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NOAA, partners find about 98% decline in VOCs concentrations in LA Basin over last 50 years despite 3x increase in use of gasoline and diesel

A scatter plot of CO, as a tracer for pollution, versus CO2, as a tracer for the amount of combustion, for 2002 and 2010. Even though the CO2 enhancements over background were comparable in the two years, about 25 ppm in 2002 and 35 ppm in 2010, the ∆CO/∆CO2 ratio for this flight track segment decreased from 24.5 ppbv/ppmv to 10.7 ppbv/ppmv. Source: Warneke et al. Click to enlarge.

In California’s Los Angeles Basin, levels of volatile organic compounds (VOCs) pollutants have decreased by about 98% since the 1960s, even as area residents now burn approximately three times as much gasoline and diesel fuel. Between 2002 and 2010 alone, the concentration of VOCs dropped by half, according to a new study by NOAA scientists and colleagues, published in the AGU’s Journal of Geophysical Research.

VOCs, primarily emitted from the tailpipes of vehicles, are a key ingredient in the formation of ground-level ozone which, at high levels, can harm people’s lungs and damage crops and other plants. The magnitude of the drop in VOC levels was surprising, even to researchers who expected some kind of decrease resulting from California’s longtime efforts to control vehicle pollution.

The reason is simple: cars are getting cleaner. Even on the most polluted day during a research mission in 2010, we measured half the VOCs we had seen just eight years earlier. The difference was amazing.

—lead author Carsten Warneke, Ph.D., Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder

The 98% drop in VOCs in the last 50 years does not mean that ozone levels have dropped that steeply; the air chemistry that leads from VOCs to ozone is more complex than that. Ozone pollution in the Los Angeles Basin has decreased since the 1960s, but levels still don’t meet ozone standards set by the Environmental Protection Agency.

Requirements for catalytic converters, use of reformulated fuels less prone to evaporate, and improved engine efficiency of new vehicles have all likely contributed to overall declines in vehicle-related pollution, including VOCs.

For the new study, Warneke and his colleagues evaluated Los Angeles air quality measurements from three sources: NOAA-led research campaigns in 2002 and 2010, which involved extensive aircraft sampling of the atmosphere; datasets from other intensive field campaigns reaching back five decades; and air quality measurements from the California Air Resources Board monitoring sites, which reach back two to three decades.

Detailed VOC measurements from two aircraft campaigns in the LA basin in 2002 and 2010 were used to show that VOC emissions have been reduced by about a factor of two in those eight years. Literature data and data from the monitoring network in the LA basin were used to show that the mixing ratios of VOCs and CO have declined almost two orders of magnitude since 1960 at an average annual rate of about 7.5% even though the fuel sale in California increased by about a factor of three over the same time period.

VOC/CO ratios have been remarkable constant in this time period despite the introduction of the catalyst and reformulated and oxygenated gasoline. This indicates that the main VOC source in the LA basin is likely gasoline vehicle emission. Ethane and propane have decreased more slowly than all other VOCs indicating that after the large reduction in vehicle emissions other sources such as the use and production of natural gas can have a significant contribution to ambient mixing ratios.

A comparison with London found that VOC mixing ratios in 2008 are at about the same level in both cities, but in London a much stronger decrease from higher mixing ratios in 1998 has led to the current mixing ratios. This suggests that the early implementation of VOC emission reduction strategies in California clearly has led to improved air quality earlier on compared to London.

—Warneke et al.

Another recent study led by CIRES and NOAA researchers and published online 4 August in Geophysical Research Letters has shown that one VOC, ethanol, is increasing in the atmosphere, consistent with its increasing use in transportation fuels.

Warneke said that he would expect the decrease in emissions of VOCs by cars to continue in Los Angeles, given that engine efficiency continues to improve and older, more polluting vehicles drop out of the fleet of all vehicles on the road.


  • Warneke, C., J. A. de Gouw, J. S. S. Holloway, J. Peischl, T. B. B. Ryerson, E. L. Atlas, D. R. Blake, M. K. Trainer, and D. D. D. Parrish (2012), Multi-Year Trends in Volatile Organic Compounds in Los Angeles, California: Five Decades of Decreasing Emissions, J. Geophys. Res., doi: 10.1029/2012JD017899, in press



Even in the north of the country;

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