Dr. Sanford Sillman
Research Scientist University of Michigan

sillman@umich.edu
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Also at this site: Observation-based methods for analyzing O 3 -NO x-VOC sensitivity. Research publications available for download. Summary of research activities. Site index with links to the author's home page , research at the University of Michigan, and related research sites elsewhere. For more complete information , along with references for work cited here: Sillman, S. , Tropospheric ozone and photochemical smog. in B. Sherwood Lollar, ed., Treatise on Geochemistry, Vol. 9: Environmental Geochemistry, Ch. 11, Elsevier, 2003. http://www.TreatiseOnGeochemistry.com

. For availability, contact the author. Sillman, S., The relation between ozone, NO x and hydrocarbons in urban and polluted rural environments. Millenial Review series, Atmos. Environ., 33, 12, 1821-1845, 1999. Additional resources : The NARSTO web site contains comprehensive reviews of ozone and particulates. The American Lung Association has information about ozone, particulates and health. For other sites with information on environmental education and health effects , click here.



### 1.1 What is ozone?. [Go to Section 1.1] 1.2 Ozone, NO x 

### and VOC. [Go to Section 1.2] 1.3 Chemistry of ozone and cause of the NO x 

### -VOC split. [Go to Section 1.3]

[Return to Table of Contents] Ozone (O 3 ) is the primary ingredient of photochemical smog , the type of air pollution that is associated with sunlight-driven chemical reactions. Ozone is associated in particular with the type of air pollution events that occur in Los Angeles and Mexico City, often associated with automobile emissions. Ozone itself is colorless and invisible, but often occurs along with other, more visible species (e.g. sulfate aerosols) in Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

1 of 8 12/14/2011 6:35 PMlarge-scale pollution events. Ozone is a pollutant of concern because it is associated with extensive health effects , most notably associated with the respiratory system. Ozone is also a pollutant of concern because it can affect both forests and agricultural crops . Ozone levels in urban areas during pollution events are believed to be high enough to affect human health. Many metropolitan areas in the U.S. are currently (2003) in violation of government health standards for ozone, and violations of health standards have been occurring since the standards were first established in the 1970's. Pollution events with high ozone are associated specifically with sunshine and warm temperatures. High ozone is very rare when temperatures are below 20 C (68 F), and are usually associated with temperatures above 30 C (86 F). High ozone is also associated with relatively light winds and conditions that suppress vertical mixing in the atmosphere (thermal inversions or subsidence layers), but the most important conditions are sunshine and high temperatures. Pollution events are not limited to cities. Typically, the highest ozone levels are found in suburban locations downwind from the city center, rather than in the city center itself. In some situations plumes with high ozone have been found in locations that are 500 km (300 mi) distant from the apparent emission sources. In addition, densely populated regions are subject to pollution events with uniformly high ozone, affecting both urban and rural areas. During these regional pollution events, elevated ozone may extend over an area as large as 1000x1000 km (600x600 mi). Events of this type have occurred in the eastern U.S., California and in western Europe (see Figure 1.1 ). Ozone as an air pollutant should not be confused with the ozone layer in the stratosphere. Ozone occurs naturally in the stratosphere - the atmospheric layer located between 20 and 60 km (15 and 40 miles) above the earth's surface. Ozone levels in the stratosphere are 50 times higher than in the most polluted city - but no one has to breathe that air. Stratospheric ozone i s beneficial to life on earth, because it blocks harmful ultraviolet radiation. Over the past 50 years human activities have caus ed a 5% decrease in the amount of stratospheric ozone and in particular have lead to the occurrence of an ozone hole over Antarctica during the southern hemisphere spring. This damage to the ozone layer was primarily the result of chlorofluorocarbons (CFCs) , which accumulate in the atmosphere and eventually affect the ozone layer. The Montreal Protocol (1987) planned for the reduction and eventual worldwide ban on the production of CFC's. None of this relates to ozone as an air pollutant. Ozone also occurs naturally at the earths surface, due in part to transport downward from the ozone layer in the stratosphere and in part to naturally occurring chemistry in the lower atmosphere (the troposphere). However, ozone during pollution events is approximately ten times the natural level. In addition, human activities have caused an increase in ozone at the global scal e, especially in the northern hemisphere. Thus, even the most remote locations in the northern hemisphere have significantly more ozone than would occur naturally. Ozone levels are typically expressed in parts per billion by volume (ppbV, or ppb) , which represent the fraction of air molecules represented by ozone. The following are typical ozone levels (mixing ratios): Natural background (pre-industrial): 10-20 ppb Remote locations in the Northern Hemisphere: 20-40 ppb (varying by season and latitude) Rural areas during region-wide pollution events 80-100 ppb Peak O 3 in urban areas during pollution events 120-200 ppb Maximum urban O 3 (Los Angeles, Mexico City) 490 ppb Stratospheric ozone layer 15000 ppb USEPA health standard for ozone 125 ppb, 1-hour exposure (proposed revision: 85 ppb, 8 hour exposure) Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

2 of 8 12/14/2011 6:35 PMFigure 1.1. Extent of an air pollution event. The figure shows peak ozone in the eastern U.S. on June 15, 1988 based on surface observations at 350 EPA monitoring sites. Ozone above 80 ppb extends continuously from Tennessee to Maine. The ozone plume from New York City and Boston extends to Cape Cod and along the coast of Maine. First printed in Sillman (1993).



### x [Return to Table of Contents] 

### 1.2.1. Ozone formation
1.2.2. Factors affecting O 3 

### -NO x 

### -VOC sensitivity
1.2.3. Uncertainty in O 3 

### -NO x 

### -VOC sensitivity predictions. Ozone formation is driven by two major classes of directly emitted precursors: nitrogen oxides (NO x) and volatile organic compounds (VOC). The relation between O 3 , NO x and VOC is driven by complex nonlinear photochemistry.
The relation between O 3 , NO x and VOC can be illustrated by isopleth plots ( Figure 1.2 ), which shows peak O 3 during the afternoon as a function of NO x and VOC mixing ratios or (as shown here) emission rates. As shown in the figure, it is possible to identify two regimes with different O 3 -NO x-VOC sensitivity. In the NO x -sensitive regime (with relatively low NO x and high VOC), O 3 increases with increasing NO x and changes little in response to increasing VOC. In the NO x -saturatedor VOC-sensitive regime O 3 decreases with increasing NO x and increases with increasing VOC. The dotted line (the ridge line ) represents a local maximum for O 3 versus NO x and VOC, separates the NO x-sensitive and VOC-sensitive regimes. The response of ambient O 3 to reductions in emissions of NO x and VOC is also shown in results for indicator ratios ( Observation-based methods, Figure 2.1 ). Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

3 of 8 12/14/2011 6:35 PMWARNING: The exact dependence of O 3 on NO x and VOC in isopleth plots varies greatly depending on assumptions and conditions used in generating the isopleth plot. The pattern (with a split into NO x-sensitive and NO x-saturated regimes) tends to recur for a wide range of conditions, but the values of VOC and NO x associated with the transition between regimes change.
Ambient NO x and VOC mixing ratios are directly related to the instantaneous rate of production of O 3 , and it may be possible to make inferences about the instantaneous production rate of Ox based on ambient NO x and VOC. A more robust form of the isopleth plot can be created by plotting the instantaneous rate of production of O 3 versus summed ambient VOC and NO x , where the VOC sum is weighted by reactivity. This is discussed in the report on Observation-based methods , elsewhere at this site.

Figure 1.2. Ozone isopleths (ppb) as a function of the average emission rate for NO x and VOC (10 12 molec. cm-2 s -1 ) in 0-d calculations. The isopleths (solid green lines) represent conditions during the afternoon following 3-day calculations with a constant emission rate, at the hour corresponding to maximum O 3 . The short blue dashed line represents the transition from VOC-sensitive to NO x-sensitive conditions. From Sillman and He (2002).



### 1.2.2 Factors affecting O 3 

### -NO x 

### -VOC sensitivity. [Top of section].
O 3 -NO x-VOC sensitivity for individual locations and events are often very uncertain (see 1.2.3. below). Generalizations about NO x-sensitive versus VOC-sensitive conditions are always very approximate and subject to many exceptions. The following are offered as broad guidelines for understanding the factors that cause O 3 -NO x-VOC sensitivity to vary for different locations and for different events.
VOC/NO x ratio. NO x-sensitive conditions are associated with high VOC/NO x ratios and VOC-sensitive conditions are associated with low VOC/NO x ratios (see Figure 1.1). Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

4 of 8 12/14/2011 6:35 PMWARNING: VOC/NO x ratios are associated with the instantaneous production rate of O 3 , not necessarily the ambient O 3 mixing ratio. Ambient O 3 is the result of photochemistry and transport over several hours (and often several days), and ambient VOC and NO x can vary greatly over time and through the upwind region in which the ozone was produced. VOC reactivity. A more reactive VOC mix is associated with NO x-sensitive conditions. NO x-sensitive versus VOC-sensitive conditions are correlated with the ratio of reactivity-weighted VOC to NO x , rather than the ratio of the simple sum of VOC to NO x (see report on Observation-based methods , elsewhere at this site. Biogenic VOC. Naturally occurring VOC (especially isoprene, emitted primarily by oaks and other deciduous trees) represents a significant fraction of total ambient VOC, especially in suburban and rural settings and when VOC is weighted by reactivity. Biogenic VOC is especially important because they are usually highly reactive. Biogenic NO x is far smaller relative to the anthropogenic source. A high rate of biogenic VOC increases the ratio of reactivity-weighted VOC to NO x and makes NO x-sensitive conditions more likely. (Chameides et al., 1988, 1992; Pierce et al., 1998).
Downwind distance. As air moves downwind from emission sources and ages photochemically, conditions tend to change from VOC-sensitive (closer to emission sources) to NO x-sensitive (further from emission sources). This occurs because NO x is removed more rapidly than VOC as an air mass moves downwind (thus increasing the VOC/NO x ratio) and because biogenic VOC becomes increasingly important as air moves downwind (Milford et al., 1989, 1994). WARNING: There are likely to be many exceptions to this rule. NO x-sensitive conditions are possible even in an urban center, and VOC-sensitive conditions can occur even at far downwind locations (Jacob et al., 1995; Kleinman et al., 2000)



### 1.2.3. Uncertainty in O 3 

### -NO x 

### -VOC sensitivity predictions [Top of section]. The split between NO x-sensitive and VOC-sensitive chemistry is a major source of uncertainty in predictions for the relation between O 3 and precursor emissions.
NO x-VOC sensitivity is difficult to predict, and there are no simple "rules of thumb" for distinguishing NO x-sensitive from VOC-sensitive conditions. O 3 -precursor sensitivity predictions are usually derived from 3-dimensional Eulerian chemistry/transport models. These models are subject to many uncertainties that affect their ability to identify NO x-sensitive vs. VOC-sensitive conditions. Different assumptions in models may lead to very different results for predicted sensitivity to NO x and VOC.
Emission inventories are often the largest source of uncertainty in model sensitivity predictions. O 3 -precursor sensitivity depends critically on emission rates for anthropogenic VOC and NO x , on the speciation of anthropogenic VOC, and on emission rates for biogenic VOC. Evaluation of chemistry/transport modelsis usually based on the ability of the model to predict O 3 in comparison with ambient measurements. However, success in simulating the observed Ox does not guarantee that models predict the O 3 -NO x-VOC sensitivity correctly. Frequently it is possible to create different model scenarios for an event that give similar O 3 but with very different predictions for NO x-VOC sensitivity (Sillman et al., 1995, Reynolds et al., 1996, Pierce et al., 1998). Effect of uncertainty. Figure 1.3 illustrates the uncertainty in predictions for the impact of reduced NO x and VOC. The figures show the reduction in peak O 3 resulting from reduced NO x and VOC, as predicted by a series of different model scenarios for two days in Atlanta. On the first day, a 35% reduction in NO x emissions is predicted to cause changes in peak O 3 ranging from a 23 ppb decrease to a 2 ppb increase, depending on which model scenario is chosen. Similarly, a 35% reduction in anthropogenic VOC is predicted to cause changes in peak O 3 ranging from a 23 ppb reduction to a 2 ppb reduction. This range of uncertainty occurs because the model chemistry may be predominantly NO x -sensitive or predominantly VOC-sensitive, depending on assumptions about emission rates and meteorology. Uncertainties are less on the second day (12-29 ppb reduction in O 3 for reduced NO x ; 0-11 ppb reduction for reduced VOC) because model uncertainties do not cause a switch from NO x-sensitive to VOC-sensitive chemistry. Observation-based methods (OBMs) can be used to reduce the uncertainty in predictions for the impact of reduced NO x and Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

5 of 8 12/14/2011 6:35 PMVOC on O 3 . OBMs may provide an independent observation-based estimate for the relative impact of NO x and VOC. The most important use of OBMs is to provide an observation-based evaluation and constraint on chemistry/transport models, in order to reduce the uncertainty in NO x-VOC predictions shown in Figure 1.3

.

(a) August 10, 1992

(b) August 11, 1992 Figure 1.3. Impact of model uncertainty on control strategy predictions for O 3 for two days in Atlanta, GA. The figures show the predicted reduction in peak O 3 resulting from 35% reductions in anthropogenic VOC emissions (crosses) and from 35% reductions in NO x (solid circles) in a series of model scenarios with varying base case emissions, wind fields and mixed layer heights. Results are plotted from tabulated values published in Sillman et al. (1995, 1997).



### x Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

6 of 8 12/14/2011 6:35 PM[Return to Table of Contents] Ozone formationoccurs through the following sequence of reactions. The sequence is almost always initiated by the reaction of various VOC or CO with the OH radical [ R1, R2 ]. This is followed by the conversion of NO to NO 2 (through reaction with HO2 or RO2 radicals), which also regenerates OH [ R3, R4 ]. NO 2 is photolyzed to generate atomic oxygen, which combines with O2 to create O 3 [ R5, R6 ].

Here, RO2 represents any of a number of chains of organics with an O2 attached (replacing H in the original chain). For example: propane (C3H8) reacts with OH [ R1 ] to generate the following RO2: C3H7O2. This reacts with NO [ R3 ] to generate C2H5CHO (propionaldehyde) and H (which combines with O2 to form HO2).
The rate of ozone formation is controlled primarily by the rate of the initial reaction of VOC with OH. NO xtitration: Elevated ozone in polluted regions is usually due to the ozone production sequence shown above. However, at nighttime and in the immediate vicinity of very large emissions of NO (e.g. power plants), ozone concentrations are depressed through the process of NO x titration. This consists of the removal of O 3 through reaction with NO [ R7 ].

During the daytime, this reaction is normally balanced by the photolysis of NO 2 [ R5, R6 ]. However, in the vicinity of large NO emissions the result is net conversion of O 3 to NO 2 . This is most common in the vicinity of large point sources. Ozone is depressed immediately downwind of these sources, and beomes elevated as the plume moves further downwind (Gillani et al., 1996). At nighttime, there is no photolysis of NO 2 reaction R7 also leads to the removal of ozone.
NO x -sensitive vs NO x -saturated: The split between NO x-sensitive and NO x-saturated regimes, illustrated in Figure 1.2 , is driven by the chemistry of odd hydrogen radicals. There are two main sinks for these radicals: peroxidesand nitric acid (HNO 3 ). When peroxides represent the dominant radical sink, chemistry is NO x -sensitive. when HNO 3 represents the dominant sink, conditions are NO x-saturated. This is described in more detail immediately below. Odd hydrogen radicalsconsist of OH, HO2 and species with the form RO2. This family is directly connected to the chemistry of ozone production, because the initial reaction of the ozone production sequence (VOC+OH, R1 ) is driven by OH and susequent steps depend on HO2 and RO2. The ozone production rates depend on the abundance of OH, which depends on sources and sinks of odd hydrogen radicals as a whole. Radical sourcesinclude photolysis of ozone, formaldehyde, and other secondary VOC. Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

7 of 8 12/14/2011 6:35 PMRadical sinksinclude formation of hydrogen peroxide ( R10 ) and organic peroxides ( R11 ) , formation of nitric acid ( R12 ) and net formation of peroxyacetyl nitrate and other organic nitrates.

NO x -sensitive regime occurs when peroxides ( R10 and R11 ) represent the dominant radical sink. In this case, ambient concentrations of HO2 and RO2 radicals will be determined by the balance between radical sources ( R8 andR9 ) and the peroxide-forming reactions ( R10 and R11 ). Because the rate of peroxide formation is quadratic in HO2, the ambient concentrations of HO2 and RO2 show little variation in response to changes in NO x and VOC. The rate of ozone formation is determined by the reaction of HO2 and RO2 with NO ( R3 and R4 ). This rate increases with increasing NO x . In polluted regions the rate of ozone formation usually is relatively unaffected by changes in VOC. In remote regions the rate of ozone formation also increases with increasing VOC. NO x -saturated (VOC-sensitive) regime occurs when nitric acid ( R12 ) represents the dominant radical sink. In this case, ambient concentrations of OH will be determined by the balance between radical sources ( R8 and R9 ) and the reaction of OH with NO 2 ( R12 ). Because the rate of nitric acid formation ( R12 ) increases with NO 2 , ambient OH decreases with increasing NO 2 . The rate of ozone formation is determined by the rate of the reaction of VOC and CO with OH ( R1 and R2 ). This rate increases with increasing VOC and decreases with increasing NO x . VOC/NO x ratios: The split between NO x sensitive and VOC-sensitive regimes is closely related to the ratio of summed VOC to NO 2 , assuming that the sums are weighted by reactivity (rVOC/NO 2 ). The formation rate of ozone and the radical source term [ R9 ] both increase with increases in the rate of the reaction of VOC with OH [ R1 ]. The rate of formation of nitric acid, a critical radical sink, depends on the rate of the reaction of NO 2 with OH [ R12 ]. The ratio of radical sources to the formation rate of nitric acid (as a radical sink) is proportional to the ratio of summed VO C (weighted by reactivity with OH] to NO 2 . When this ratio is high, peroxides become the dominant radical sink and conditions are NO x-sensitive. When this ratio is low, nitric acid becomes the dominant radical sink and conditions are NO x -saturated.
WARNING: VOC/NO x ratios are associated with the instantaneous production rate of O 3 , not necessarily the ambient O 3 mixing ratio. Ambient O 3 is the result of photochemistry and transport over several hours (and often several days), and ambient VOC and NO x can vary greatly over time and through the upwind region in which the ozone was produced. Ozone production efficiency per NO xis equal to the ratio of the rate of ozone formation to the rate of removal of NO x (largely by chemical conversion to HNO 3 ). The ozone formation rate is proportional to the rate of the reaction of VOC with OH. The conversion of NO x to nitric acid is proportional to the rate of the reaction of NO 2 with OH. The ozone production efficiency is equal to the ratio of these rates. Therefore, it is also related to the ratio of summed VOC(weighted by reactivit y with OH) to NO 2 . [Top of page] Ozone-NOx-VOC sensitivit y http://www-personal.umich.edu/~sillman/ozone.ht m

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