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CMU study finds SOA levels in cities like LA will remain high despite cleaner cars; nonlinear relationship between SOA and NOx

The findings of a study by researchers at Carnegie Mellon University, with colleagues at UC Berkeley, suggest that changing atmospheric NOx levels over the next two decades will likely significantly reduce the effectiveness of stricter new gasoline vehicle emissions standards in reducing concentrations of secondary organic aerosol (SOA). The study is published in Proceedings of the National Academy of Sciences (PNAS).

Secondary organic aerosol is a major component of atmospheric fine particles, which negatively affect the human body and the earth’s climate. SOA production depends on both precursor concentrations (e.g., intermediate volatility organic compound (IVOC) emissions from vehicles) and atmospheric chemistry (SOA yields due to the photo-oxidation of exhaust). The study, led by CMU Professor Allen Robinson (earlier post), found a strongly nonlinear relationship between SOA formation and the ratio of non-methane organic gas to oxides of nitrogen (NOx) (NMOG:NOx). As an example, changing the NMOG:NOx from 4 to 10 ppbC/ppbNOx increased the SOA yield from dilute gasoline vehicle exhaust by a factor of 8.

As a result, Although organic gas emissions from gasoline vehicles in urban areas such as Los Angeles are expected to fall significantly over the next two decades, the team predicted no reduction in SOA production resulting from these lower emissions due to the effects of changing NMOG:NOx on SOA yields.

For the study, the team characterized tailpipe emissions from on-road gasoline vehicles and their SOA production using dynamometer testing at the California Air Resources Board’s (CARB) Haagen-Smit Laboratory. The test fleet comprised 59 on-road gasoline vehicles, spanning a wide range of model years and emission control technologies/standards. All of these vehicles were tested for primary emissions. A subset of these vehicles (n=25) was tested for SOA formation in a smog chamber.

Overall, we found that new and stricter regulations for gasoline vehicle tailpipe emissions will not be that effective at reducing human exposure to secondary organic aerosol because of changing NOx levels. This feedback illustrates the complex coupling between different pollutants, which must be accounted for in models used to develop control strategies.

About 23 million Americans live in areas that violate the current federal standard for fine particulate matter. SOA is a major component of those particles, and bringing these areas into compliance—so that all Americans breathe cleaner, safer air—will likely require addressing SOA pollution.

—Allen Robinson


  • Yunliang Zhao, Rawad Saleh, Georges Saliba, Albert A. Presto, Timothy D. Gordon, Greg T. Drozd, Allen H. Goldstein, Neil M. Donahue and Allen L. Robinson (2017) “Reducing secondary organic aerosol formation from gasoline vehicle exhaust” PNAS doi: 10.1073/pnas.1620911114



Seems no matter which way one cuts it the fossil fuels should stay buried.


"fossil fuels should stay buried" is the not a viable way. A better strategy carbon capture and re-purposing fuel to be feed-stock for graphene, and carbon tax.

This will ensure full support of the fossil fuel industry.

Otherwise reducing demand will lead to lower prices which feeds back to encourage demand.


Looking at it another way, regulators should be/should have been focusing on VOC emission reductions more than NOx emission reductions instead of the other way around.

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