Contradictory California Studies Question Diesel Cleanliness

Two studies published in the past year in California have had contradictory conclusions about the cleanliness of diesel fumes. Both studies looked at secondary organic aerosols, which is a major contributor to smog

A study published in October in the Proceedings of the National Academy of Sciences of the United States of America found that diesel engines produced more a pollutant called secondary organic aerosol than gasoline engines. 

The study found that diesel engines produce 65% to 90% of vehicular-derived secondary organic aerosol. It specifically took air quality samples in a tunnel in Bakersfield, California. 

"We can now say that, while both motor vehicle sources are important for these 'secondary' particles, diesel is responsible for a larger proportion, especially in regions such as the San Joaquin Valley with a lot of diesel use," said study author Allen Goldstein.

However, a study published in March in Geophysical Research Letters found the exact opposite - that gasoline engines produce more secondary organic aerosol. This study examined ground level secondary organic aerosol levels in the Los Angeles Basin.

The study found that diesel emissions were reduced by 54% on the weekends, but secondary organic aerosols were roughly the same, actually slightly higher, on the weekend. The scientists came to the conclusion that diesels were not the major contributors to secondary organic aerosols to air quality. 

Diesel fuel accounts for about 21% of on-road fuel use in the US.

The major difference between these studies is that the most recent study looked at air quality readings within a tunnel, and the March study looked at air quality in general. It appears that diesel engines may produce more secondary organic aerosols, but in the real world diesel-fueled vehicles do not make up a large enough percentage to make a contribution to smog. 

Source: LA Times, Geophysical Research Letters, Proceedings of the National Academy of Sciences of the United States of America