Atmospheric importance of Nitrate ester

Atmospheric Occurrence

Atmospheric Abundance

Alkyl nitrates (RONO₂) compounds are widespread in the boundary layer, accounting for a large portion of total reactive nitrogen oxides in both urban and rural environments and across various hydrocarbon mixtures. In addition, alkyl nitrates are typically complex, multifunctional molecules that are still challenging to detect using chromatographic methods at the concentrations found under ambient conditions. An observational study of alkyl nitrates was carried out, during which air samples were collected around midday in summer under sunny conditions. In total, 43 (C₆–C₁₃) alkyl mononitrates, 24 (C₃–C₆) alkyl dinitrates, and 19 (C₂–C₆) hydroxy alkyl nitrates were identified. In the urban air samples, the summed concentrations of 15 (C₆–C₁₀) alkyl mononitrates ranged from 2.9 to 11.0 pptv. The total levels of 21 (C₃–C₆) alkyl dinitrates were between 2.3 and 10.5 pptv, and the sum of 7 (C₂–C₄) hydroxy alkyl nitrates ranged from 7.3 to 28 pptv. These findings indicate that alkyl dinitrates, hydroxy alkyl nitrates, and alkyl mononitrates make an important contribution to the NO_y compound budget. Moreover, no significant differences in either the concentrations or patterns of organic nitrates were observed between the urban air of cities. Organic nitrates, however, occur at comparatively low concentrations. Short-chain alkyl nitrates (C₄–C₆) exhibit mixing ratios of about 0.2–2.5 pptv, with a pronounced minimum in the tropics and much lower levels in the Southern Hemisphere. For the longer and more functionalized species, the summed mixing ratio of 36 long-chain alkyl mononitrates (C₇–C₁₃) lies between 0.02 and 0.43 pptv, that of 23 alkyl dinitrates (C₃–C₆) ranges from 0.005 to 1.08 pptv, and that of 7 hydroxy alkyl nitrates (C₂–C₄) ranges from 0.005 to 1.07 pptv.

Alkyl Nitrates Abundance in Variant Geographic Regions

Concentrations (pptv) / Region	Urban area	Marine air
Alkyl mononitrates	2.9 – 11.0	0.02 – 0.43
Alkyl dinitrates	2.3 – 10.5	0.005 – 1.08
Hydroxy alkyl nitrates	7.3 – 28	0.005 – 1.07

Formation Mechanisms

There are two primary mechanisms for producing RONO₂ in the atmosphere:

(1) oxidation of hydrocarbons initiated by hydroxyl radicals (OH) in the presence of NO_x during the daytime.

(2) oxidation of alkenes initiated by nitrate radicals (NO₃) at night.

Methyl nitrate (CH₃ONO₂), ethyl nitrate (C₂H₅ONO₂), and possibly propyl nitrate (C₃H₇ONO₂) are released directly from the oceans, and concentrations of these compounds reaching up to several tens of ppt have been observed in remote marine environments. Apart from these marine emissions, direct emissions are generally not regarded as a major source. RONO₂ exhibit vapor pressures covering a very wide range, they occur in the atmosphere in both gaseous form and aerosol particles.

OH-Initiated RONO₂ Formation

When OH reacts with a saturated hydrocarbon (1), it first abstracts a hydrogen atom, and almost immediately afterward, oxygen (O₂) adds to the resulting radical to form an RO₂ radical (2).

RH + OH → R • + H₂O (1)

R • + O₂ → RO₂ • (2)

Two distinct product pathways (3a and 3b) arise from the reaction between RO₂ and NO when NO_x is present.

RO₂ • + NO → RO • + NO₂ (3a)

RO₂ • + NO → RONO₂ (3b)

The dominant pathway 3a, produces NO₂ and an alkoxy radical and thereby continues the NO_x and HO_x catalytic cycles. In contrast, the less frequent pathway 3b, terminates the chain by generating a stable monofunctional organic nitrate. Reaction 3b is a termolecular process and represents only a minor pathway (typically <5%) compared with the major, bimolecular pathway, reaction 3a.

NO₃-Initiated RONO₂ Formation

During daylight hours, the OH-initiated pathway described previously is the primary source of RONO₂. At night, in contrast, RONO₂ formation occurs through reactions of NO₃ with alkenes and phenols. These nighttime reactions have high nitrate formation amounts. Although the NO₃ pathway constitutes only a small portion of overall organic oxidation relative to OH, this pathway can still account for up to about 50% of the regional RONO₂ burden.

The process begins with NO₃ adding to a carbon–carbon double bond (4). The alkyl radical formed in this step then rapidly reacts with O₂, analogous to reaction 2, yielding a peroxy radical (5).

R₁=R₂ + NO₃ → R₁(ONO₂)- R₂ • (4)

R₁(ONO₂)- R₂ • → R₁(ONO₂)- R₂O₂ • (5)

Once formed, this peroxy radical can react with HO₂, RO₂, or NO₃, and these reactions yield stable aldehyde- or alcohol-nitrate products. In contrast, pathways involving NO (3a and 3b) are negligible in the atmosphere because NO₃ cannot accumulate where NO is present; NO and NO₃ rapidly react together, producing two NO₂ molecules. Nitrooxy-peroxynitrates (R₁(ONO₂)–R₂O₂NO₂) can arise from the reaction of nitrooxy-peroxy radicals with NO₂ and have been detected as transient species in laboratory experiments. However, their strong thermal instability and the lack of ambient detections indicate that they mainly influence how laboratory results are interpreted rather than playing a direct role in atmospheric chemistry. NO₃ is also theoretically capable of oxidizing alkanes through hydrogen atom abstraction, but this reaction proceeds so slowly that it is not considered important under atmospheric conditions.

Atmospheric Removal

For gas phase, a RONO₂ molecule has several possible fates: it may be transported by atmospheric winds, take part in further chemical reactions, deposit onto the surface, or partition into the aerosol phase. How quickly any of these processes occur depends on the molecule’s specific structure, which controls both its reactivity and its tendency to deposit or move into particles.

Nitrates that contain multiple functional groups, either formed that way in the first oxidation step or made multifunctional after successive reactions, are expected to be particularly reactive. Molecules that still possess a carbon–carbon double bond or have abstractable hydrogen atoms also tend to be processed more rapidly. As one example, first-generation isoprene nitrates react quickly with both O₃ and OH, and all eight isomers could survive daytime oxidation but no more than a few hours. With continued oxidation, the rate of resulting higher-generation products would differ from those of the parent nitrate.

The detailed products formed when RONO₂ are oxidized by OH, O₃, or NO₃ are not generally well constrained, but the overall chemistry can be thought of as following two main pathways:

1. Oxidation preserves the nitrate group and produces a stable multifunctional nitrate, R₁ONO₂. Here the original carbon skeleton R has been altered so that the new backbone R₁ carries additional functional groups (e.g., carbonyls, alcohols, or extra nitrate groups).
2. Oxidation removes the nitrate functionality, releasing NO₂ and effectively stripping the nitrate group from the molecule.

RONO₂ compounds absorb near-UV radiation (λ < 340 nm) and break down through photodissociation, yielding RO• radicals and NO2 with quantum efficiencies close to unity. Experimental studies have determined photolysis rates for numerous C₁–C₅ alkyl and cycloalkyl nitrates, as well as for several difunctional organic nitrates.

For the smaller members of this family (C₁–C₄), these photolysis rates are sufficiently fast that they rival oxidation by OH. Under typical summer surface conditions with a 0° solar zenith angle, for instance, the photolytic lifetime is on the order of 3 days for t-butyl nitrate and about 10 days for methyl nitrate. On the other hand, most other RONO₂ photolyze much more slowly than they react with OH, so for larger nitrooxy alkanes and alkenes, photolysis contributes little, if at all, to their atmospheric loss.

References

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