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The impact of meteorological parameters on urban air quality
Atmospheric Environment 86 (2014) 58e67
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
The impact of meteorological parameters on urban air quality Nicole R. Ramsey*, Petra M. Klein, Berrien Moore III University of Oklahoma, USA
h i g h l i g h t s Comparison of ozone concentrations from two hot, dry years and two cool, wet years. Hot, dry summers experience higher monthly minimum and maximum ozone concentrations. Hot, dry years have three times as many exceedance days as cool, wet years. Human exposure to harmful levels of ozone is doubled during hot, dry years.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2013 Received in revised form 6 October 2013 Accepted 6 December 2013 Available online 1 January 2014
Previous studies have shown that global climate change will have a significant impact on both regional and urban air quality. As air temperatures continue to rise and mid-latitude cyclone frequencies decrease, the overall air quality is expected to degrade. Climate models are currently predicting an increased frequency of record setting heat and drought for Oklahoma during the summer months. A statistical analysis was thus performed on ozone and meteorological data to evaluate the potential effect of increasing surface temperatures and stagnation patterns on urban air quality in the Oklahoma City Metropolitan area. Compared to the climatological normal, the years 2011 and 2012 were exceptionally warm and dry, and were therefore used as case study years for determining the impact of hot, dry conditions on air quality. These results were then compared to cooler, wetter summers to show how urban air quality is affected by a change in meteorological parameters. It was found that an increase in summertime heat and a decrease in summertime precipitation will lead to a substantial increase in both the minimum and maximum ozone concentrations as well as an increase in the total number of exceedance days. During the hotter, drier years, the number of days with ozone concentrations above the legal regulatory limit increased nearly threefold. The length of time in which humans and crops are exposed to these unsafe levels was also doubled. Furthermore, a significant increase was noted in the overnight minimum ozone concentrations. This in turn can lead to significant, adverse affects on both health and agriculture statewide. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ozone concentrations Climate change Urban air quality Health impacts Oklahoma City
1. Introduction Tropospheric ozone is one of the leading photochemical pollutants in the atmospheric boundary layer. Because of the photochemical nature of ozone production, the largest concentrations typically occur during the early to mid-afternoon hours when incoming solar radiation is maximized (Bloomer et al., 2010). Reciprocally, the lowest concentrations generally occur during the overnight hours due to scavenging of ozone through reactions with nitrous oxide and volatile organic compounds (Böhm et al., 1991). Many studies, including Logan (1985), have shown that monthly
* Corresponding author. E-mail address: [email protected] (N. R. Ramsey). 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.12.006
average ozone concentrations reach a maximum between March and August and a minimum during the winter. This broad sixmonth period of elevated ozone concentrations from spring through summer has increased over time. Between 1990 and 1994, ozone concentrations in the United States peaked during the summer months but now elevated ozone concentrations have been noted earlier (Bloomer et al., 2010). High ozone days, or days in which the maximum eight-hour (8h) ozone concentration is greater than 75 ppb, in the Oklahoma City (OKC) region often occur when conditions are hot and sunny and winds are light out of the south, southeast, or east (Association of Central Oklahoma Governments, 2008). This connection between certain meteorological parameters and elevated ozone concentrations has been well documented in the literature for other study areas. In general, an increase in surface temperature and a decrease
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in wind speed are both associated with higher ozone values (Bloomer et al., 2010; Camalier et al., 2007; Ngan and Byun, 2011). One of the primary formation mechanisms for ozone is via photochemical processes. Therefore, the presence of sunlight and the amount of available radiation are important in determining the rate of ozone formation (Logan, 1985; Arya, 1999). Furthermore, an increase in relative humidity is usually associated with an increase in cloud cover and a decrease in atmospheric stability, which in turn reduces the photochemical processes that are vital to the formation of ozone (Camalier et al., 2007). Similarly, higher rainfall values result in lower ozone concentrations because rainfall acts as a pollution ventilation mechanism via the removal of ozone from the lower atmosphere (Ludwig and Shelar, 1978; Tarasova and Karpetchko, 2003; Banta and Coauthors, 2005). This influence of meteorological parameters on ozone concentrations will become increasingly important as the climate changes. A study conducted by Fang et al. (2013) showed that when the emission of air pollutants is held constant, higher ozone concentrations could be expected over populated regions due to climate change during the 21st century. Wu et al. (2008) showed that an increase in the frequency of stagnation periods is expected during the 21st century. Furthermore, a decrease in mid-latitude cyclone frequency across southeast Canada along with a weakening in the global circulation will lead to a decline in the number of frontal passages moving through the United States (Wu et al., 2008; Fang et al., 2013). This has significant ramifications on ozone concentrations and air quality since frontal passages are a major source of pollution ventilation. General circulation models also predict that an increase in the number of heat waves will lead to an increase in surface temperature and a decrease in cloud cover. This is expected to cause ozone concentrations in polluted areas to increase by 1 to 10 parts per billion (ppb) (Jacob and Winner, 2009). An increase in temperature is also expected to result in a 12-day increase in the total number of days during which the 8-h ozone concentrations exceed 80 ppb (Murazaki and Hess, 2006). The U.S. Environmental Protection Agency (EPA) is in charge of reducing air pollution and improving air quality in the United States with the goal of protecting humans and animals from elevated exposure to harmful pollution levels. The National Ambient Air Quality Standards (NAAQS) were designed by the EPA to limit the anthropogenic emission of ozone and five other harmful pollutants (United States Environmental Protection Agency Office of Air Quality Planning and Standards, 2012). Nonattainment designations are subsequently determined by taking the “annual fourth highest daily maximum 8-h concentration, averaged over 3 years” (United States Environmental Protection Agency, 2011). If this value is greater then 75 ppb, that region will be designated as a non-attainment area and will be required to reduce pollution concentrations below the legal limit. Similarly, the World Health Organization (WHO) advises that 8-h ozone concentrations should remain below 50 ppb in order to satisfactorily protect humans from the harmful effects of ozone (World Health Organization, 2011). Elevated surface ozone concentrations can cause harm to human health and agricultural productivity. Short-term exposure to ozone concentrations above the EPA’s 1-h ozone regulation standard further results in chest tightness, dry coughing, and rapid and shallow breathing (Beckett, 1991). At high concentrations, surface ozone can lead to increases in respiratory ailments, hospitalizations, and morbidity among children and the elderly (Beckett, 1991). Fang et al. (2013) showed that changes in ozone concentrations due to climate change could increase premature mortality from respiratory diseases in people over 30 by 6300 deaths. Increased ozone concentrations also negatively affect the stomatal conductance of wheat crops, resulting in a substantial relative yield
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loss (Van Dingenen et al., 2009). Wheat, one of the primary crops produced in Oklahoma (United States Department of Agriculture, 2011), is extremely sensitive to ozone exposure. Avnery et al. (2011) showed that wheat is more sensitive to frequent exposure to high ozone concentrations then it is to long-term exposure to moderate ozone concentrations. In this study, ozone was found to inhibit wheat growth by reducing the amount of photosynthesis and the number of physiological functions that the plant can undergo. This weakens the plant, decreases the quality of the crop, and reduces the amount of crop yield. The harmful effects of elevated ozone concentrations will become increasingly important since climate models are predicting warmer, drier, more stagnant conditions to become more prevalent in the OKC Metropolitan area over the next century (Jacob and Winner, 2009). Surface ozone concentrations are exacerbated under warm, dry, stagnant conditions, so a shift towards these weather patterns could result in dangerously high ozone concentrations during the summer months. By comparing ozone concentrations during cooler, wetter years against those from warmer, drier years, this research shows how ozone concentrations and the number of exceedance days could evolve under a changing climate if emission rates and patterns are not adapted. The data and methods used in this study, as well as a discussion of the meteorological and air quality patterns observed during the ten-year (10y) study period from 2003 to 2012, are described in Section 2. Based on the trends noted in these patterns, four case study years were selected and discussed in detail in Section 3, followed by conclusions in Section 4. 2. Data and methods The Oklahoma Mesonet is a network of 120 automated stations that monitor upwards of seventeen different meteorological parameters in 5-min intervals statewide (McPherson and Coauthors, 2007). Three monitoring sites, Norman, Minco, and Spencer, are located in the greater OKC Metropolitan region (Fig. 1) and were used during this study to collect data on temperature, wind speed, wind direction, relative humidity, solar radiation, and precipitation. In order to determine how variations in meteorological parameters under a changing climate will influence ozone concentrations in OKC, climate data from the Oklahoma Mesonet were analyzed from 2003 to 2012. Fig. 2 shows how 2004 and 2007 were unseasonably cool and wet compared to the climatological average while 2011 and 2012 were unseasonably warm and dry compared to the climatological average, especially during the summer months. These four years were selected as case study years to serve as proxies for the two different climates: cool and wet (2004 and 2007) and warm and dry (2011 and 2012). Similarly, the average monthly radiation was much higher and the dewpoint temperature was much lower during the peak summer months in the warm, dry years compared to the cool, wet years (not shown). The EPA maintains the Air Quality System (AQS) database, which includes air quality data collected by both the EPA, and state, local, and tribal governments, and uses it to monitor air quality and pollution levels (United States Environmental Protection Agency, 2013). Out of more then 10,000 monitoring stations nationwide, six state-maintained, ozone-monitoring stations are located in and around the OKC Metropolitan region (Fig. 1). The southern most monitoring site, Goldsby, is located in a rural setting on land used for agricultural purposes. OSDH, or OKC Central, is an urban site located in the heart of downtown. The remaining four stations, Yukon, OCC, Choctaw, and Moore, are located in a suburban setting on land zoned for either residential or commercial use. Throughout the 10-y study period, OCC, OSDH, and Choctaw have the highest
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Fig. 1. Location of the six AQS monitoring sites and three Oklahoma Mesonet stations in the Oklahoma City area. Goldsby is a rural AQS monitoring sites. Moore, Choctaw, Yukon, and OCC are suburban AQS monitoring sites. OSDH is an urban AQS monitoring sites (the site ID is noted in parentheses). SPEN, MIN, and NRMN show the location of the Oklahoma Mesonet stations.
monthly average ozone concentrations (Fig. 3). The predominant wind direction is southerly during the summer months, so these stations are often located downstream from the urban core. The average monthly 1-h ozone concentration time series plot, as shown in Fig. 3, clearly highlights the increase of summertime ozone concentrations during the warm, dry years of 2011 and 2012, while the cool, wet years of 2004 and 2007 have the lowest summertime ozone concentrations. Since the two warm, dry case study years fall at the end of our study period, one could suspect that the increase in ozone is caused by an increase in the anthropogenic emission of ozone precursors throughout the study period. However, summertime concentration values during 2006, which was another hot and dry year (Fig. 2), are comparable to those seen during the summer of 2011. While a change in emission patterns
and transportation pathways could have led to an increase in ozone concentrations, the close resemblance of meteorological and air quality patterns between 2006 and the two warm, dry case study years of 2011 and 2012 strongly indicates that changes in meteorological parameters were dominant. Therefore, ozone data during the four case study years are further analyzed to both quantify the differences in concentrations during the cool, wet years versus the warm, dry years and to determine the potential ramifications of perceived changes on policy, health, and agriculture. 3. Results Most studies have focused on the correlation between maximum ozone and various meteorological parameters because
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Fig. 2. Monthly averages of 1.5-m air temperature for the entire year (top) and precipitation for the high ozone months (bottom), averaged for the three central Oklahoma Mesonet stations for 2003e2012.
of the pronounced peak in ozone concentrations during the late afternoon hours and the marked decline during the overnight hours. Although this is the typical diurnal cycle associated with ozone, elevated concentrations and secondary maxima are frequent to the OKC metropolitan area (Klein et al., 2013). Because of the prolonged exposure time and exacerbated impacts felt by both humans and agriculture associated with elevated nighttime concentrations, our study investigated the change in distribution for both maximum and minimum concentrations during the four study years. In addition, we determined the number of days and the length of time during which EPA and WHO threshold values are exceeded as a way to better assess the potential impacts on human health. 3.1. Maximum ozone concentrations To emphasize the difference between warm, dry years and cool, wet years, monthly average maximum one-hour (1-h) ozone
concentrations at the ozone-monitoring site north of the OKC central business district (OCC) are compared in Fig. 4. The average maximum one-hour (1-h) ozone concentration is upwards of 25 ppb higher during the summer months of 2011 and 2012 compared to 2004 and 2007. The figure also shows that ozone concentrations experienced during 2006 most closely resembled those present during 2011 and 2012. As discussed above, this indicates that the elevated ozone concentrations experienced during 2011 and 2012 were likely not caused by an increase in anthropogenic emissions after 2007. Histograms of maximum 8-h concentrations from the four case study years for the summer months of June through September are compared in Fig. 5. During the hot and dry years, the concentration distribution is clearly shifted towards higher values. To further analyze possible causes for the shift in the concentration distribution, scatter plots between the maximum daily summertime 8-h ozone concentration and the corresponding 8-h average temperature and wind speed are shown in Fig. 6 for the Goldsby and OCC monitoring sites. Goldsby and OCC were chosen
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Fig. 3. Average monthly 1-h ozone concentrations from 2003 to 2012 for the six ozone-monitoring stations located in central Oklahoma.
because they are primarily located upwind and downwind of OKC, respectively, and because the lowest (Goldsby) and highest (OCC) concentrations within the domain are often observed at these sites (also see Fig. 9). Red points indicate data from 2011 and 2012, while blue points indicate data from 2004 and 2007. At both monitoring sites, maximum daily 8-h ozone concentrations are positively correlated with air temperature, and the trends are generally similar during all four case study years. However, the higher temperatures recorded during 2011 and 2012 increase the temperature range, which leads to higher maximum ozone concentrations. The correlations between maximum daily 8-h ozone concentrations and wind speed are much weaker, but as the wind speeds increase, maximum ozone concentrations tend to decrease due to the dilution effect (Camalier et al., 2007). The shift in the concentration distribution towards higher values, as seen in Fig. 6, results in a dramatic increase in the number of high ozone days (Fig. 7). During the cool, wet years, ozone concentrations exceed the WHO’s standard an average of 27% of the days each year while this percentage is nearly doubled during the warm, dry years. Similarly, only 1.5% of the days have 8-h ozone concentrations above the EPA’s regulatory level of 75 ppb during the cool, wet years, while this threshold is exceeded on approximately 6% of the days during the warm, dry years. These percentages correspond to less then 10 days during both 2004 and 2007 and over 27 days during both 2011 and 2012 that are labeled as an exceedance day (Table 1), or a day in which the maximum 8-h ozone concentration from one or more monitoring stations was greater than the EPA’s regulatory threshold of 75 ppb. This indicates that if the climate does in fact warm and become more stagnant, the number of exceedance days could increase significantly if emissions are not further reduced. Our study also confirmed the lengthening of the elevated ozone season due to climatic changes that has been previously reported (e.g. Bloomer et al., 2010). Fig. 8 shows the total number of days per month that at least one station experienced 8-h ozone concentrations above 75 ppb. With the exception of one day in April, all exceedance days from the cool, wet years occurred during the traditional high ozone months of June through September. 2011 and 2012 saw an extension of this season, with exceedance days
occurring in March and persisting through October. A high ozone season that once lasted four months is now doubled. Table 2 shows the total number of cases and the average length of each case in which the 8-h ozone concentration exceeded 75 ppb for multiple days in a row. During both 2004 and 2007, there was only one case in which the ozone concentration exceeded 75 ppb for multiple days in a row. During 2011 and 2012 though, there were five and four cases, respectively, that experienced multiple days in a row of ozone concentrations above the federal regulatory limit. The average length of each case study also nearly doubled. In accordance with the regulatory standard for ozone, Fig. 9 shows the AQS for each of the six monitoring stations from 2005 (average of the fourth highest concentration during 2003e2005) through 2012 (average of the fourth highest concentration during 2010e2012). Goldsby, the farthest south and most rural station, always has the lowest AQS value. OCC on the other hand is the most northern station and experiences the highest AQS throughout the year. Every station, with the exception of OCC, has never exceeded the old standard of 80 ppb. But, as of 2012, all six monitoring stations are in exceedance of the new legal limit of 75 ppb.
3.2. Minimum ozone concentrations In addition to changes in the daytime maximum ozone concentration, an increase in the minimum ozone concentration is also observed during the hotter, drier years. Fig. 10 shows the distribution of minimum 8-h ozone concentrations, which typically occur just before sunrise, from each of the four case study years for OCC. Most notable is the definitive shift towards higher minimum 8-h ozone concentrations during the warm, dry years. Minimum 8h ozone concentrations range from 15 to 25 ppb during the cool, wet years and 25 to 35 ppb during the warm, dry years. If ozone concentrations do not decrease during the overnight hours, humans, animals, and plants will be exposed to elevated ozone concentrations for longer periods of time. This prolonged exposure to elevated ozone concentrations could lead to irreversible lung damage and significant crop damage (Brookes et al., 1989; Van Dingene et al., 2009).
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Fig. 4. Average maximum 1-h ozone concentration from the cool, wet years (2004 and 2007) and the warm, dry years (2011 and 2012) for OCC. Additionally, data for 2006, another hot and dry year, is also shown.
Fig. 11 shows the relationship between the minimum daily 8-h ozone concentration and the corresponding 8-h average temperature and wind speed during the summer months of June through September. Red points indicate data from 2011 and 2012, while blue points (in web version) indicate data from 2004 and 2007. Contrary to the increase in the linear regression line seen between maximum ozone and temperature, there is a distinct shift in the vertical of the minimum linear regression line. For a given minimum temperature, the minimum 8-h ozone concentration is higher during the
warmer, drier years. While the dilution effect reduced maximum ozone concentrations at higher wind speeds, the opposite can be said for the relationship between minimum ozone concentration and wind speed. As minimum wind speeds increase and nighttime stability decreases, elevated ozone from aloft is mixed down and increases the overnight minimum ozone concentrations. The observed increase in the minimum ozone concentrations for a warmer, drier climate cannot be explained by changes in the corresponding temperature or wind speed values, i.e. similar
Fig. 5. Histograms of maximum 8-h ozone concentrations for the four case study years.
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Fig. 6. Correlation between maximum 8-h ozone concentration at the OCC (top) and Goldsby (bottom) sites with the corresponding 8-h averages of temperature (left) and wind speed (right) during the summer months. Data from 2011 and 2012 are shown in red while data from 2004 and 2007 are shown in blue. The corresponding correlation coefficients r are shown in the plots (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 7. Percentage of days from each case study year with 8-h ozone concentrations in exceedance of the WHO and EPA’s standards of 50 ppb and 75 ppb, respectively.
Table 1 Total number of days per year with ozone concentrations above the specified criteria. Year 2004 2007
Maximum 8-h ozone >75 ppb
8-h Ozone >50 ppb for entire day
6 9
6 4
Total wet & cool
15
10
2011 2012
30 27
20 17
Total dry & hot
57
37
nighttime values of wind and temperature will lead to higher nighttime ozone concentrations in a warmer, drier climate. Further studies will be required to identify the cause of these changes. The observed increase in minimum concentrations potentially prolongs the exposure of humans to unhealthy ozone concentration levels. Therefore, we determined the number of days during which the 8-h minimum ozone concentration remained above the 50 ppb threshold set by the WHO to protect humans from the harmful effects of ozone for an entire day during each study year. During 2004 and 2007, 8-h averaged ozone concentrations were continuously higher than 50 ppb for 10 days; this number jumps to 37 days during 2011 and 2012 (Table 1). Hence, there is a sharp increase in both the number of days during which the minimum 8h ozone concentration exceeds the WHO’s guideline value and in
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Fig. 8. Number of exceedance days per month for each of the four case study years.
Number of cases
Average length (days)
Number of cases
Average length (days)
are no cases in which ozone concentrations for multiple consecutive days remain above 50 ppb during the cool, wet years, but there are nine cases during the warm, dry years. This indicates that the warmer, drier years are experiencing elevated ozone concentrations in excess of the WHO’s desired 8-h maximum ozone concentration threshold for multiple days in a row.
2004 2007
1 1
2.0 3.0
0 0
0 0
4. Conclusions
Total wet & cool
2
2.5
0
0
2011 2012
5 4
3.6 4.0
5 4
3.0 2.5
Total 9 dry & hot
3.8
9
2.75
Table 2 Number and average length of study periods per year with ozone concentrations above the specified criteria for multiple days in a row. Year
Maximum 8-h ozone >75 ppb 8-h Ozone >50 ppb for entire day
the corresponding number of days during which humans are exposed to harmful concentration levels for an entire day. Table 2 shows the total number of cases in which 8-h ozone concentrations from multiple consecutive days remain above 50 ppb. There
A statistical analysis was conducted to examine the effects of changing meteorological parameters on ozone concentrations in OKC. Ozone concentrations from two cool, wet years were compared to those from two warm, dry years in order to determine how a shift towards a warmer, drier climate would affect ozone concentrations and health. There is a nearly threefold increase in the number of exceedance days between the cool, wet years and the warm, dry years. OKC has never been designated as out of attainment for ozone, but the results of this study suggest that it might be at risk for being designated a non-attainment region in the near future. Not only has the total number of exceedance days
Fig. 9. The AQS for each of the six monitoring stations from 2003 to 2012.
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Fig. 10. Histograms of the minimum 8-h ozone concentrations for the four case study years.
increased across the OKC Metropolitan area, but the length of the season in which these exceedance days occurred has doubled. It has also been shown that multiple exceedance days in a row are much more common during the warmer, drier years as compared to the cooler, wetter ones. This indicates that humans and plants will be more frequently exposed to harmful levels of ozone for longer periods of time in a warmer, drier climate. Because the EPA sets regulatory standards based upon protecting human health and welfare, people in OKC are more at risk for developing negative health effects under the elevated concentrations. Similarly, an increase in the exposure of crops to elevated ozone concentrations can also lead to an increase in the relative yield loss of wheat crops.
A significant increase was further noted in the minimum ozone concentrations during the warmer, drier years as compared to the cooler, wetter ones. Many studies have correlated an increase in maximum temperature and maximum solar radiation to an increase in maximum ozone concentrations, but no studies have looked at the impact of changing meteorological parameters on minimum ozone concentrations. This research shows that for a given temperature and wind speed, minimum ozone concentrations are higher in a warmer, drier climate than in a cooler, wet climate. A nearly four-fold increase was also noted in the number of days in which the 8-h ozone concentrations remained above the WHO’s threshold for maximum 8-h ozone concentration. Further
Fig. 11. Same as Fig. 5 but for minimum 8-h ozone concentrations.
N. R. Ramsey et al. / Atmospheric Environment 86 (2014) 58e67
research is still needed to determine the exact cause and effect of this increase in minimum ozone concentration. References Association of Central Oklahoma Governments, 2008. Central Oklahoma 8-Hour Ozone Flex Program, p. 44. Avnery, S., Mauzerall, D.L., Liu, J., Horowitz, L.W., 2011. Global crop yield reductions due to surface ozone exposure: 2. Year 2030 potential crop production losses and economic damage under two scenarios of O3 pollution. Atmos. Environ. 45, 2297e2309. Arya, S.P., 1999. Air Pollution Meteorology and Dispersion. Oxford University Press, p. 310. Banta, R.M., Coauthors, 2005. A bad air day in Houston. Bull. Am. Meteorol. Soc. 86, 657e669. Beckett, W.S., 1991. Ozone, air pollution, and respiratory health. Yale J. Biol. Med. 64, 167e175. Bloomer, B.J., Vinnikov, K.Y., Dickerson, R.R., 2010. Changes in seasonal and diurnal cycles of ozone and temperature in the eastern U.S. Atmos. Environ. 44, 1e9. Böhm, M., McCune, B., Vandetta, T., 1991. Diurnal curves of tropospheric ozone in the Western United States. Atmos. Environ. 25A, 1577e1590. Brookes, K.A., Adams, W.C., Schelegle, E.S., 1989. 0.35 ppm O3 exposure induces hyperresponsiveness on 24-h reexposure to 0.20 ppm O3. J. Appl. Physiol. 66, 2756e2762. Camalier, L., Cox, W., Dolwisck, P., 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmos. Environ. 41, 7127e 7137. Fang, Y., Mauzerall, D.L., Liu, J., Fiore, A.M., Horowitz, L.W., 2013. Impacts of 21st century climate change on global air pollution-related premature mortality. Clim. Change. http://dx.doi.org/10.1007/s10584-013-0847-8. Jacob, D.J., Winner, D.A., 2009. Effect of climate change on air quality. Atmos. Environ. 43, 51e63. Klein, P.M., Hu, X.-M., Xue, M., 2013. Impacts of mixing processes in the nocturnal atmospheric boundary layer on urban ozone concentrations. Bound-Layer Meteorol.. http://dx.doi.org/10.1007/s10546-013-9864-4.
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Ludwig, F.L., Shelar, E., 1978. Effects of weather fronts on ozone transport. In: Morris, A.L., Barras, R.C. (Eds.), Air Quality Meteorology and Atmospheric Ozone. American Society for Testing and Materials, p. 636. ASTM STP 653. Logan, J.A., 1985. Tropospheric ozone: seasonal behavior, trends, and anthropogenic influence. J. Geophys. Res. 90, 10463e10482. McPherson, R.E., Coauthors, 2007. Statewide monitoring of the mesoscale environment: a technical update on the Oklahoma Mesonet. J. Atmos. Ocean. Technol. 24, 301e321. Murazaki, K., Hess, P., 2006. How does climate change contribute to surface ozone change over the United States? J. Geophys. Res. 111 http://dx.doi.org/10.1029/ 2005JD005873. Ngan, F., Byun, D., 2011. Classification of weather patterns and associated trajectories of high-ozone episodes in the HoustoneGalvestoneBrazoria area during the 2006/06 TexAQS-II. J. Appl. Meteor. 50. http://dx.doi.org/10.1175/ 2010JAMC2483.1. Tarasova, O.A., Karpetchko, A.Y., 2003. Accounting for local meteorological effects in the ozone time-series of Loozero (Kola Peninsula). Atmos. Chem. Phys. Discuss. 3, 941e949. United States Department of Agriculture, 2011. 2011 State Agricultural Overview Oklahoma. Available online at: http://www.nass.usda.gov/Statistics_by_State/ Ag_Overview/AgOverview_OK.pdf. United States Environmental Protection Agency, 2011. Ground Level Ozone Quick Finder. Available online at: http://www.epa.gov/airquality/ozonepollution/. United States Environmental Protection Agency, 2013. Technology Transfer Network (TTN) Air Quality System (AQS). Available online at: http://www.epa.gov/ttn/ airs/airsaqs/. United States Environmental Protection Agency Office of Air Quality Planning and Standards, 2012. Our Nation’s Air e Status and Trends through 2010, p. 27. Van Dingenen, R., Dentener, F.J., Raes, F., Krol, M.C., Emberson, L., Cofala, J., 2009. The global impact of ozone on agricultural crop yields under current and future air quality legislation. Atmos. Environ. 43, 604e618. World Health Organization, 2011. Air Quality and Health. Available online at: http:// www.who.int/mediacentre/factsheets/fs313/en/index.html. Wu, S., Mickley, L.J., Leibensperger, E.M., Jacob, D.J., Rind, D., Streets, D.G., 2008. Effects of 2000e2050 global change on ozone air quality in the United States. J. Geophys. Res. 113. http://dx.doi.org/10.1029/2007JD008917.
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
The impact of meteorological parameters on urban air quality Nicole R. Ramsey*, Petra M. Klein, Berrien Moore III University of Oklahoma, USA
h i g h l i g h t s Comparison of ozone concentrations from two hot, dry years and two cool, wet years. Hot, dry summers experience higher monthly minimum and maximum ozone concentrations. Hot, dry years have three times as many exceedance days as cool, wet years. Human exposure to harmful levels of ozone is doubled during hot, dry years.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2013 Received in revised form 6 October 2013 Accepted 6 December 2013 Available online 1 January 2014
Previous studies have shown that global climate change will have a significant impact on both regional and urban air quality. As air temperatures continue to rise and mid-latitude cyclone frequencies decrease, the overall air quality is expected to degrade. Climate models are currently predicting an increased frequency of record setting heat and drought for Oklahoma during the summer months. A statistical analysis was thus performed on ozone and meteorological data to evaluate the potential effect of increasing surface temperatures and stagnation patterns on urban air quality in the Oklahoma City Metropolitan area. Compared to the climatological normal, the years 2011 and 2012 were exceptionally warm and dry, and were therefore used as case study years for determining the impact of hot, dry conditions on air quality. These results were then compared to cooler, wetter summers to show how urban air quality is affected by a change in meteorological parameters. It was found that an increase in summertime heat and a decrease in summertime precipitation will lead to a substantial increase in both the minimum and maximum ozone concentrations as well as an increase in the total number of exceedance days. During the hotter, drier years, the number of days with ozone concentrations above the legal regulatory limit increased nearly threefold. The length of time in which humans and crops are exposed to these unsafe levels was also doubled. Furthermore, a significant increase was noted in the overnight minimum ozone concentrations. This in turn can lead to significant, adverse affects on both health and agriculture statewide. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ozone concentrations Climate change Urban air quality Health impacts Oklahoma City
1. Introduction Tropospheric ozone is one of the leading photochemical pollutants in the atmospheric boundary layer. Because of the photochemical nature of ozone production, the largest concentrations typically occur during the early to mid-afternoon hours when incoming solar radiation is maximized (Bloomer et al., 2010). Reciprocally, the lowest concentrations generally occur during the overnight hours due to scavenging of ozone through reactions with nitrous oxide and volatile organic compounds (Böhm et al., 1991). Many studies, including Logan (1985), have shown that monthly
* Corresponding author. E-mail address: [email protected] (N. R. Ramsey). 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.12.006
average ozone concentrations reach a maximum between March and August and a minimum during the winter. This broad sixmonth period of elevated ozone concentrations from spring through summer has increased over time. Between 1990 and 1994, ozone concentrations in the United States peaked during the summer months but now elevated ozone concentrations have been noted earlier (Bloomer et al., 2010). High ozone days, or days in which the maximum eight-hour (8h) ozone concentration is greater than 75 ppb, in the Oklahoma City (OKC) region often occur when conditions are hot and sunny and winds are light out of the south, southeast, or east (Association of Central Oklahoma Governments, 2008). This connection between certain meteorological parameters and elevated ozone concentrations has been well documented in the literature for other study areas. In general, an increase in surface temperature and a decrease
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in wind speed are both associated with higher ozone values (Bloomer et al., 2010; Camalier et al., 2007; Ngan and Byun, 2011). One of the primary formation mechanisms for ozone is via photochemical processes. Therefore, the presence of sunlight and the amount of available radiation are important in determining the rate of ozone formation (Logan, 1985; Arya, 1999). Furthermore, an increase in relative humidity is usually associated with an increase in cloud cover and a decrease in atmospheric stability, which in turn reduces the photochemical processes that are vital to the formation of ozone (Camalier et al., 2007). Similarly, higher rainfall values result in lower ozone concentrations because rainfall acts as a pollution ventilation mechanism via the removal of ozone from the lower atmosphere (Ludwig and Shelar, 1978; Tarasova and Karpetchko, 2003; Banta and Coauthors, 2005). This influence of meteorological parameters on ozone concentrations will become increasingly important as the climate changes. A study conducted by Fang et al. (2013) showed that when the emission of air pollutants is held constant, higher ozone concentrations could be expected over populated regions due to climate change during the 21st century. Wu et al. (2008) showed that an increase in the frequency of stagnation periods is expected during the 21st century. Furthermore, a decrease in mid-latitude cyclone frequency across southeast Canada along with a weakening in the global circulation will lead to a decline in the number of frontal passages moving through the United States (Wu et al., 2008; Fang et al., 2013). This has significant ramifications on ozone concentrations and air quality since frontal passages are a major source of pollution ventilation. General circulation models also predict that an increase in the number of heat waves will lead to an increase in surface temperature and a decrease in cloud cover. This is expected to cause ozone concentrations in polluted areas to increase by 1 to 10 parts per billion (ppb) (Jacob and Winner, 2009). An increase in temperature is also expected to result in a 12-day increase in the total number of days during which the 8-h ozone concentrations exceed 80 ppb (Murazaki and Hess, 2006). The U.S. Environmental Protection Agency (EPA) is in charge of reducing air pollution and improving air quality in the United States with the goal of protecting humans and animals from elevated exposure to harmful pollution levels. The National Ambient Air Quality Standards (NAAQS) were designed by the EPA to limit the anthropogenic emission of ozone and five other harmful pollutants (United States Environmental Protection Agency Office of Air Quality Planning and Standards, 2012). Nonattainment designations are subsequently determined by taking the “annual fourth highest daily maximum 8-h concentration, averaged over 3 years” (United States Environmental Protection Agency, 2011). If this value is greater then 75 ppb, that region will be designated as a non-attainment area and will be required to reduce pollution concentrations below the legal limit. Similarly, the World Health Organization (WHO) advises that 8-h ozone concentrations should remain below 50 ppb in order to satisfactorily protect humans from the harmful effects of ozone (World Health Organization, 2011). Elevated surface ozone concentrations can cause harm to human health and agricultural productivity. Short-term exposure to ozone concentrations above the EPA’s 1-h ozone regulation standard further results in chest tightness, dry coughing, and rapid and shallow breathing (Beckett, 1991). At high concentrations, surface ozone can lead to increases in respiratory ailments, hospitalizations, and morbidity among children and the elderly (Beckett, 1991). Fang et al. (2013) showed that changes in ozone concentrations due to climate change could increase premature mortality from respiratory diseases in people over 30 by 6300 deaths. Increased ozone concentrations also negatively affect the stomatal conductance of wheat crops, resulting in a substantial relative yield
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loss (Van Dingenen et al., 2009). Wheat, one of the primary crops produced in Oklahoma (United States Department of Agriculture, 2011), is extremely sensitive to ozone exposure. Avnery et al. (2011) showed that wheat is more sensitive to frequent exposure to high ozone concentrations then it is to long-term exposure to moderate ozone concentrations. In this study, ozone was found to inhibit wheat growth by reducing the amount of photosynthesis and the number of physiological functions that the plant can undergo. This weakens the plant, decreases the quality of the crop, and reduces the amount of crop yield. The harmful effects of elevated ozone concentrations will become increasingly important since climate models are predicting warmer, drier, more stagnant conditions to become more prevalent in the OKC Metropolitan area over the next century (Jacob and Winner, 2009). Surface ozone concentrations are exacerbated under warm, dry, stagnant conditions, so a shift towards these weather patterns could result in dangerously high ozone concentrations during the summer months. By comparing ozone concentrations during cooler, wetter years against those from warmer, drier years, this research shows how ozone concentrations and the number of exceedance days could evolve under a changing climate if emission rates and patterns are not adapted. The data and methods used in this study, as well as a discussion of the meteorological and air quality patterns observed during the ten-year (10y) study period from 2003 to 2012, are described in Section 2. Based on the trends noted in these patterns, four case study years were selected and discussed in detail in Section 3, followed by conclusions in Section 4. 2. Data and methods The Oklahoma Mesonet is a network of 120 automated stations that monitor upwards of seventeen different meteorological parameters in 5-min intervals statewide (McPherson and Coauthors, 2007). Three monitoring sites, Norman, Minco, and Spencer, are located in the greater OKC Metropolitan region (Fig. 1) and were used during this study to collect data on temperature, wind speed, wind direction, relative humidity, solar radiation, and precipitation. In order to determine how variations in meteorological parameters under a changing climate will influence ozone concentrations in OKC, climate data from the Oklahoma Mesonet were analyzed from 2003 to 2012. Fig. 2 shows how 2004 and 2007 were unseasonably cool and wet compared to the climatological average while 2011 and 2012 were unseasonably warm and dry compared to the climatological average, especially during the summer months. These four years were selected as case study years to serve as proxies for the two different climates: cool and wet (2004 and 2007) and warm and dry (2011 and 2012). Similarly, the average monthly radiation was much higher and the dewpoint temperature was much lower during the peak summer months in the warm, dry years compared to the cool, wet years (not shown). The EPA maintains the Air Quality System (AQS) database, which includes air quality data collected by both the EPA, and state, local, and tribal governments, and uses it to monitor air quality and pollution levels (United States Environmental Protection Agency, 2013). Out of more then 10,000 monitoring stations nationwide, six state-maintained, ozone-monitoring stations are located in and around the OKC Metropolitan region (Fig. 1). The southern most monitoring site, Goldsby, is located in a rural setting on land used for agricultural purposes. OSDH, or OKC Central, is an urban site located in the heart of downtown. The remaining four stations, Yukon, OCC, Choctaw, and Moore, are located in a suburban setting on land zoned for either residential or commercial use. Throughout the 10-y study period, OCC, OSDH, and Choctaw have the highest
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Fig. 1. Location of the six AQS monitoring sites and three Oklahoma Mesonet stations in the Oklahoma City area. Goldsby is a rural AQS monitoring sites. Moore, Choctaw, Yukon, and OCC are suburban AQS monitoring sites. OSDH is an urban AQS monitoring sites (the site ID is noted in parentheses). SPEN, MIN, and NRMN show the location of the Oklahoma Mesonet stations.
monthly average ozone concentrations (Fig. 3). The predominant wind direction is southerly during the summer months, so these stations are often located downstream from the urban core. The average monthly 1-h ozone concentration time series plot, as shown in Fig. 3, clearly highlights the increase of summertime ozone concentrations during the warm, dry years of 2011 and 2012, while the cool, wet years of 2004 and 2007 have the lowest summertime ozone concentrations. Since the two warm, dry case study years fall at the end of our study period, one could suspect that the increase in ozone is caused by an increase in the anthropogenic emission of ozone precursors throughout the study period. However, summertime concentration values during 2006, which was another hot and dry year (Fig. 2), are comparable to those seen during the summer of 2011. While a change in emission patterns
and transportation pathways could have led to an increase in ozone concentrations, the close resemblance of meteorological and air quality patterns between 2006 and the two warm, dry case study years of 2011 and 2012 strongly indicates that changes in meteorological parameters were dominant. Therefore, ozone data during the four case study years are further analyzed to both quantify the differences in concentrations during the cool, wet years versus the warm, dry years and to determine the potential ramifications of perceived changes on policy, health, and agriculture. 3. Results Most studies have focused on the correlation between maximum ozone and various meteorological parameters because
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Fig. 2. Monthly averages of 1.5-m air temperature for the entire year (top) and precipitation for the high ozone months (bottom), averaged for the three central Oklahoma Mesonet stations for 2003e2012.
of the pronounced peak in ozone concentrations during the late afternoon hours and the marked decline during the overnight hours. Although this is the typical diurnal cycle associated with ozone, elevated concentrations and secondary maxima are frequent to the OKC metropolitan area (Klein et al., 2013). Because of the prolonged exposure time and exacerbated impacts felt by both humans and agriculture associated with elevated nighttime concentrations, our study investigated the change in distribution for both maximum and minimum concentrations during the four study years. In addition, we determined the number of days and the length of time during which EPA and WHO threshold values are exceeded as a way to better assess the potential impacts on human health. 3.1. Maximum ozone concentrations To emphasize the difference between warm, dry years and cool, wet years, monthly average maximum one-hour (1-h) ozone
concentrations at the ozone-monitoring site north of the OKC central business district (OCC) are compared in Fig. 4. The average maximum one-hour (1-h) ozone concentration is upwards of 25 ppb higher during the summer months of 2011 and 2012 compared to 2004 and 2007. The figure also shows that ozone concentrations experienced during 2006 most closely resembled those present during 2011 and 2012. As discussed above, this indicates that the elevated ozone concentrations experienced during 2011 and 2012 were likely not caused by an increase in anthropogenic emissions after 2007. Histograms of maximum 8-h concentrations from the four case study years for the summer months of June through September are compared in Fig. 5. During the hot and dry years, the concentration distribution is clearly shifted towards higher values. To further analyze possible causes for the shift in the concentration distribution, scatter plots between the maximum daily summertime 8-h ozone concentration and the corresponding 8-h average temperature and wind speed are shown in Fig. 6 for the Goldsby and OCC monitoring sites. Goldsby and OCC were chosen
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Fig. 3. Average monthly 1-h ozone concentrations from 2003 to 2012 for the six ozone-monitoring stations located in central Oklahoma.
because they are primarily located upwind and downwind of OKC, respectively, and because the lowest (Goldsby) and highest (OCC) concentrations within the domain are often observed at these sites (also see Fig. 9). Red points indicate data from 2011 and 2012, while blue points indicate data from 2004 and 2007. At both monitoring sites, maximum daily 8-h ozone concentrations are positively correlated with air temperature, and the trends are generally similar during all four case study years. However, the higher temperatures recorded during 2011 and 2012 increase the temperature range, which leads to higher maximum ozone concentrations. The correlations between maximum daily 8-h ozone concentrations and wind speed are much weaker, but as the wind speeds increase, maximum ozone concentrations tend to decrease due to the dilution effect (Camalier et al., 2007). The shift in the concentration distribution towards higher values, as seen in Fig. 6, results in a dramatic increase in the number of high ozone days (Fig. 7). During the cool, wet years, ozone concentrations exceed the WHO’s standard an average of 27% of the days each year while this percentage is nearly doubled during the warm, dry years. Similarly, only 1.5% of the days have 8-h ozone concentrations above the EPA’s regulatory level of 75 ppb during the cool, wet years, while this threshold is exceeded on approximately 6% of the days during the warm, dry years. These percentages correspond to less then 10 days during both 2004 and 2007 and over 27 days during both 2011 and 2012 that are labeled as an exceedance day (Table 1), or a day in which the maximum 8-h ozone concentration from one or more monitoring stations was greater than the EPA’s regulatory threshold of 75 ppb. This indicates that if the climate does in fact warm and become more stagnant, the number of exceedance days could increase significantly if emissions are not further reduced. Our study also confirmed the lengthening of the elevated ozone season due to climatic changes that has been previously reported (e.g. Bloomer et al., 2010). Fig. 8 shows the total number of days per month that at least one station experienced 8-h ozone concentrations above 75 ppb. With the exception of one day in April, all exceedance days from the cool, wet years occurred during the traditional high ozone months of June through September. 2011 and 2012 saw an extension of this season, with exceedance days
occurring in March and persisting through October. A high ozone season that once lasted four months is now doubled. Table 2 shows the total number of cases and the average length of each case in which the 8-h ozone concentration exceeded 75 ppb for multiple days in a row. During both 2004 and 2007, there was only one case in which the ozone concentration exceeded 75 ppb for multiple days in a row. During 2011 and 2012 though, there were five and four cases, respectively, that experienced multiple days in a row of ozone concentrations above the federal regulatory limit. The average length of each case study also nearly doubled. In accordance with the regulatory standard for ozone, Fig. 9 shows the AQS for each of the six monitoring stations from 2005 (average of the fourth highest concentration during 2003e2005) through 2012 (average of the fourth highest concentration during 2010e2012). Goldsby, the farthest south and most rural station, always has the lowest AQS value. OCC on the other hand is the most northern station and experiences the highest AQS throughout the year. Every station, with the exception of OCC, has never exceeded the old standard of 80 ppb. But, as of 2012, all six monitoring stations are in exceedance of the new legal limit of 75 ppb.
3.2. Minimum ozone concentrations In addition to changes in the daytime maximum ozone concentration, an increase in the minimum ozone concentration is also observed during the hotter, drier years. Fig. 10 shows the distribution of minimum 8-h ozone concentrations, which typically occur just before sunrise, from each of the four case study years for OCC. Most notable is the definitive shift towards higher minimum 8-h ozone concentrations during the warm, dry years. Minimum 8h ozone concentrations range from 15 to 25 ppb during the cool, wet years and 25 to 35 ppb during the warm, dry years. If ozone concentrations do not decrease during the overnight hours, humans, animals, and plants will be exposed to elevated ozone concentrations for longer periods of time. This prolonged exposure to elevated ozone concentrations could lead to irreversible lung damage and significant crop damage (Brookes et al., 1989; Van Dingene et al., 2009).
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Fig. 4. Average maximum 1-h ozone concentration from the cool, wet years (2004 and 2007) and the warm, dry years (2011 and 2012) for OCC. Additionally, data for 2006, another hot and dry year, is also shown.
Fig. 11 shows the relationship between the minimum daily 8-h ozone concentration and the corresponding 8-h average temperature and wind speed during the summer months of June through September. Red points indicate data from 2011 and 2012, while blue points (in web version) indicate data from 2004 and 2007. Contrary to the increase in the linear regression line seen between maximum ozone and temperature, there is a distinct shift in the vertical of the minimum linear regression line. For a given minimum temperature, the minimum 8-h ozone concentration is higher during the
warmer, drier years. While the dilution effect reduced maximum ozone concentrations at higher wind speeds, the opposite can be said for the relationship between minimum ozone concentration and wind speed. As minimum wind speeds increase and nighttime stability decreases, elevated ozone from aloft is mixed down and increases the overnight minimum ozone concentrations. The observed increase in the minimum ozone concentrations for a warmer, drier climate cannot be explained by changes in the corresponding temperature or wind speed values, i.e. similar
Fig. 5. Histograms of maximum 8-h ozone concentrations for the four case study years.
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Fig. 6. Correlation between maximum 8-h ozone concentration at the OCC (top) and Goldsby (bottom) sites with the corresponding 8-h averages of temperature (left) and wind speed (right) during the summer months. Data from 2011 and 2012 are shown in red while data from 2004 and 2007 are shown in blue. The corresponding correlation coefficients r are shown in the plots (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Fig. 7. Percentage of days from each case study year with 8-h ozone concentrations in exceedance of the WHO and EPA’s standards of 50 ppb and 75 ppb, respectively.
Table 1 Total number of days per year with ozone concentrations above the specified criteria. Year 2004 2007
Maximum 8-h ozone >75 ppb
8-h Ozone >50 ppb for entire day
6 9
6 4
Total wet & cool
15
10
2011 2012
30 27
20 17
Total dry & hot
57
37
nighttime values of wind and temperature will lead to higher nighttime ozone concentrations in a warmer, drier climate. Further studies will be required to identify the cause of these changes. The observed increase in minimum concentrations potentially prolongs the exposure of humans to unhealthy ozone concentration levels. Therefore, we determined the number of days during which the 8-h minimum ozone concentration remained above the 50 ppb threshold set by the WHO to protect humans from the harmful effects of ozone for an entire day during each study year. During 2004 and 2007, 8-h averaged ozone concentrations were continuously higher than 50 ppb for 10 days; this number jumps to 37 days during 2011 and 2012 (Table 1). Hence, there is a sharp increase in both the number of days during which the minimum 8h ozone concentration exceeds the WHO’s guideline value and in
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Fig. 8. Number of exceedance days per month for each of the four case study years.
Number of cases
Average length (days)
Number of cases
Average length (days)
are no cases in which ozone concentrations for multiple consecutive days remain above 50 ppb during the cool, wet years, but there are nine cases during the warm, dry years. This indicates that the warmer, drier years are experiencing elevated ozone concentrations in excess of the WHO’s desired 8-h maximum ozone concentration threshold for multiple days in a row.
2004 2007
1 1
2.0 3.0
0 0
0 0
4. Conclusions
Total wet & cool
2
2.5
0
0
2011 2012
5 4
3.6 4.0
5 4
3.0 2.5
Total 9 dry & hot
3.8
9
2.75
Table 2 Number and average length of study periods per year with ozone concentrations above the specified criteria for multiple days in a row. Year
Maximum 8-h ozone >75 ppb 8-h Ozone >50 ppb for entire day
the corresponding number of days during which humans are exposed to harmful concentration levels for an entire day. Table 2 shows the total number of cases in which 8-h ozone concentrations from multiple consecutive days remain above 50 ppb. There
A statistical analysis was conducted to examine the effects of changing meteorological parameters on ozone concentrations in OKC. Ozone concentrations from two cool, wet years were compared to those from two warm, dry years in order to determine how a shift towards a warmer, drier climate would affect ozone concentrations and health. There is a nearly threefold increase in the number of exceedance days between the cool, wet years and the warm, dry years. OKC has never been designated as out of attainment for ozone, but the results of this study suggest that it might be at risk for being designated a non-attainment region in the near future. Not only has the total number of exceedance days
Fig. 9. The AQS for each of the six monitoring stations from 2003 to 2012.
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Fig. 10. Histograms of the minimum 8-h ozone concentrations for the four case study years.
increased across the OKC Metropolitan area, but the length of the season in which these exceedance days occurred has doubled. It has also been shown that multiple exceedance days in a row are much more common during the warmer, drier years as compared to the cooler, wetter ones. This indicates that humans and plants will be more frequently exposed to harmful levels of ozone for longer periods of time in a warmer, drier climate. Because the EPA sets regulatory standards based upon protecting human health and welfare, people in OKC are more at risk for developing negative health effects under the elevated concentrations. Similarly, an increase in the exposure of crops to elevated ozone concentrations can also lead to an increase in the relative yield loss of wheat crops.
A significant increase was further noted in the minimum ozone concentrations during the warmer, drier years as compared to the cooler, wetter ones. Many studies have correlated an increase in maximum temperature and maximum solar radiation to an increase in maximum ozone concentrations, but no studies have looked at the impact of changing meteorological parameters on minimum ozone concentrations. This research shows that for a given temperature and wind speed, minimum ozone concentrations are higher in a warmer, drier climate than in a cooler, wet climate. A nearly four-fold increase was also noted in the number of days in which the 8-h ozone concentrations remained above the WHO’s threshold for maximum 8-h ozone concentration. Further
Fig. 11. Same as Fig. 5 but for minimum 8-h ozone concentrations.
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