62
J. M. Pro spero
2.4.4
Organic Nitrogen Aerosols
Organic nitrogen could play an important role as an N-nutrient source. Measurements
of dissolved organic nitrogen (DON) in precipitation from various continental and
marine locations including the North Atlantic (Cornell et al. 1995) show that concentrations are often comparable to that of dissolved inorganic nitrogen (DIN, mainly NO;
and NH;). Organic nitrogen in aerosols and precipitation can result from reactions of
gas phase species and by mechanical processes that release particles directly (e.g. plant
fragments, sea spray droplets). The protein in pollens or oceanic surface films can
degrade in the atmosphere to lower molecular weight organic species such as amino
acids or primary amines. Because many different techniques have been used to study
organic nitrogen, it is difficult to make direct comparisons of results. Most research
has focused on the DON fraction.
More recently, Neff et al. (2001) reviewed the atmospheric organic nitrogen (AON)
cycle, including both reduced and oxidized forms. They found that AON comprises
about 1/3 of the total nitrogen deposited to the surface but that this fraction varies
widely from region to region. The sources of AON do not appear to be dominated by
pollution; there are no strong correlations between fluxes of nitrate and AON or ammonium and AON. They suggest that the reduced and bacterially-related forms of AON
do not appear to play an important role in the overall flux of AON to the surface of the
EartlI; however, they suggest that both dust and organic nitrates such as PAN may be
important. Neff et al. estimate a global AON flux between 10 and 50 Tg N yr- 1 .
These studies show that there are large unresolved uncertainties in the organic nitrogen cycle. Nonetheless, it is clear that organic nitrogen is an important aspect of
local and global atmospheric nitrogen budgets.
2.4.5
Trends in Nitrate and Ammonium in Pollution Aerosols
There are certain regions in the world where the emissions of NO x and NH3 are very
high relative to those of SOx. Examples are southern California, Western Europe and
India. Under such conditions, the SO;- aerosol is neutralized by NH 3 , and NH 4 N0 3 is
found in tlIe submicrometre size fraction. NH4N03 concentrations can exceed tlIose of
(NH4hSO 4 and tlIereby play an important role in radiative forcing on regional scales (ten
Brink et al. 1996). This has important implications about future trends in air quality and
radiative forcing. Air quality regulations have been most effective in reducing tlIe emissions of SOx tlIan of NOx. This is largely because the major sources of SOx are industry
and power plants that are easier to regulate than the principal sources of NO x , usually
cars and trucks. Thus, the ratio of NOx/SOx in emissions will continue to increase as
will the importance of NH4N03 aerosol. A substantial increase in the concentration of
NH4N03 has already been observed in the European plume emerging over tlIe eastern
NortlI Atlantic (Andreae et al. 2000; Raes et al. 2000). On a global scale over the coming
decades, the emissions of NOx and NH3 are expected to increase, while tlIose of SOx are
expected to remain relatively stable; tlIus, NH 4 N0 3 could play an increasingly important
role in radiative processes. This factor could also be important witlI regard to the deposition of nutrients to tlIe ocean, especially in coastal waters and regional seas.
J. M. Pro spero
2.4.4
Organic Nitrogen Aerosols
Organic nitrogen could play an important role as an N-nutrient source. Measurements
of dissolved organic nitrogen (DON) in precipitation from various continental and
marine locations including the North Atlantic (Cornell et al. 1995) show that concentrations are often comparable to that of dissolved inorganic nitrogen (DIN, mainly NO;
and NH;). Organic nitrogen in aerosols and precipitation can result from reactions of
gas phase species and by mechanical processes that release particles directly (e.g. plant
fragments, sea spray droplets). The protein in pollens or oceanic surface films can
degrade in the atmosphere to lower molecular weight organic species such as amino
acids or primary amines. Because many different techniques have been used to study
organic nitrogen, it is difficult to make direct comparisons of results. Most research
has focused on the DON fraction.
More recently, Neff et al. (2001) reviewed the atmospheric organic nitrogen (AON)
cycle, including both reduced and oxidized forms. They found that AON comprises
about 1/3 of the total nitrogen deposited to the surface but that this fraction varies
widely from region to region. The sources of AON do not appear to be dominated by
pollution; there are no strong correlations between fluxes of nitrate and AON or ammonium and AON. They suggest that the reduced and bacterially-related forms of AON
do not appear to play an important role in the overall flux of AON to the surface of the
EartlI; however, they suggest that both dust and organic nitrates such as PAN may be
important. Neff et al. estimate a global AON flux between 10 and 50 Tg N yr- 1 .
These studies show that there are large unresolved uncertainties in the organic nitrogen cycle. Nonetheless, it is clear that organic nitrogen is an important aspect of
local and global atmospheric nitrogen budgets.
2.4.5
Trends in Nitrate and Ammonium in Pollution Aerosols
There are certain regions in the world where the emissions of NO x and NH3 are very
high relative to those of SOx. Examples are southern California, Western Europe and
India. Under such conditions, the SO;- aerosol is neutralized by NH 3 , and NH 4 N0 3 is
found in tlIe submicrometre size fraction. NH4N03 concentrations can exceed tlIose of
(NH4hSO 4 and tlIereby play an important role in radiative forcing on regional scales (ten
Brink et al. 1996). This has important implications about future trends in air quality and
radiative forcing. Air quality regulations have been most effective in reducing tlIe emissions of SOx tlIan of NOx. This is largely because the major sources of SOx are industry
and power plants that are easier to regulate than the principal sources of NO x , usually
cars and trucks. Thus, the ratio of NOx/SOx in emissions will continue to increase as
will the importance of NH4N03 aerosol. A substantial increase in the concentration of
NH4N03 has already been observed in the European plume emerging over tlIe eastern
NortlI Atlantic (Andreae et al. 2000; Raes et al. 2000). On a global scale over the coming
decades, the emissions of NOx and NH3 are expected to increase, while tlIose of SOx are
expected to remain relatively stable; tlIus, NH 4 N0 3 could play an increasingly important
role in radiative processes. This factor could also be important witlI regard to the deposition of nutrients to tlIe ocean, especially in coastal waters and regional seas.