Chemistry Reference
In-Depth Information
shortwave radiation [
6
]. The indirect effects affect the climate system through the
clouds and mostly with the cloud albedo effect and the cloud lifetime effect. The
cloud albedo effect (Twomey effect) is the resulting change in cloud radiative
properties due to changes in cloud droplet number concentration (CDNC) [
7
];
the lifetime effect (Albrecht effect) is connected to the changes in cloud properties
and in drizzle and precipitation [
8
]. The aerosol indirect effects are controlled by
the ability of particles to activate to cloud droplets (i.e., to be cloud condensational
nuclei, CCN) within a cloud [
9
]. This ability is a strong function of particle size,
water supersaturation, and particle hygroscopicity (i.e., chemical composition). An
extensive overview of the processes affecting the ability of particles to act as CCN
is provided in literature [
10
], with the conclusion that the particle size is the
dominant aerosol property on cloud droplet activation, making particle number
size distribution the key factor in aerosol-climate interactions.
1.1 Properties of Aerosol Number Concentrations
and Size Distributions
In the submicron range, the aerosol population typically consists of several
subpopulations, so-called modes, which are indicative of different loss and forma-
tion processes in the atmosphere. The smallest aerosol particles are formed from
gas phase vapors via nucleation, forming the nucleation mode. In slightly larger
sizes, the Aitken mode particles are produced via growth from nucleation mode and
a wide variety of combustion and natural sources. The largest submicron particles
are in the accumulation mode, with a major source being growth from Aitken mode,
e.g. via cloud processing and condensation. Only the smallest tail of the coarse
mode can be seen in the submicron range, and the overall effect of coarse particles
in the submicron particle range number concentrations is generally very small.
Often as a rough estimate the particles with diameters between 30 and 100 nm are
considered to belong to Aitken mode and particles between 100 nm and 1
mtothe
accumulation mode. The particle modes can move in the size axis, and they are not
always visible in the measured size spectra, especially nucleation mode is often
absent during nighttime.
As particle number size distributions can be complex, and the instruments used
generate large amount of size distribution data, which can be hard to effectively
describe, a common method is to calculate integrated particle number
concentrations for specific aerosol particle diameter ranges, depending on which
part of the particle number size spectrum is needed for the application. In this
chapter, three different ranges are used (Fig.
1a
):
m
• Number concentration of particles between 30 and 50 nm of dry diameter
(N
30-50
), given in particles per cubic centimeter (cm
3
). These particles repre-
sent smaller end of the Aitken mode particles and are representative of recently
emitted or formed nanoparticles, which have high probability to end up in
