A large fraction of Los Angeles' smog is ammonium nitrate particles, which are formed when ammonia reacts with nitrogen oxides that are produced in large quantities in automobile emissions. The ammonia is generated by cars with certain types of catalytic converters – and by bacteria that consume cattle waste. To provide an accounting of these ammonia sources in Los Angeles, researchers from the University of Colorado-Boulder and the National Oceanographic and Atmospheric Administration organized eighteen flights of a plane carrying instruments that could make detailed atmospheric measurements.

Some flew transects across the area just downwind of urban Los Angeles, and others flew across the area downwind of the large dairy farms east of the city.

Downwind of the city, the measured ratio of ammonia to carbon monoxide matched that of emissions from downtown traffic. Using the average concentration of ammonia and the area covered, they estimated that vehicles were contributing 62±24 metric tons of ammonia per day to the air above Los Angeles.

Downwind of the dairies, the story was different. There, where carbon monoxide was absent, the ammonia was more concentrated, but present in smaller areas. In total, estimates of the ammonia from dairies ranged (depending on the flight) from 33±16 to 176±88 metric tons per day.

That’s considerably higher than previous estimates that relied on extrapolations from average emissions per animal.


That means that, contrary to expectations, traffic and dairy farms appear to be about equally responsible for a sizeable fraction of the haze over Los Angeles.

Actually, it’s a little more complex than that – because the ammonia downwind of the dairies is much more concentrated, conditions favor greater NH4NO3 particle formation there. So, even for equal amounts of total ammonia, dairies will have an outsized impact on air quality. PD

—From Ars Technica (Click here to read the full article.)