CHAPTER 2 . The Chemical and Physical Properties of Marine Aerosols: An Introduction
59
species. Most NOx is emitted into the atmosphere as NO, but there is rapid cycling in
the atmosphere between NO and N02 on the time scale of a minute. Because of this
rapid cycling, it is customary to think of the atmospheric chemistry of these species
in terms of NOx, although at night-time (i.e. in the absence of sunlight), NO x is present
entirely as N02• The main sink of NO x is oxidation to HN03. In the daytime, the reaction is through N02 with OH; at night it is through N0 2 with 0 3 to produce the N0 3 -
radical, which reacts with N02 to form N20 S' which subsequently reacts with H20 to
from HN03. HN03 is extremely soluble in water and highly reactive. Consequently, it
is rapidly removed from the atmosphere through precipitation and by direct deposition to surfaces. Over the ocean, HN0 3 reacts rapidly with sea-salt aerosol particles. It
is because of these various factors, especially the latter, that the NOx-NO; system has
relatively little impact on the radiative forcing of climate over the oceans, a subject
discussed in greater detail below.
The dominant reduced nitrogen species in the atmosphere are NH: and NH3 (which,
as a pair, are commonly referred to as NHx). Ammonia is important in atmospheric
chemistry because it is by far the most important gaseous species available for the
titration of acidic aerosol particles - primarily SO~- particles. It is important to note
that in the atmosphere, there is relatively little conversion or exchange between
NOxlNO y species and the NHx forms of nitrogen. In particular, NH3 is quite resistant
to oxidation under normal circumstances, although in tropical regions model results
suggest that significant amounts ofNH3 may be oxidized due to high OH and low sulphate concentrations (Dentener and Crutzen 1994).As a result, the NHx and NOx cycles
follow quite different routes and have completely different fates in the marine aerosol
cycle as discussed below. Also in contrast to the NO x cycle, where the ocean is invariably the sink (for NO x as NO;), the ocean can serve as either a source or a sink for
NH 3 , depending on the relative concentrations in the ocean and in the atmosphere
(Quinn et al. 1996).
2.4.1
Global Budgets of NOyand NHx
The global budget of NOy (see Prospero et al. 1996; Holland et al. 1999) is dominated
by anthropogenic sources (Table 2.3). Energy production (combustion of coal, petroleum products, natural gas) produces about 20 Tg N yr- l as NO x ' This source has been
growing at a steady rate - from 1960 to 1986, at about 2.7% per year. The next largest
source is biomass burning, which produces about 9 Tg Nyr- l . The primary natural
sources of NOx are biological fixation (about 6 Tg Nyr- l ) and lightning (about
3 Tg N yr- l ). Note that in contrast to the sulphur cycle, which had a substantial oceanic source (i.e. DMS), there are no significant oceanic (water column) sources of NO xOn the other hand, lightning and the stratosphere can be substantial sources of NOx
over the oceans. The present-day emissions ofNO y are roughly 40 Tg Nyr-I, while the
preindustrial rate was 8 Tg N yr- l . Thus, human activities have resulted in a five-fold
increase in nitrogen emissions largely due to energy production and biomass burning.
Because of the highly reactive nature of NO x species, they have a relatively short
residence time in the atmosphere: about one day. Consequently, pollutant NOx is not
directly transported in large quantities over the oceans; the concentration of NOx is
59
species. Most NOx is emitted into the atmosphere as NO, but there is rapid cycling in
the atmosphere between NO and N02 on the time scale of a minute. Because of this
rapid cycling, it is customary to think of the atmospheric chemistry of these species
in terms of NOx, although at night-time (i.e. in the absence of sunlight), NO x is present
entirely as N02• The main sink of NO x is oxidation to HN03. In the daytime, the reaction is through N02 with OH; at night it is through N0 2 with 0 3 to produce the N0 3 -
radical, which reacts with N02 to form N20 S' which subsequently reacts with H20 to
from HN03. HN03 is extremely soluble in water and highly reactive. Consequently, it
is rapidly removed from the atmosphere through precipitation and by direct deposition to surfaces. Over the ocean, HN0 3 reacts rapidly with sea-salt aerosol particles. It
is because of these various factors, especially the latter, that the NOx-NO; system has
relatively little impact on the radiative forcing of climate over the oceans, a subject
discussed in greater detail below.
The dominant reduced nitrogen species in the atmosphere are NH: and NH3 (which,
as a pair, are commonly referred to as NHx). Ammonia is important in atmospheric
chemistry because it is by far the most important gaseous species available for the
titration of acidic aerosol particles - primarily SO~- particles. It is important to note
that in the atmosphere, there is relatively little conversion or exchange between
NOxlNO y species and the NHx forms of nitrogen. In particular, NH3 is quite resistant
to oxidation under normal circumstances, although in tropical regions model results
suggest that significant amounts ofNH3 may be oxidized due to high OH and low sulphate concentrations (Dentener and Crutzen 1994).As a result, the NHx and NOx cycles
follow quite different routes and have completely different fates in the marine aerosol
cycle as discussed below. Also in contrast to the NO x cycle, where the ocean is invariably the sink (for NO x as NO;), the ocean can serve as either a source or a sink for
NH 3 , depending on the relative concentrations in the ocean and in the atmosphere
(Quinn et al. 1996).
2.4.1
Global Budgets of NOyand NHx
The global budget of NOy (see Prospero et al. 1996; Holland et al. 1999) is dominated
by anthropogenic sources (Table 2.3). Energy production (combustion of coal, petroleum products, natural gas) produces about 20 Tg N yr- l as NO x ' This source has been
growing at a steady rate - from 1960 to 1986, at about 2.7% per year. The next largest
source is biomass burning, which produces about 9 Tg Nyr- l . The primary natural
sources of NOx are biological fixation (about 6 Tg Nyr- l ) and lightning (about
3 Tg N yr- l ). Note that in contrast to the sulphur cycle, which had a substantial oceanic source (i.e. DMS), there are no significant oceanic (water column) sources of NO xOn the other hand, lightning and the stratosphere can be substantial sources of NOx
over the oceans. The present-day emissions ofNO y are roughly 40 Tg Nyr-I, while the
preindustrial rate was 8 Tg N yr- l . Thus, human activities have resulted in a five-fold
increase in nitrogen emissions largely due to energy production and biomass burning.
Because of the highly reactive nature of NO x species, they have a relatively short
residence time in the atmosphere: about one day. Consequently, pollutant NOx is not
directly transported in large quantities over the oceans; the concentration of NOx is