CHAPTER 2 • The Chemical and Physical Properties of Marine Aerosols: An Introduction
61
nental sources of the NHx budget, the emission rates today are about four to five
times greater than the preindustrial rates, a factor similar to that found for NOx emissions.
2.4.2
Concentrations of Nitrate and Ammonium in the Marine Atmosphere
The impact of pollution sources of NO; and NH! is evident in Table 2.1. In remote ocean
regions, nitrate aerosol concentrations are a few ten's of flg m -3 or less (see for example,
American Samoa, Mawson, Palmer Station); in contrast, off the coast of Asia and in
the North Atlantic, concentrations are ten to one hundred times greater. The same is
true for NH! concentrations.
The NO; time series in Fig. 2.5 show dramatic evidence of the impact of polluted
air masses in the form of sharp "spikes" in the NO; concentrations. These are most
visible in the record from Heimaey where aerosol concentrations are quite low except
on those occasions when polluted air is advected into the region, usually from Europe
(Prospero et al. 1995). Note that the peaks in NO; concentrations coincide with those
of nss-SO~-. The Bermuda aerosol time series also shows a lot of sharp peaks, much
more than at Heimaey, because of transport from North American pollution sources;
furthermore, there is little evidence of a seasonal cycle.
At Barbados (Fig. 2.5), the NO; time series is relatively smooth, although some sharp
peaks are evident. Barbados is affected by pollution sources in Europe and North Africa; Savoie et al. (1989b) estimate that approximately half of the NO; and nss-SO~- is
natural and half is anthropogenic. There is a clear seasonal cycle in NO; at Barbados;
the spring maximum and sporadic peaks during winter are attributed to the transport of biomass burning products from Africa (Savoie et al. 1989b).
2.4.3
Nitrate and Ammonium Aerosol Properties
Aerosol nitrate has a more complex chemistry than sulphate, because under acid conditions NO; can be volatilized as HN03, which can subsequently undergo further
chemical reactions (including photochemistry). In contrast, once SO~- enters the aerosol phase, it is essentially locked in the particle. Because of the volatility of NO;, the
size distribution of NO; aerosol is very dependent on the chemical properties of the
ambient aerosol. Under most conditions on the continents and almost invariably over
the oceans, the submicrometre of aerosol is dominated by SO~-, which is only partially
neutralized by NH;; as a consequence, NO; is found almost exclusively in the
supramicrometre size distribution, where there is little SO~- and a relatively high concentration of basic material such as sea-salt and mineral dust. Over the ocean, the size
distribution of NO; typically follows the surface area distribution of sea salt (Li-Jones
and Prospero 1998; Murphy and Thomson 1997; Gard et a1.1998). Because of the higher
settling velocities of large sea salt particles, the marine NO; aerosol has a shorter residence time than aerosol nss-SO~-. Per unit mass, coarse particles are less efficient light
scatterers than submicrometre particles; thus, in general, over the oceans the radiative impact of nitrate is insignificant relative to that of sulphate (Yang et al. 1994; LiJones and Prospero 1998).
61
nental sources of the NHx budget, the emission rates today are about four to five
times greater than the preindustrial rates, a factor similar to that found for NOx emissions.
2.4.2
Concentrations of Nitrate and Ammonium in the Marine Atmosphere
The impact of pollution sources of NO; and NH! is evident in Table 2.1. In remote ocean
regions, nitrate aerosol concentrations are a few ten's of flg m -3 or less (see for example,
American Samoa, Mawson, Palmer Station); in contrast, off the coast of Asia and in
the North Atlantic, concentrations are ten to one hundred times greater. The same is
true for NH! concentrations.
The NO; time series in Fig. 2.5 show dramatic evidence of the impact of polluted
air masses in the form of sharp "spikes" in the NO; concentrations. These are most
visible in the record from Heimaey where aerosol concentrations are quite low except
on those occasions when polluted air is advected into the region, usually from Europe
(Prospero et al. 1995). Note that the peaks in NO; concentrations coincide with those
of nss-SO~-. The Bermuda aerosol time series also shows a lot of sharp peaks, much
more than at Heimaey, because of transport from North American pollution sources;
furthermore, there is little evidence of a seasonal cycle.
At Barbados (Fig. 2.5), the NO; time series is relatively smooth, although some sharp
peaks are evident. Barbados is affected by pollution sources in Europe and North Africa; Savoie et al. (1989b) estimate that approximately half of the NO; and nss-SO~- is
natural and half is anthropogenic. There is a clear seasonal cycle in NO; at Barbados;
the spring maximum and sporadic peaks during winter are attributed to the transport of biomass burning products from Africa (Savoie et al. 1989b).
2.4.3
Nitrate and Ammonium Aerosol Properties
Aerosol nitrate has a more complex chemistry than sulphate, because under acid conditions NO; can be volatilized as HN03, which can subsequently undergo further
chemical reactions (including photochemistry). In contrast, once SO~- enters the aerosol phase, it is essentially locked in the particle. Because of the volatility of NO;, the
size distribution of NO; aerosol is very dependent on the chemical properties of the
ambient aerosol. Under most conditions on the continents and almost invariably over
the oceans, the submicrometre of aerosol is dominated by SO~-, which is only partially
neutralized by NH;; as a consequence, NO; is found almost exclusively in the
supramicrometre size distribution, where there is little SO~- and a relatively high concentration of basic material such as sea-salt and mineral dust. Over the ocean, the size
distribution of NO; typically follows the surface area distribution of sea salt (Li-Jones
and Prospero 1998; Murphy and Thomson 1997; Gard et a1.1998). Because of the higher
settling velocities of large sea salt particles, the marine NO; aerosol has a shorter residence time than aerosol nss-SO~-. Per unit mass, coarse particles are less efficient light
scatterers than submicrometre particles; thus, in general, over the oceans the radiative impact of nitrate is insignificant relative to that of sulphate (Yang et al. 1994; LiJones and Prospero 1998).