(14) Lamothe, P. J., Dzubay, T. G., Stevens, R. K., “The General Motors/Environmental Protection Agency Sulfate Dispersion Experiment”, EPA-600/3-76-035, 1-28, 1976. (15) Cobourn, W. G., Husar, R. B., Husar, J. D., Atmos. Enuiron., 12, 89-98 (1978). (16) Newman, L., ibid., 12,113-26 (1978). (17) Tanner, R. L., Newman, L., J . Air Pollut. Control Assoc., 26, 737-47 (1976). (18) Scaringelli, A. P., Rehme, A., Anal. Chem., 41,707-13 (1969). (19) Seymore, M. D., Clayton, Jr., J. W., Fernando, Q., ibid., 49, 1429-32 (1977). (20) Eatough, D. J., Hansen, L. D., Izatt, R. M., Mangelson, N. F., in “Methods and Standards for Environmental Measurements”, NBS Special Publ. 464, W. H. Kirchhoff, Ed., pp 643-9, National Bureau
of Standards, Washington, D.C., Nov. 1977. (21) Hansen, L. D., Whiting, L., Eatough, D. J., Jensen, T. E., Izatt, R. M., Anal. Chem., 48,634-8 (1976). (22) Nielson, K. K., Hill, M. W., Mangelson, N. F., Adu. X-ray Anal., 19,511-20 (1976). (23) Eatough, D. J., Major, T., Ryder, J., Hill, M., Mangelson, N. F., Eatough, N. L., Hansen, L. D., Atmos. Enuiron., 12, 263-71 (1978). Received for reuieu: November 18,1577. Accepted May 30,1578. Work supported by the Energy Research and Development Administration, Contract E Y 765-02-2588, and by the Electric Power Research Institute, Contract TPS-76-64 7.
Ozone Generation over Rural Areas lvar S. A. Isaksen”, Bystein Hov, and Eigil Hesstvedt Institute of Geophysics, University of Oslo, Oslo, Norway ~~
~~
Calculations of ozone in the polluted atmosphere outside urban areas are presented. Different types of pollution situations are considered: a strongly polluted urban plume and less polluted air in rural areas. In an urban plume, ozone is generated on a time scale of a few hours. The chemical lifetime of ozone increases strongly as the polluted air masses are transported over rural areas (>1day); this allows for transport over long distances once high concentrations of ozone are formed within the plume. Maximum concentrations well above 100 ppb can be expected downwind of urban areas on sunny summer days with stable weather conditions. Over rural areas, ozone is generated on a time scale of some days. During anticyclonic weather situations, ozone concentrations in excess of 100 ppb may build up. High ozone concentrations, often in excess of air quality standards (e.g., the National Ambient Air Quality Standard in the US.states 80 ppb as a maximum hourly average to be exceeded once a year or less), are frequently observed over large rural areas in industrialized countries. This has raised interesting questions about the degree of anthropogenic influence on tropospheric ozone. Measurements in urban plumes provide evidence that elevated ozone concentrations are correlated with increased release of hydrocarbons and nitrogen oxides within the urban area, and that ozone is transported over long distances ( I -4).Furthermore, measurements (5,6)have shown that ozone has accumulated in rural areas in the midwestern U.S., well outside any identifiable plume in quiet, sunny weather, with maximum values during the sunlit periods. This indicates that ozone was formed as a result of chemical activity within the region. It is interesting to note that hydrocarbons of both natural (isoprene) and anthropogenic (aromatic compounds) origin were present in the ozone-rich air masses. The concentrations of the hydrocarbon species (a few ppb) were considerably lower than what can commonly be found in urban air (7). Simulation of the chemical turnover in an urban-rural atmosphere requires an adequate representation of the interaction between pollution chemistry and natural tropospheric chemistry. In this paper we will discuss the generation of ozone on the wide range of time scales typical of air masses which have been exposed to emission over urban and rural areas. We will show that the time scale of ozone formation in an urban plume is a few hours, and that the chemical lifetime of ozone
over rural regions is long enough to allow for transport over large areas, once ozone is formed. Furthermore, accumulation of ozone may occur over large rural areas during anticyclonic ‘weather which persists for several days, since the time scale of ozone formation during such situations is a few days. Our chemical model was described in detail previously (8, 9). The numerical procedure used to solve the set of timedependent differential equations deduced from the chemical reaction scheme is a version of an explicit numerical scheme ( 1 0 , I I ) . Extensive comparisons with Gear’s method (12) for the type of simulations discussed in this paper show good agreement for all model species at all times, most of the time within 1%.Gear’s method is not applied here because of its demand on computer time and storage. The method used here requires a factor of the order of 10 less computer time and storage. The diurnal variation of direct and scattered solar radiation is computed by the simplified scheme given by Isaksen et al. (13).This scheme improves the accuracy of solar flux determinations compared to calculations when scattering is omitted, without being very time consuming. The solar fluxes are computed for a clear sky at 40’ latitude, summer. We assume an ozone column density of 312 Dobson units and a surface temperature of 28 OC.
0013-936X/78/0912-1279$01.00/0 @ 1978 American Chemical Society
Chemistry and Emissions A complete list of the chemical reactions with references considered in our model is included in the supplementary material. Here we shall only outline the reactions of primary importance for the ozone budget. Odd oxygen and nitrogen oxides are strongly coupled through the set of chemical reactions:
+ NO NO2 + hu 0 3
0
--*
-
-j
+0 2 +M
+ 0 2 R7.9 NO + 0 RlO.hu 0 3 + M R1.4.M
NO2
[The numbering of the chemical reactions follows the NBS system (141.1 These reactions proceed without consuming either odd oxygen or nitrogen oxides (NO,). Nevertheless, they usually determine the internal equilibrium between NO, NO2, and 0 3 . Ozone is produced when reactions RlO.hu and R1.4.M follow a reaction where NO is converted to NO2 without consuming an odd oxygen molecule (0 or 0 3 ) . Such conversion Volume 12, Number 12, November 1978 1279
takes place through the reaction of NO with HOz and organic peroxy radicals:
+ HOz NO + ROz
NO
+ OH NO2 + RO
NO2
-
+
R9.20
CH4
C2H2
C2H.q
C3n6
C4H10
CBH14
R9.a
0.1
0.2
0.2
0.1
0.1
0.1
I t is important to notice that NO, acts as catalyst and may go through several ozone-forming cycles before it is removed by real loss reactions. High concentrations of peroxy radicals are maintained in polluted air through the decomposition of hydrocarbons which mainly occurs by reactions of the type OH
+ HC
-
products R19.6
where peroxy radicals are formed in the subsequent reactions. We have discussed in detail the chemical reactions which determine the production of radicals previously (8, 9). In particular, a scheme for the decomposition of rn-xylene is proposed in (9). Crutzen (15, 16) has shown that ozone is generated in a similar way in the natural background atmosphere, where water vapor serves as a source for odd hydrogen radicals through the reaction with excited state atomic oxygen. The natural production of ozone is also enhanced by the decomposition of methane which eventually leads to the formation of CH3Oz and HO2 radicals. Uncertainties in the rates of some of the key reactions represent a large obstacle when background ozone levels in the troposphere are to be estimated (17). Following the argument above, ozone destruction occurs whenever NO2, formed by reaction R7.9, reacts to give products other than odd oxygen. In polluted air this will mainly take place through the formation of nitric acid: M
OH + NOz -+ H N 0 3 R10.19 and to a smaller extent through the formation of PAN CH3COO2
-
+ NO2
CH3COOzN02 R10.59q
On a time scale of a few days, nitric acid is removed through heterogeneous reactions, while PAN is broken down through the strongly temperature-dependent thermal dissociation CH3COOzN02
-
CH3COOz
+ NO2
R59s
The direct loss of ozone through the reaction
-
O3 + H 0 2
OH
+ 2OZ
R7.20
is unimportant in polluted air. In rural air masses, however, where the ratio 03/N02often is large, it becomes a major loss. Similarly, the reaction of O(1D) with water vapor removes odd oxygen effectively over rural areas (18). In the absence of solar radiation a t night, ozone is lost through reactions with nitrogen oxides. Reaction R7.9 followed by
-
+ 0 2 R7.10 NO3 + NO NO2 + NO2 R9.11 NO3 + NO2 NO + 0 2 + NO2 R1O.ll NOz
+ O3
NO3
+
form a sequence of catalytic reactions which may rather effectively destroy ozone over rural areas. Based on published data from OECD (19) we find release rates of NO, of 4.5 X 1012 molecules cm-2 s-l to represent an upper limit for strongly polluted areas, while values around 1.5 X 10 12molecules cmV2 seem to be typical for a large number of urban areas. Emission rates of 2-6 X 10" molecules cm-2 s-l, may be considered typical as average values in industrialized countries (19, 20). NO, emissions are assumed 1280
Environmental Science & Technology
Table 1. Adopted Composition of Anthropogenic Hydrocarbon Emissions (by Volume) ceHlo
0.2
to consist of 99% NO and 1%NOz, and emission of CO is set to 10 times the emission of NO, (by volume). The HC/NO, ratio in the gas emissions is known to vary rather strongly from one area to another, depending on what type of combustion process is responsible for the emission (29). Our model calculations are therefore carried out for different HC/NO, emission ratios. The adopted composition of the anthropogenic hydrocarbon emissions is given in Table I. I t represents a selection of hydrocarbons typical of anthropogenic mixtures which are dominated by auto exhaust (7,9) and may therefore serve as a reasonable approximation in an urban plume. The composition is less representative for hydrocarbons found over rural areas ( 5 ) ,where a variety of combustion sources and probably also natural processes contribute to the atmospheric hydrocarbon mixture. We are here primarily concerned with the ozone-forming potential of the HC-NO,-air mixture, and the types of hydrocarbons present initially are not critical for the ozone formation, provided a major part of the hydrocarbons is oxidized (21). This occurs on a time scale of the order of some days. We will consider the chemical activity in rural air masses over a time period lasting several days [typical time scale of anticyclonic weather situations over the continental U.S. and Europe (22)].This is sufficient time for most of the anthropogenic hydrocarbons to be oxidized (with the exception of species like methane, ethane, and acetylene). This should also be valid for HC-mixtures consisting of hydrocarbons released from the biosphere (e.g., isoprene), since they probably are oxidized rather rapidly (23), and may have the potential to form ozone. The estimation of ozone generation over rural areas should therefore not depend critically on the HC-composition chosen.
Ozone Generation We will consider the generation of ozone over rural areas in two general cases: in an urban plume where the ozone chemistry is dominated by the anthropogenic release of nitrogen oxides and hydrocarbons within the urban area; and in a rural area with emissions from diffuse anthropogenic and natural sources with typical source strength amounting to a few percent of the source strength within an urban area. In this case, water vapor and methane may be important precursors for ozone. Urban Plume. An air mass passes over an urban area in the early morning hours and is exposed to solar radiation throughout the day. The lateral dimension of the plume is large enough to neglect horizontal dilution on the time scale we consider (-10 h). This assumption is justified by field measurements (3).The vertical extension of the plume is limited by an inversion a t the top of the well-mixed surface layer. Within the plume, vertical mixing is fast enough to give homogeneous distribution of the emitted gases. The deepening of the mixing layer is believed to take place by vertical mixing through the inversion layer. The time variation of the concentration of a chemical compound C in a volume of unit base area and height h (the mixing height) is obtained from the continuity equation:
(1) a = dh/dt expresses the expansion of the mixing layer, and
[C,] the concentration of the species outside the plume. Clean
8 n a a
i I \
10
RO,
v
0
+ 0 L
~
9
12
15
18
21
C
time ( h ) Figure 1. Development of NO, NOn, OH, HOP,and Ron(sum of organic peroxy radicals) for air mass that remains for 2 h in urban area with NO, release rate of 5 X IO7 molecules s-' and a HC/NO, ratio of 2 in emissions
air is assumed to surround the plume. [C,] is chosen accordingly. Observations show that h may vary markedly with time and location ( 2 4 ) ,particularly during the summer. Typical values for a are 0.01-0.05 m/s, for h from a few hundred meters in the morning to 1000-1500 m in the afternoon. Our selection of (Y and h is based on observations by White et al. ( 3 ) ,and seems to represent a continental summer situation rather well. a takes the value 0.036 m/s between 9 h and 17h ; a t other times it is equal to zero. h is set equal to 300 m before g h ; afterward it is determined by a. Our adopted values for (Y and h result s-l. P& and L c h [c]are in d h - r a t i o s smaller than 1.2 X the chemical production and loss terms, respectively. P, is the pollution source given by
P,
*
=-
(2) h where IJ is the flux of the pollutant considered. We assume that the sources are evenly distributed. The term Lhet+[C]gives the loss rate due t o heterogeneous processes, either by ground removal or by absorption on particles. Ground removal may be an important sink for ozone. A decay constant given as v d / h is adopted, where v d = 0.002 m/s is the deposition velocity. Ground removal is found to be an unimportant loss process for other gases (e.g.,NO2, PAN, aldehydes, ketones) and is therefore neglected in these calculations. Nitrogen oxides, aldehydes, ketones, and several others are assumed to be removed by absorption on particles. Such loss is of importance only as long as the characteristic time is comparable to or shorter than the gas-phase loss. This heterogeneous loss rate is approximated by using a constant value of 2 X s-l for L h e t . The time the air mass remains over the source region, and hence the amount of pollution released into it, depends on the depth of the source region and on the wind speed. With wind speeds of 3-5 m/s and horizontal depths of the urban region in the range 30-80 km, the time A t , needed to pass over the urban area is 2-8 h. We will discuss ozone generation in an urban plume, when IJ and Ats are varied within the limits mentioned above. Characteristic times for generation and loss of ozone will be discussed. Observational evidence ( 3 )indicates a rather rapid generation in the plume (a time scale of only a few hours) followed by a long period (-1 day) where the ozone concentrations remain a t elevated levels provided certain meteorological conditions prevail (2, 25, 26). Nitric oxide, nitrogen dioxide, hydroxyl, and peroxy radicals
9
12
15
18
21
time ( h ) Figure 2. Net hourly rate of ozone formation per NO, molecule emitted, vs. time, in plume outside source region Numbers on curves refer to case numbers defined in Table i l
are key components in the formation and destruction of ozone. Their variation with time in the plume is shown in Figure 1. The results are characteristic of these gases in an urban plume over rural areas: NO concentrations drop rapidly, and the peroxy radical concentrations increase due to their dependence on NO. This behavior is determined by several factors: conversion of NO to NO2 becomes more effective with time due to an increase in the ozone concentrations, and NO, concentrations are reduced, both as a result of dilution and as a result of conversion to " 0 3 through reaction R10.19. Furthermore, production of peroxy radicals through hydrocarbon decomposition is effective throughout the day. Peroxy radical concentrations are found to vary strongly in the plume, depending on the release rates of HC and NO,, and on the ratio HC/NO,. Maximum values in the range 0.2-3 ppb are in agreement with observations in industrialized areas (27). OH depends less on the degree of pollution. Maximum values are in the range 2-4 X ppb in all calculations reported here, in agreement with observations (28).OH concentrations of this magnitude lead to conversion rates of NO, to "03 up to 35%/h during the early afternoon. In Figure 2 we present the net rate of ozone formation in the plume relative to the release of NO, in the urban area. The production reaches its maximum value well outside the urban area, and remains high for several hours after the air mass leaves the source region. The production of ozone is more effective the smaller the total emission of precursors, i.e., in situations with low chemical activity. Furthermore, high HC/NO, ratios in the emissions favor a much more effective ozone production than low ratios, and the ozone production peaks earlier. For a HC/NO, ratio of 0.01, ozone production increases for about 7 h. I t is delayed by the large amounts of nitrogen oxides, which suppress the buildup of peroxy radicals. Peroxy radical production is larger for high HC/NO, ratios, and hence more NO to NO2 conversions will take place before the mixture is chemically deactivated. From Figure 2 it is also seen that with a HC/NO, ratio equal 2 and with the emission rates of NO, chosen, 5-9 ozone molecules are produced for each NO,-molecule released from 9 a.m. to 5 p.m. during the first day of transport. We have here considered the production of ozone in the plume during the first day after the air mass passed over the source region. The anthropogenic hydrocarbon emissions Volume 12, Number 12, November 1978
1281
300 r
Table II. Fraction of Hydrocarbons Left after 8 h over Rural Areas (i.e., at 5 p.m.) 5
1
2
3
4
2
2
4
0.01
x
107
2
x
108
1
x
108
1
x
108
2
8
2
2
0.04 0.51 0.14 0.97 0.67 0.26
0.01 0.42 0.17 0.97 0.64 0.28
0.03 0.55 0.20 0.98 0.72 0.33
0.31 0.86 0.59 0.99 0.92 0.72
"- I 200
E
x
E O1
l 5 OI 1 -
consist of species with rather different chemical lifetimes, some of them comparable to and even longer than the time period considered (8 h). A large part of the HC-mixture will therefore remain unactivated a t the end of the first day, and may contribute to ozone formation as it is broken down chemically the following days. Table I1 gives the amount of the different HC compounds present in the plume a t 17h relative to the amount which was present when the polluted air left the source region. Propylene is seen to be removed effectively, while most of the other compounds have a potential for further ozone generation. The ozone-forming potential of the primary pollutants in the plume will therefore be considerably higher than what can be the impression from Figure 2. The chemical lifetime of ozone in the plume is of great interest for the question of ozone transport. I t increases with time after the air mass has left the source region, and amounts to several days in the early afternoon. We therefore conclude that ozone formed in urban plumes has a sufficiently long chemical lifetime to remain at elevated concentrations for days over rural areas, and that the ozone concentrations are determined by mixing with cleaner air rather than by chemical processes. The concentration of ozone in the plume (see Figure 3) depends strongly on the amount of pollutants emitted in the urban area, and on the HC/NO, ratio in the emissions. For HC/NO, ratios larger than 0.5, the concentrations of ozone in the plume are strongly enhanced above the rural background level, in most cases well above 80 ppb. I t is important to notice that the buildup occurs during the first hours of passage over rural areas, whereafter ozone is found to remain a t high levels throughout the day. This results in transport over large distances (>lo0 km). I t is only for low HC/NO, ratios in the emissions (
