Although carbon dioxide and methane concentrations are being studied quite extensively, oxygenated hydrocarbons(acetone and methanol) have rarely been studied. It will be shown that these types of molecules can play an important part in HO_x, NO_x and ozone cycling.

Data was collected using a Reduction Gas Detector. This data was collected in February-March 1994 during the NASA Pacific Exploratory Mission. From this mission the latitudinal distribution of acetone and peroxyacetylnitrate (PAN) in the troposphere was found. Acetone concentrations in the troposphere were on the order of 500ppt at northern latitudes and declined to approximately 200 ppt at southern latitudes. PAN concentrations were around 100ppt in the northern troposphere and around 10 ppt in the southern. Although the PAN concentrations were quite variable. Approximately 700 ppt of methanol was found in the northern latitudes and 400ppt in the southern.

The main removal process for methanol and acetone is most likely the reaction of OH radicals and photolysis. Based on published OH radical rate coefficients the average lifetimes of acetone and methanol can be estimated as 16 days. In order to balance the removal rate and the measured concentrations, a global source of about 45 Tg/yr methanol, and 50 Tg/yr acetone is calculated.

The reasons why all of this is important are the reactions that they can undergo as shown below: Methanol produces free radicals: CH₃OH + OH +(O) = CH₂O + HO₂ + H₂O Acetone can easily react with OH radical and be photolysed to produce free radicals and stable molecules like PAN:

CH₃COCH₃ + hv + (2O) = CH₃COO + CH₃O

CH₃COO + NO = CH₃COONO (PAN)

CH₃COO + HO₂ = CH₃COOH + O3 (67%)
= CH₃COOH + O (33%)

CH₃COO + NO + (O) = CH₃O + CO + NO

CH₃O + NO = CH₃O + NO

CH₃O + O = CH₂O + HO₂

CH₂O + hv = CO + H₂ (50%)
= 2HO₂ + CO (30%)
CH₂O + OH = CHO + H₂O (20%)

There are two main conclusions from these reactions:

The second conclusion is that acetone and methanol both have reaction pathways that produce free radicals (HO₂ for example.) Acetone in an important and previously unrecognized source of HO_x free radicals. By estimating the rate of the acetone reactions, acetone may increase HO_x radicals in the upper troposphere by 30%. HO_x radicals are crucial in ozone production. Thus the presence of acetone and molecules like it increase the production of ozone.

Anthropogenic sources of methanol and acetone have not been rigorously studied. From a recent study however 2.7 Tg/yr acetone, and 3.8 Tg/yr methanol were found to be be emitted by man. Burning of biomass also gives off large quantities of acetone and methanol to the troposphere. Natural sources(such as from oceans and forests) are becoming more recognized as larger sources than originally thought.

In conclusion, oxygenated hydrocarbons were found to be in fairly high concentrations in the troposphere. This is important because they participate in photochemistry of the troposphere by the formation of free radicals. These free radicals effect the rate of ozone formation. The sources of the oxygenated hydrocarbons are complex as are the reactions they undergo. Unfortunately these oxygenated hydrocarbons have not been extensively studied but should be in the future.

Reference

Singh, Hanwant and M. Kanakidou, 1995: High Concentrations and photochemical fate of oxygenated hydrocarbons in the global troposphere. Nature, 378, November 2.