1D: Alkyl Nitrates

Alkanes are ubiquitous components of the troposphere as a result of their long lifetimes and large emissions related to oil, petroleum and natural gas production. Oxidation of these alkanes by OH leads to formation of oxygenated VOCs (OVOCs), and organic nitrates, RONO2.

OH + RH (+ O2) → H2O + RO2
RO2 + NO → RO + NO2                                 (1-α)
RO2 + NO (+ M) → RONO2 (+ M)              α
RO → → OVOC + HO2

The organic nitrates are of particular importance, as their formation yields and ultimate fate determine the lifetime and distribution of NOx, and hence of ozone production. Higher nitrate yields imply less available NOx, and consequently a lower ozone production rate.

It has been known for many years that the yields of organic nitrates increase with the size of the alkane, with increasing pressure, and with decreasing temperature. Yet, these nitrate formation yields remain uncertain, even from fairly simple alkanes. In general, it is thought that the yields of nitrates from secondary RO2 are roughly twice those of primary and tertiary peroxy radicals, on the basis of data published in the mid to late 1980s. However, there is increasing evidence that the nitrate yields are in fact, largely independent of the nature of the RO2 radical involved.

Experiments have been conducted in the ACOM 50-L chamber by scientists Tyndall and Orlando to study the yields of alkyl nitrates from a series of linear and branched alkanes. The starting concentrations of alkane, NO and NO2 were measured by in situ FTIR spectrometry, while the concentrations of nitrate and carbonyl products were measured by gas chromatography. Nitrate yields from unbranched compounds (butane, pentane and cyclohexane) agreed well with those from the earlier studies. However, when branched compounds were used (2-methyl butane, 2-methyl pentane and 3-methl pentane) there was no discernible difference in nitrate yields from primary, secondary or tertiary sites. Work is continuing to synthesize authentic samples of alkyl nitrates to quantify these larger alkyl nitrate yields.

Figure 1: Region of gas chromatogram
Figure 1: Region of gas chromatogram showing production of alkyl nitrates as a function of retention time on the column. Nitrates are produced from the oxidation of 2-methyl pentane and show (from left to right) 2 secondary, 1 tertiary, and 2 primary nitrates.