Atmospheric particles contain electrolytes, for example the salts
and NaCl and acids such as H2
, which are
soluble in water. Their presence can lead to some or all of the particle
existing as an aqueous solution, depending on the atmospheric conditions.
The amount of water in an aerosol particle depends on the nature of the
electrolytes, the quantities present, and on the ambient relative humidity and temperature.
The aerosol water determines the concentrations of the dissolved
electrolytes and consequently the ion activities. These in turn control
equilibria of volatile species such as NH3 and HNO3 with the
surrounding atmosphere, and the formation of solid salts within the aerosol
This lesson will help you to understand the relationship between the water
content of a particle and relative humidity, and the influence on ion
activities, for systems containing a single electrolyte at constant temperature.
In part (a) of the lesson the output of the model, and the meaning of the
various quantities presented, will be explained.
Before starting, ensure that this browser window occupies only the
left half of your screen. You should leave enough
space for another browser window where you can
enter data into E-AIM
and read the results. If your screen is too small for
two windows, print out this tutorial and use this window to enter data and
's results. In these lessons we assume that you will
have two browser windows open.
link to open a second browser window containing the data
input page for "simple" calculations using Model III (http://www.aim.env.uea.ac.uk/aim/model3/model3a.php). Arrange
the windows on your screen so that both are visible and the left window
contains this text.
This consists of the four sets of calculations described in the links below,
which should be done in the order listed.
You have completed Lesson 1, and learned:
How to enter data into E-AIM and interpret the results for simple
The uptake of water by a soluble electrolyte, and therefore its
concentration in the aqueous phase, is controlled by the
ambient relative humidity. The activity of any aqueous phase species
is equal to its activity coefficient multiplied by its mole fraction,
and in a system at equilibrium the water activity is equal to the
relative humidity (expressed as a fraction).
The concentration of an aqueous particle, at a fixed relative humidity,
is independent of the amount of solute. However, this concentration (or
uptake of water) can differ markedly between solutes
(NH4NO3 and HNO3 in the
examples here). These differences are greatest at low relative humidities.
The equilibrium partial pressures of water and soluble gases such
as NH3 and HNO3, and the saturation ratios
of solids, can be calculated from the water and ion activities and the
appropriate equilibrium constants.
The activity coefficients of ions in solution tend to unity as
the relative humidity tends to 100% and the solution becomes
very dilute. In the very concentrated solutions that exist at moderate
to low relative humidities activity coefficients of ions vary widely.
For example, ion activity coefficients in NH4NO3
and HNO3 solutions here show opposite trends with
Now proceed to Lesson 2, which examines how
solid phases form in aqueous aerosols at different relative