Study Concludes That Climate Change Alone Could Erode US Improvements in Ground Level Ozone Events Resulting from Reduced Emissions
Global climate change by itself can significantly worsen the severity and frequency of high ground-level ozone (O3) events over most locations in the US, even with relatively small changes in average O3 air quality, according to a new modeling study by researchers at Carnegie Mellon University.
While changes in US anthropogenic emissions will play the most important role in attaining (or not) near-term US O3 air quality standards, high-O3 increases due to climate change alone can moderately erode an improvement in O3 made under a scenario of reduced US emissions, they concluded. A paper on the study by Pavan Racherla and Peter Adams was published online 30 December 2008 in the ACS journal Environmental Science & Technology.
A number of projects are trying to ascertain the impact of global change on future US air quality, with some differing results and recommendations.
While the net effect of future climate change alone is a decrease in the global tropospheric O3 due to increased absolute humidity, there could be ground-level O3 increases in some polluted regions due to increased NOx under warmer temperatures, increased biogenic volatile organic compounds (VOCs) emissions, and circulation changes. Non-US anthropogenic emissions affect US O3 concentrations by influencing the US O3 background which, for example, contributes an average 15-30 ppbv to afternoon O3 mixing ratios in surface air in the eastern US and 25-35 ppbv in the western US (based on model simulations for the summer of 1995). Thus, future increases in non-US anthropogenic emissions, especially CH4 and nitrogen oxides (NOx), could increase US O3 concentrations.
...some studies find that anthropogenic emissions changes (US and worldwide) will influence future US O3 air quality the most, others find that climate change will play an equally—if not more—influential role; others point to reductions in CH4 emissions as a cost-effective method of achieving both global (and US) O3 air quality and climate objectives. Thus, there is a need for analyses that consider the multitude of factors influencing future US air quality under a common framework.—Racherla and Adams
Racherla and Adams Racherla and Adams focused on the combined effects of changes in climate and anthropogenic emissions worldwide. They did not perform a detailed scenario analysis of the O3 effects of CH4 and non-US emissions, but focused instead on quantifying their role in a high-emissions scenario.
The team used the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 and B1 scenarios, which have overall higher and lower O3-precursor emissions for the US, respectively, to examine the sensitivity of US O3 to regional emissions increases (decreases). The A2 climate scenario lies at the upper extreme of projected climate change for the 21st century.
They ran total of eight model simulations using the two scenarios with a “unified” model of global climate, gas-phase chemistry, and aerosols including sulfate, nitrate, ammonium, black carbon, primary and secondary organic aerosol, mineral dust, and sea salt.
Racherla and Adams suggested three policy implications of their findings:
Although US emissions will be the most important determinant of meeting ozone standards, policy makers must plan appropriately for O3 background increases due to projected increases in global CH4 abundance and non-US anthropogenic emissions, as well as potential local enhancements that they could cause.
Climate change, by itself, significantly worsens the severity and frequency of high-O3 episodes over most locations in the US. There appears to be a general consensus among both regional and global modeling studies on this, they note. If projected US emissions reductions (as in the B1 scenario) do materialize, the high-O3 increases due to future climate change alone will moderately erode the gains made.
The effect of climate change on high- and average-O3 increases with anthropogenic emissions.
The above finding that the O3 effect of climate change increases with anthropogenic emissions utilized has important regulatory implications. It implies, for example, that the benefits of anthropogenic emissions reductions under a future climate will be 2-fold. The O3 decreases due to the NOx/VOC emissions reductions themselves, which is a direct benefit, and the minimized O3 increase due to climate change itself, which could be viewed as an indirect benefit.
Conversely, the O3 increases due to anthropogenic emissions increases will be amplified by future climate change, which could be viewed as a climate “penalty”. These findings provide strong incentives for more-than-planned emissions reductions at locations that are currently O3-nonattainment.—Racherla and Adams
Pavan N. Racherla and Peter J. Adams. US Ozone Air Quality under Changing Climate and Anthropogenic Emissions. Environ. Sci. Technol., Article ASAP, doi: 10.1021/es800854f