58
J. M. Pro spero
the cycle of particle production and transport. This process might well serve as the
climate-controlling engine of the pre-human marine atmosphere and the present-day
remote marine atmosphere.
2.3.6
Impact on Climate
A question that concerns us today is the degree to which humans have perturbed this
natural aerosol production cycle over the world ocean and the impact on radiative
processes. It has been clearly demonstrated that pollution aerosols modify cloud properties, often dramatically. This effect is readily visible as the appearance of "ship tracks"
in stratus cloud over the ocean. These were first detected in satellite images (Coakley
et al. 1987). It was noted that thin stratus cloud decks often were crossed by long thin
lines of denser and brighter cloud. These tracks often appeared in regions where ship
traffic was heavy. It was subsequently demonstrated that the exhaust from ships was
entrained in clouds; the high concentration of exhaust aerosols resulted in a cloud that
had higher concentrations of smaller droplets. This increased the scattering of sunlight and (hence) caused the clouds to appear "whiter". The increase in light scatter
with increasing particle concentrations is known as the Twomey effect (Twomey 1991).
This effect has been shown to be widespread, and it is central to the study of the role
of anthropogenic aerosols in climate forcing.
There are still many unresolved issues about gas-to-particle conversion processes
in the marine atmosphere. Although this discussion has focused on the DMS-SOrSO~aerosol system, it does demonstrate that there are many aspects of the aerosol formation process that are poorly characterized and that similar problems apply to other
types of gas-aerosol systems.
The concern about the role of anthropogenic sulphate particles in climate has provided the impetus for a renewed interest in aerosol chemistry. The interest in marine
SO~- stimulated a strong research effort into oceanic sources, in particular the role of
D MS. After more than a decade of intensive research, there are still many unanswered
questions about the marine sulphur cycle and the relative importance of natural and
anthropogenic sources in the atmospheric chemistry of sulphur over the oceans. Of
particular interest is the linkage between marine primary productivity and the impact that it has on the atmospheric sulphur cycle. If climate is indeed changing as a
result of human activities (or natural ones for that matter), then we might expect some
affects on ocean productivity. Thus, the possible feedback to the sulphur cycle has
important implications for future climate trends.
2.4
The Oceanic Atmospheric Cycle of Nitrates and Ammonium
Nitrate and ammonium are important to marine chemistry because of their possible
impact on the oceanic nutrient cycle. The important precursor species of aerosol NO;
are NO and N02 (collectively referred to as NO x )' There are a number of organic nitrates that may playa significant role, in particular peroxyacetylnitrate (PAN). The
entire ensemble of reactive gas phase N-species is generally referred to as NO y , which
is defined as the sum of NOx, HN03, PAN and other related minor (mostly organic)
J. M. Pro spero
the cycle of particle production and transport. This process might well serve as the
climate-controlling engine of the pre-human marine atmosphere and the present-day
remote marine atmosphere.
2.3.6
Impact on Climate
A question that concerns us today is the degree to which humans have perturbed this
natural aerosol production cycle over the world ocean and the impact on radiative
processes. It has been clearly demonstrated that pollution aerosols modify cloud properties, often dramatically. This effect is readily visible as the appearance of "ship tracks"
in stratus cloud over the ocean. These were first detected in satellite images (Coakley
et al. 1987). It was noted that thin stratus cloud decks often were crossed by long thin
lines of denser and brighter cloud. These tracks often appeared in regions where ship
traffic was heavy. It was subsequently demonstrated that the exhaust from ships was
entrained in clouds; the high concentration of exhaust aerosols resulted in a cloud that
had higher concentrations of smaller droplets. This increased the scattering of sunlight and (hence) caused the clouds to appear "whiter". The increase in light scatter
with increasing particle concentrations is known as the Twomey effect (Twomey 1991).
This effect has been shown to be widespread, and it is central to the study of the role
of anthropogenic aerosols in climate forcing.
There are still many unresolved issues about gas-to-particle conversion processes
in the marine atmosphere. Although this discussion has focused on the DMS-SOrSO~aerosol system, it does demonstrate that there are many aspects of the aerosol formation process that are poorly characterized and that similar problems apply to other
types of gas-aerosol systems.
The concern about the role of anthropogenic sulphate particles in climate has provided the impetus for a renewed interest in aerosol chemistry. The interest in marine
SO~- stimulated a strong research effort into oceanic sources, in particular the role of
D MS. After more than a decade of intensive research, there are still many unanswered
questions about the marine sulphur cycle and the relative importance of natural and
anthropogenic sources in the atmospheric chemistry of sulphur over the oceans. Of
particular interest is the linkage between marine primary productivity and the impact that it has on the atmospheric sulphur cycle. If climate is indeed changing as a
result of human activities (or natural ones for that matter), then we might expect some
affects on ocean productivity. Thus, the possible feedback to the sulphur cycle has
important implications for future climate trends.
2.4
The Oceanic Atmospheric Cycle of Nitrates and Ammonium
Nitrate and ammonium are important to marine chemistry because of their possible
impact on the oceanic nutrient cycle. The important precursor species of aerosol NO;
are NO and N02 (collectively referred to as NO x )' There are a number of organic nitrates that may playa significant role, in particular peroxyacetylnitrate (PAN). The
entire ensemble of reactive gas phase N-species is generally referred to as NO y , which
is defined as the sum of NOx, HN03, PAN and other related minor (mostly organic)