Changes in Biogenic VOC’s

The work of Drew Purves and Arlene Fiore investigates the impact of recent changes in both natural and anthropogenic precursors on surface ozone smog. Ozone is created by the reaction of volatile organic compounds (VOC), which are released by automobiles and industry, with nitrogen oxides (collectively known as NOx), which are released during fossil fuel combustion, in the presence of sunlight. Over recent decades efforts have been made to reduce anthropogenic VOC and NOx emissions (e.g. via low-VOC fuels, catalytic converters, and vapor recovery equipment on gasoline pumps and engine technology), resulting in net decreases in VOC, and increases in NOx that were substantially lower than the increase in energy use and driver miles.

The complicating factor is that VOC also have a “natural” source – trees, which swamp anthropogenic VOC emissions on the regional scale. All trees emit VOC, but some release higher amounts than others. Purves’ work shows that a combination of forest regrowth on abandoned farmland and increases in plantation forests in the Eastern US caused rapid increases in biogenic VOC emissions between the 1980s and 1990s. The average increase in heatwave emissions of biogenic VOC was three times greater than the decrease in anthropogenic VOC. This result would seem to imply that forest re-growth and plantation forestry cancelled out some or all of the ozone improvements achieved via legislative reduction of VOC.

Figure 7. Estimated decadal change in heatwave emission rate mid-1980s to mid-1990s for isoprene and monoterpenes (BVOC), compared with decadal change in anthropogenic VOC emissions. Insets give percentage changes (scale from -30% to 130% decadal change). Average change in emission rate over all grid cells is given in parentheses above each map.


Prediction of Future Ozone Pollution

Fiore is applying chemical transport models to examine the implications of the recent changes in biogenic VOC found by Purves. Consistent with prior work over the past decade, the model results show that surface ozone over the eastern United States responds most strongly to reported changes in anthropogenic NOx emissions from the mid-1980s to the mid-1990s.

The magnitude and spatial pattern of the ozone response to NOx controls, however, depends strongly upon the assumed emissions of isoprene, a highly reactive VOC that is emitted abundantly from forests in the eastern United States. Fiore’s simulations show that the increases in isoprene in the southeastern United States reported by Purves could actually decrease ozone concentrations. This result calls into question the conventional wisdom in air quality management, that increases in VOC should either increase ozone (if sufficient NOx is available) or have little impact on ozone (in low-NOx settings). Fiore’s findings, along with those of other groups, imply that the expected isoprene emission increases in a warmer future climate may not raise surface ozone concentrations as much as might be anticipated from the strong correlation of ozone pollution events with temperature, particularly if more stringent controls on anthropogenic NOx emissions are implemented.