Chemical Transformations And Speciation Of Reactive Nitrogen

Almost all reactive nitrogen is introduced into the atmosphere as NO, but within minutes, NO reaches equilibrium with N02. This NO, is subsequently transformed into other NOy species that can be removed, transported, or recycled back to NO,. A general outline of these transformations is represented in Figure 1, which accompanies the following discussion of the behavior and importance of various NOy species.

1. NOx {NO + N02). During the day, NO and N02 experience rapid interconversion via the following simple reaction scheme:

This reaction sequence is a null cycle that serves no purpose photochemically other than to partition NO, into NO and N02. NO may also be converted to N02 by hydroperoxy radicals (H02) that result from the oxidation of CO as well as organic

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2 CHEMICAL TRANSFORMATIONS AND SPECIATION OF REACTIVE NITROGEN

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Figure 1 General schematic of reactive nitrogen chemistry in the troposphere arranged to emphasize dominant pathways in the presence of sunlight and in darkness.

peroxy radicals (RO:. where R denotes a CH, or higher organic grouping) that result from the oxidation of hydrocarbons. This cycle of NO-NO: interconversion has impacts outside the NOi-NQ system,

Here, two key impacts of NOv interconversion are the formation of 03 and the regeneration of OH from HO,,

The conversion of N0V into other longer-lived NOt reservoir species is accomplished almost exclusively through reactions involving N0; (see Figure 1). Thus, the lifetime of NO, in the atmosphere relies in part on the partitioning of N0( between its constituents, NO and NO-,, As shown in Figure 2, the fraction of N0( existing in the form of NO; changes dramatically w ith altitude. At the surface, NO. tends to be predominantly in the form of NO: since reaction (I) proceeds at a faster rate than reaction (2). Here. N0V lifetimes are typically one day or less. Reaction (1). however, has a strong temperature dependence and is about 5 times slower at the cold tempera tures of the upper troposphere, thus shifting the NOx equilibrium in favor of NO. To a lesser degree, the increase in reaction (2) with altitude (~50%) also contributes to an NOx partitioning that favors NO at high altitude. As N02 becomes a smaller fraction of NOx with increasing altitude, the lifetime of NOx lengthens. In the upper troposphere, NOx lifetimes can be a few days to a week. The longer lifetime of NOx at high altitude tends to enhance its per-molecule efficiency in the production of ozone since NO can be cycled through reaction (4) more times before being lost. Although efficiency is increased at high altitude, the ozone production rate per molecule of NO^ is slower owing to the lower abundance of H02, which generally decreases with altitude.

2. NOt,. The nitrate radical, N03, photolyzes within a few seconds in sunlight; thus, it is of negligible importance to the daytime photochemistry of the atmosphere. N03 is formed by the reaction of N02 with 03.

Overnight at the surface, a significant fraction of N02 may be converted by this reaction. The strong temperature dependence of reaction (5), however, slows conversion rates by an order of magnitude for the upper free troposphere, where only a small fraction of N02 may be converted overnight. Given its concentration and high

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Figure 2 Fraction of NO, in the form of N02 as a function of altitude. Data based on concurrent measurements of NO and N02 conducted during NASA's PEM-Tropics A field campaign (Bradshaw et al., 1999).

2 CHEMICAL TRANSFORMATIONS AND SPECIATION OF REACTIVE NITROGEN 65

reactivity, N03 can be competitive with OH as an oxidant of dimethylsulfide (DMS) in the marine boundary layer of coastal regions. In general, however, marine NOx levels are insufficient to support more than a minor role for N03 in DMS oxidation. In the continental boundary layer, N03 can be important in the oxidation of unsaturated hydrocarbons, e.g., olefins and biogenic hydrocarbons such as isoprene.

3. N205. N205 is formed during periods of darkness by the reaction of N03 and N02.

Here, M represents an inert third body, typically N2 or 02. N205 is a thermally labile species; therefore, the equilibrium represented by reaction (6) favors N03 near the surface. Larger concentrations of N205 exist at high altitude where it is favored by colder temperatures, although its concentration is still limited by the slowdown in reaction (5). N205 also has a lifetime due to photolysis of several hours, but this is not sufficient to prevent significant daytime concentrations in the upper troposphere. N205 represents a loss of NOx through its heterogeneous conversion to nitric acid (HN03) on aerosol surfaces.

4. HN03. Nitric acid is believed to be the major reservoir species for NOx. It is formed primarily by the reaction of N02 and OH.

HN03 can also result from the heterogeneous reaction of N205 on aerosols or the reaction of N03 with certain hydrocarbons. HN03 is efficiently removed from the atmosphere through dry deposition and rainout processes. HN03 may also be recycled back into NOx through either reaction with OH or photolysis with a lifetime of a few weeks. At low altitude these two processes are slow compared to wet removal, but they can be important at high altitude where wet removal is less frequent.

5. HONO. Nitrous acid is formed by the gas-phase reaction of NO and OH. It also photolyzes within minutes and thus is a negligibly small component of NO,,. Although the details are not fully understood, evidence exists for nighttime formation of HONO, possibly through heterogeneous processes, based on elevated nighttime observations of HONO. Substantial buildups of HONO may take place in special environments such as overnight in polluted air rich in NOv or in polar regions with extended periods of darkness. For these special conditions, HONO may for a short time be the dominant source of OH through its rapid photolysis during sunrise periods.

6. H02N02. Pernitric acid is a thermally labile species resulting from the reaction of H02 and N02. As with N205, it is favored at the cold temperatures of the upper troposphere. In the upper troposphere, H02N02 concentrations are limited by reaction with OH and photolysis resulting in a lifetime of only a couple of days, but concentrations should approach that of NO,. H02N02 may also return to NO, through thermal decomposition in descending air masses.

7. PAN. Peroxyacetylnitrate is the most common organic nitrogen-containing species resulting from the reaction of N02 with the CH3C03 radical. The CH3C03 radical results from oxidation of a wide range of hydrocarbons, but at high altitude oxidation of acetone appears to be predominantly responsible. While loss in the lower troposphere is dominated by thermal decomposition, loss in the upper troposphere occurs through photolysis with a lifetime of 1 to 2 months. At high altitude, PAN is thermally stable and serves as an effective reservoir for global-scale transport of NO,,. In remote regions, thermal decomposition of PAN in descending air masses can be a dominant source of NO,. Somewhat analogous to H02N02, organic peroxy radicals (R02) can react with N02 to form organic species (R02N02) with properties similar to those of PAN.

8. CH-i0N01. Although not depicted in Figure 1, methyl nitrate represents the most common member of a family of alkyl nitrates that can result from NO, in the presence of hydrocarbon oxidation. There is also evidence that these species are emitted from the ocean in small amounts. Loss is primarily through photolysis to yield NO,.

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