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The newly discovered dynamic role of primary organic aerosols

April 2nd, 2007 by Hasenkopf · 1 Comment

An article by Robinson et al. appeared in Science last month and presented laboratory evidence that primary organic aerosols (the term given to aerosols directly injected into the atmosphere from a source) may generate more secondary aerosols (those that form from the oxidation of gas-phase precursors) than previously thought from atmospheric chemistry models.

Currently, primary organic aerosols are statically represented in models, while Robinson et al. believe their laboratory work demonstrates that they must be represented more dynamically as semi-volatile aerosols that lose their low-volatility material. The low-volatility material then oxidizes, and recondenses to form secondary organic aerosols.

The figure below does a pretty good job illustrating the authors’ point that primary organic aerosols do indeed contain a lot of semi-volatile material. The graph shows how diesel exhaust aerosol mass is not conserved as it is diluted with air. The line would be horizontal and have a constant normalized organic aerosols emission factor of 1 if the condensed mass of the aerosol was conserved while it underwent dilution.

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The authors then showed that secondary aerosols could be created from the volatile gases given off from diluted primary aerosols (diesel gas was again used) by exposing the primary aerosols to UV light and using an aerosol mass spectrometer to monitor the primary and secondary aerosols content. The results, shown in the figure below, initially confused me because I didn’t understand why, if the secondary aerosols were forming from the primary, the total mass was not constant and the primary aerosol was decreasing. But I think what is going on is that since the diesel exhaust is pre-diluted before interacting with the UV-light, the sample has already been partitioned into the gas and condensed phase, so the primary organic aerosol content is constant, while the secondary increases beyond expected levels (which is indicated by the blue portion).

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The big picture of their work is summarized by their last figure which compares the fraction of secondary organic aerosols for the eastern portion of the US to their revised fraction of SOA for the same area.

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There are big emissions-regulations implications in this work – apparently diesel exhaust can cause more secondary aerosol generation than previously realized. Also, it has been believed that urban and rural populations were exposed to mostly primary and secondary organic aerosols, respectively. However, this work implies that both populations are exposed primarily to secondary organic aerosols.

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