
bility and subsequent environmental behavior of both NPs and aquatic col-
