Organic atmospheric: FTIR assesses pollution

Aerosols and particulates

Fourier transform infrared spectroscopy has been used to analyse submicron particles from 14 regions in North America, Asia, South America, and Europe. The measurements were used to identify characteristic organic functional group compositions of fuel combustion, terrestrial vegetation, and ocean bubble bursting sources. The information helps account for more than a third of organic mass in the atmosphere.

Laboratory experiments have shown that there are various mechanisms through which so-called secondary organic aerosols (SOAs) might form in the atmosphere. Such particles have an important effect on air quality and also the absorption and reflection of radiation from the Earth's surface including the oceans ultimately affecting the balance of climate and global temperatures through a shift in surface albedo. Models have suggested that much of the organic matter in the atmosphere is due to human activity, combustion of fossil fuels, for instance, but there are many other ways in which SOAs might be generated.

Now, Lynn Russell and Ranjit Bahadur of the Scripps Institution of Oceanography, at the University of California and Paul Ziemann of the Air Pollution Research Center and Department of Chemistry, at the University of California, Riverside, California, explain that the majority of the organic matter consists of alkyl, carboxylic acid, hydroxyl, and carbonyl groups. Their study of the organic functional groups formed by combustion and vegetation emissions could help explain the way in which SOAs are generated and so not only help improve climate models but also point to potential remediation methods should anthropogenic sources be to blame.

Spectroscopy versus spectrometry

The team has now used FTIR studies to compare the measured organic functional group composition of atmospheric aerosols with those estimated from chamber studies. Their approach should be able to provide an assessment of the extent to which chamber measurements are valid in predicting atmospheric SOA formation. The FTIR approach circumvents the intrinsic issue associated with mass spectrometry fragments in that it is more specific.

The researchers point out that although the FTIR approach does not identify individual molecules, which would otherwise act as direct evidence for specific reaction pathways in the atmosphere, by quantifying the major functional groups it puts limits on the types of compounds that can form in the atmosphere.

Not only did the team determine the general cross section of functional groups in the organic matter, but they also demonstrated that such groups formed from combustion and vegetation emissions are similar to the secondary products that have been identified in chamber studies. Carbonyl groups are mostly lacking in the observed SOA associated with combustion, which they explain is consistent with alkane rather than aromatic precursors. The absence of organonitrate groups is explained because they are hydrolysed in humid ambient conditions.

Combustion sources

Data from remote forests puts a different perspective on organic matter than that observed downwind of urban areas. The ratios of carboxylic acid, organic hydroxyl, and non-acid carbonyl groups are similar to those observed for isoprene and monoterpene chamber studies simulating forest vegetation whereas esters replace the acid and hydroxyl groups, leaving only non-acid carbonyl groups in the urban downwind samples. They add that the non-acid carbonyl groups indicate a link between organic matter from biogenic and biomass burning and that this is consistent with chamber studies on monoterpenes and isoprene.

The studies help corroborate chamber studies and simultaneously offer new insights into the way in which SOAs might be generated and the effect of human activity on the various components.

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