Research Changes in the Regional Emissions of Greenhouse Gases and Ozone-Depleting Compounds M. ASLAM K. KHALIL* Department of Physics, Portland State University, P. O. Box 751, Portland, Oregon 97207 REINHOLD A. RASMUSSEN Department of Environmental Science and Engineering, Oregon Graduate Institute, 20000 Northwest Walker Road, Beaverton, Oregon 97006
In the wake of the Kyoto and Montreal Protocols, there is a need to verify whether policies to reduce emissions are working. We present data showing that emissions of ozone-depleting compounds, such as the chlorofluorocarbons and methyl chloroform, are decreasing from some regions of the United States but emissions of the greenhouse gases do not appear to be declining.
Introduction Trace gases play a major role in determining the Earth’s climate and other environmental features such as the ozone layer that are crucial to human life and welfare. It is wellknown that human activities including agriculture, industrial processes, and burning fossil fuels are capable of causing global warming and depletion of stratospheric ozone. International agreements such as the Montreal Protocol, banning the production of chlorofluorocarbons and related compounds, and the Kyoto Protocol, aimed at limiting emissions of gases that can cause global warming, are the first steps in managing the environment. Once agreements are reached, it is necessary to create observational methods that can show whether the controls are actually reducing emissions and in which regions or countries these reductions are occurring. Toward this goal, we discuss the results of an experiment to evaluate the changes of emissions from two selected regions in the United States.
Experimental Design Sampling Regime. As a part of the GLOBE project (Global Observations to Benefit the Environment), we took measurements of greenhouse gases and ozone-depleting compounds at schools in Portland, OR, and on the East Coast in Burlington, VT, and Ashburnham near Boston, MA. Once a week, triplicate air samples were collected in 0.8 L internally electropolished canisters over 10-20 min and sent to our laboratory for analysis (1). We measured the concentrations of key greenhouse gases (CO2, CH4, and N2O) and ozonedepleting compounds (CCl3F, CCl2F2, CH3CCl3, and CCl4) using standard gas chromatographic techniques (1, 2). Carbon monoxide was also measured in these experiments, since it has an indirect effect on environmentally active gases. The measurements were taken over 2 years between January * Corresponding author phone: (503)725-8396; fax: (503)725-8550; e-mail:
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1996 and January 1998. During the same time, we also took measurements of these gases at two long-term background sites at comparable latitudes: Cape Meares, OR (45° N, 124° W) and Minqin, China (38.4° N, 103.1° E). The site at Cape Meares is west of Portland and has been established to represent the marine concentrations of atmospheric trace gases at middle northern latitudes (3). Readers may obtain the complete data from the supporting archive.
Results Data from the four Portland area schools were pooled (West Coast). Although the east coast schools were not as close to each other as the Portland schools, we pooled the data because there were insufficient measurements at each school for the analyses discussed here. For the background concentrations of the trace gases we used data from Cape Meares as representative of the entire middle latitudes. To analyze the data, we considered two measuressthe excess of concentrations at our sites compared with the background (δC) and the relative change of this excess from one year to the next ∆(δC). The average excess δC ) C - CCM is shown in Figure 1 where C is the concentration at our sites and CCM is the concentration at Cape Meares (δC is calculated for every month and then averaged over the duration of the experiment). In all cases we report the mean values of the variables and the associated 90% confidence limits based on the t-statistic. In the regions we studied, the emissions of greenhouse gases and ozone-depleting compounds come primarily from urban or industrial areas in the vicinity. Any agricultural emissions from around the sites would primarily affect N2O. The gases of interest, therefore, will be more concentrated in these source regions (4-7). The figure shows clear excesses of the major ozone-depleting chlorocarbons (F-12, F-11, F-113, and CH3CCl3) suggesting sources around both the East Coast and the West Coast sites where we took our samples. There are no excesses for carbon tetrachloride (CCl4), which serves as a control since it is not used in the United States. Furthermore, at the background continental site of Minqin, there are no significant excesses of these compounds compared with Cape Meares during the period of our measurements. This is consistent with the fact that Minqin is distant from most major urban and industrial areas where chlorocarbons or greenhouse gases are emitted. Similar results are seen for the greenhouse gases (CH4, CO2, and N2O). The agreement between the two distant background sites establishes that concentrations measured there are representative of the nonurban atmosphere for middle northern latitudes and hence suitable for use as the background concentrations here. The second variable we calculated is the difference of the excess between one year and the next relative to the base year: ∆(δC) ) [δC(2) - δC(1)]/δC(1), where δC(2) and δC(1) are the observed annual average excesses during year 2 and year 1, respectively (we have only 2 years of data). This difference reflects changes in the emissions or the locations of the sources, assuming that the annual average meteorological dispersion does not change much from one year to the next. Using basic principles of mass balance, we can relate the measured variables (∆(δC)) to changes in the emissions. We imagine a spatial grid surrounding our sites. The emission from each grid region (i) causes the concentration of the gas involved to increase by an amount δCi (g/cm3) ) Si(t) 10.1021/es0341539 CCC: $27.50
2004 American Chemical Society Published on Web 12/05/2003
FIGURE 1. Excess over background levels of (a) greenhouse gases and (b) ozone-depleting compounds, in urban areas where measurements were taken. The results shown are an average excess of concentrations over Cape Meares, a clean-air continental site on the Oregon coast. Data for Minquin in the Gansu Province of China represents a clean-air continental site, where concentrations are only slightly different from Cape Meares. Carbon tetrachloride represents a control case since there are no known emissions near the locations where measurements were taken. The results for CO and CH3CCl3 were multiplied by 0.02 and 0.2, respectively, to fit on the graph. (g/s)/Di(t) (cm3/s) above the background level. Here Si(t) is the emission around time t and Di(t) is the dilution factor that depends on the meteorological conditions and distance of the grid point from the site. It may change by time of day, and season. For distant sources the lifetime of the gas may also affect the relationship. The gases we are dealing with are sufficiently long-lived that we have neglected losses during transport from the sources to our sites. The observed excess (δC) is related to the total source (S) in our region by
δC ) g‚S
(1)
∑(S /D )/∑S ]
g)[
i
i
i
Here g is the sum (or integral) of the conjoined normalized spatial distribution of the sources given by Si/∑Si and the dispersion factors 1/Di. We can do away with diurnal, seasonal, and day to day changes in the emissions and dilution factors by taking annual averages of eq 1. Assuming that the emissions and the dilution factors are uncorrelated, the variables in eq 1 can be taken to be annual averages, which will be assumed here. As δC changes from year to year, some or all of the change can be attributed to changing total emissions from our region of study (S) and the rest to
FIGURE 2. Calculated reductions of emissions of (a) greenhouse gases and (b) ozone-depleting compounds at the locations where measurements were taken. The data show decreases of carbon monoxide as well as some chlorocarbons that are affected by the Montreal Protocol. The result for CO from the East Coast schools was multiplied by 0.1 to make it fit on the graph. the change of the source distribution and dispersion factor “g” as follows:
∆(δC%) ) ∆(S%) + S(2)/S(1)∆(g%)
(2)
Here ∆x ) {[x(2) - x(1)]/x(1)} × 100%, x being S or g. The observed values of the variable ∆(δC%) are shown in Figure 2. According to eq 2, it reflects the change of emissions (∆S), or the change of source locations relative to our sites (∆g), or a combination of these factors, assuming that the annual average meteorological dispersion does not change from year to year. If the changes in dispersion were solely responsible for the observed relative change ∆(δC) then the expected value would be more or less the same for all the gases, which is clearly not the case. The calculations show evidence that, during the course of our measurements, methane and carbon monoxide emissions went down at the East Coast locations, but not at the West Coast locations. Slight increases of N2O were observed at both locations, but these did not reach statistical significance (10% level). For the ozone-depleting compounds, which are under the control of the Montreal Protocol, there is evidence that F-12 emissions decreased from the East Coast locations, and although a decrease of emissions is indicated from the West Coast locations, it is not statistically significant. Similarly for F-11, we were not able to verify decreases or VOL. 38, NO. 2, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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increases of emissions during the time of our measurements. For methyl chloroform, there is clear evidence of substantial decreases in emissions from both regions. Carbon tetrachloride, which is not used around the U.S. sites, shows no change (from zero emissions) and serves as a control for our method. The magnitudes of the observed changes in concentrations (∆(δC%)) are as follows: East Coast, ∆(δC(CH4)) ) -35 ( 20%, ∆(δC(CO)) ) -55 ( 30%, ∆(δC(F-12)) ) -50 ( 30%, and ∆(δC(CH3CCl3)) ) -50 ( 24%; West Coast, ∆(δC(CH3CCl3)) ) -45 ( 20%. These changes however cannot be equated with like changes in the emissions (∆(S%)) because of the complications introduced by the effect of the location of the sources as indicated in eq 2. Nonetheless, changes in the location and dispersion factor (∆(g%)) alone cannot explain the results, particularly for the Portland area, where data from several sites were averaged and where there are no known cases of emissions remaining the same but sources moving further away which could cause an observed decrease of concentrations without a commensurate decrease in total emissions from the region of interest.
Discussion We have demonstrated that by measuring the concentrations of greenhouse gases and ozone-depleting compounds in or around the regions where emissions occur, we can establish whether the emissions are increasing or decreasing. Some remaining issues are discussed next. Because of the short span of our data, we can only estimate the change in emissions that occurred over the one year that separates the middles of the two years of measurements. Over one year the changes are not expected to be very large and hence may not be statistically detectable. We have therefore been able to detect only the changes that are relatively large. Policies for implementing reductions generally span many years, and hence slow changes occur over these years requiring that air measurements be taken over several years, or several years apart, to detect the effect of controls. The methods discussed here are easily extended to longer time series. In practice it is important to know the region where the calculated reductions in emissions are occurring. For the West Coast analysis, the Cape Meares site also represents the local backgroundsit is upwind of sources as prevailing winds come from over the vast stretches of the Pacific Ocean. Geographical information shows that the reductions of emissions can be ascribed to the Portland urban area, since there are no other major source regions nearby. For the East coast schools it is more difficult to determine the region represented by the calculated reductions in emissions except
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that it is a potentially large region upwind of the two school sites. To more narrowly define the regions where the changes occurred requires local upwind sites or meteorological backtrajectory analysis. This issue of the lack of a local background or upwind site extends also to the lifetime of the gases involved. For shorter lived gases, the changes observed tend to be more closely related to nearby sources, but for longlived gases, the changes in emissions that are occurring far upwind may influence the calculations.
Acknowledgments We thank the teachers at the GLOBE Schools who participated in this experiment: W. Mitman, Madison High School; C. Watson, Kelly Elementary School; R. Coats, Jackson Middle School; W. Costello, Champlain Valley Union High School; S. Griffin, Oakmont Regional High School. We thank Bob Dalluge and Don Stearns for laboratory measurements at OGI and Martha J. Shearer at PSU. Major funding for this project was provided by the National Science Foundation (NSF Grant GEO-9696080). Funding for the work at Minqin, China and Cape Meares, OR was provided by a grant from the Office of Science (BER) U.S. Department of Energy (Grant DE-FG03-97ER62401). Additional support was provided by the Andarz Co. and the Biospherics Research Corp.
Supporting Information Available Excel spreadsheet detailing additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Rasmussen, R. A.; Khalil, M. A. K. In Proceedings of the NATO Advanced Study Institute on Atmospheric Ozone: Its Variation and Human Influences; Aiken, A. C., Ed.; U.S. Department of Transportation: Washington, D.C., 1980; pp 209-231. (2) Rasmussen, R. A.; Lovelock, J. E. J. Geophys. Res., C13: Oceans Atmos. 1983, 88, 8369-8378. (3) Prinn, R. G.; Simmonds, P. G.; Rasmussen, R. A.; Rosen, R. D.; Alyea, F. N.; Cardelino, C. A.; Crawford, A. J.; Cunnold, D. M.; Fraser, P. J.; Lovelock, J. E. J. Geophys. Res., C13: Oceans Atmos. 1983, 88, 8353-8367. (4) Singh, H. B.; Salas, L. J.; Shigeishi, H.; Crawford, A. Atmos. Environ. 1977, 11, 819-828. (5) Blake, D. R.; Woo, V. H.; Tyler, S. C.; Rowland, F. S. Geophys. Res. Lett. 1984, 11, 1211-1214. (6) Khalil, M. A. K.; Rasmussen, R. A. Chemosphere 1989, 19, 13831386. (7) Khalil, M. A. K.; Rasmussen, R. A. J. Air Waste Manage. Assoc. 1990, 40, 1143-1146.
Received for review February 20, 2003. Revised manuscript received September 11, 2003. Accepted October 27, 2003. ES0341539
