Chemical Reactions on Urban Films

Experimental Studies

At this time, there have been no laboratory studies reported investigating reactions on "real" urban films, i.e. films allowed to develop outdoors in an urban setting. Most studies have involved oxidation of single-component (e.g.. self-assembled monolayers, oleic acid or solid PAH - see Rudich (2) for a recent review) organic films by ozone, though some very recent work on OH and N03 has been reported. (10-13) In all cases studied to date, the reaction rate is significantly faster than the corresponding gas-phase value.

We have recently reported on an extensive study of the reaction kinetics of gas-phase ozone with six PAH compounds embedded in organic films composed of octanol or decanol, mixed with vacuum grease and (for some experiments) other organic compounds. (14) Octanol was chosen as the primary proxy for the "true" film, because of the strong relationship between the K0a and KFA values for the compounds chosen here, as discussed above. Our studies have used laser-induced fluorescence detection of the PAH compound to track its concentration as a function of time following exposure to ozone. The full results are given in Kahan et al. (14) and shown in Tables I and II; some key points are summarized here.

Table I. Heterogeneous reaction rates for PAHs in organic films with gas-phase ozone ([03] =50 ppb)

PAH

Aob,(10V)

Benzo[a]pyrene

28.3

Anthracene

16.5

Naphthalene

5.93

Pyrene

4.61

Phenanthrene

2.64

Fluoranthene

<0.92

The first important result is that the reactions appear to take place at the surface of the organic film. This is indicated by the non-linear dependence of the measured pseudo-first order rate coefficients on the gas-phase ozone concentration, which show a Langmuir-Hinshelwood form in all cases. Figures land 2 illustrate the kinetic results for reaction with benzo[a]pyrene. The data shown in Figure 2 are well fit to a Langmuir-Hinshelwood model, which implies that the ozone reagent undergoes equilibrium (or, at least steady-state) partitioning between the gas phase and the surface and that the reaction takes place between surface-bound ozone and surface adsorbed PAH. Consistent with this we found that anthracene is surface active in octanol (14) (although not nearly so much as in water (15)% confirming the availability of the PAHs for reaction at the air-organic film interface.

The finding that the reaction of ozone with PAHs takes place at the organic film surface was unexpected, given the solubilities of the compounds in octanol. We and others have previously reported the Langmuir-Hinshelwood kinetic mechanism for such ozonation reactions on water (16-19) and solid surfaces (20-23). This surface reactivity has also very recently been found on liquid

Table II. Effect of composition of organic film on anthracene kinetics ([03] = 50 ppb)

Substrate

*obs(10'V)*

Quartz Plate

18.0

Silicone Grease

16.3

Vacuum Grease

17.2

Decanol

16.2

Corn Starch

20.5

Stearic Acid

23.5

Oleic Acid (1:1 v/v%)

5.6

Squalene (1:1 v/v%)

1.8

aerosol particles composed of organic substrates (unreactive towards ozone), (21) and so appears to be a general feature of these reactions. This has important consequences for how such reactions are treated in atmospheric chemistry models - whether it is the exposed surface area or the available volume of substrate which is considered.

The second key result is that the presence of other compounds did not affect the observed reaction kinetics, except when the octanol solvent was mixed with an equal volume of oleic acid. Here, the loss rate of anthracene was diminished by about 70%, perhaps due to scavenging of ozone by the highly reactive fatty acid, or to the formation of a polymer "shell" by radical reactions involving the oleic acid and its reaction products with ozone. However, the independence of the loss rates on the presence of other compounds, representative of the classes identified in the organic portion of urban surface films, suggests that laboratory experiments on simplified film proxies could yield results which have relevance to real situations. Some modeling results which address the importance of such processes are discussed below.

Modelling Studies

We incorporated the ozonation kinetic results outlined above into a modified multimedia partitioning model, in order to estimate the overall importance of surface reactions to the fate of PAHs in an urban setting. The MUM-Fate model, developed by Diamond and co-workers, (24, 25) is based on

Figure 1. Loss kinetics ofbenzofajpyrene in an organic film due to reaction with gas-phase ozone.

Time (minutes)

Figure 1. Loss kinetics ofbenzofajpyrene in an organic film due to reaction with gas-phase ozone.

Figure 2. Dependence of the measured first-order rate coefficients for heterogeneous benzofajpyrene loss on the gas-phase ozone concentration. The curve shows a fit to a Langmuir-Hinshelwood kinetic model.

Ozone Concentration (10*^ molecule cm"^)

Figure 2. Dependence of the measured first-order rate coefficients for heterogeneous benzofajpyrene loss on the gas-phase ozone concentration. The curve shows a fit to a Langmuir-Hinshelwood kinetic model.

the level III steady-state fugacity model of Mackay. (26) It consists of seven media compartments: lower air from 0-50 m, which contains aerosol particles; upper air of 50-500 m, which also contains aerosol particles; surface soil; surface water; sediment; vegetation and the film on impervious surfaces. The six PAHs whose heterogeneous ozonation kinetics were studied by us were allowed to enter the model through direct emission into the lower air compartment. Chemical transfer between compartments is characterized by chemical- and media-specific transport parameters. Chemical loss from the model urban system occurs through advective flows from air and water, reactions in all media, burial from sediment, and leaching from soil. Full details are given in Kwamena et al. (27)

Several different model environments with differing amounts of impervious surface area (and hence surface film) were studied. The model results suggest that the reactive fate of PAHs in the urban environment is determined by the interplay between their mass distribution, governed by partitioning out of the air compartment, and their lifetimes in the different environmental compartments. Advection was the dominant loss process in all model environments for all PAHs investigated. However, films coating impervious surfaces were found to play an important role for the heavier, lower volatility PAHs, which accumulate more easily in organic condensed phases. For one such compound, benzo[a]pyrene, -75% of the chemical loss in highly urbanized model environments occurred via film reaction; amounting to -30% of the total loss (i.e. considering both advective and reactive loss process) under stagnant air conditions. The more volatile PAHs remained almost entirely in the air compartment, and so were minimally affected by reactions on urban films; their chemical loss fate was dominated by gas-phase reactions with OH.

These model results strongly suggest that under certain environmental conditions, heterogeneous reactions on urban films may play an important role in determining the fates of low volatility PAHs. Although most of the PAH mass was exported from the model urban environment, the film reactions give rise to products which are also associated with the films. These products are probably more highly oxidized than the parent compounds, and hence likely to remain in the condensed phase.

It should be noted that the only heterogeneous reactions included in the model were those of the PAHs with ozone, for which the kinetic parameters were available. There have been only a handful of studies reported of the reactions of other atmospheric oxidants with organic films. In almost all cases, the films were composed of a single organic compound (either liquid or a self-assembled monolayer). Under these conditions, the reaction rates were found to be quite high, with reaction probabilities towards OH typically > 10% per collision with the organic film surface. (10) Reaction efficiencies towards N03 radical seem to be lower, (11, 12) but its high nighttime abundance suggests that it may be a key oxidant of compounds in such films. As more such reactions are studied experimentally, it will be important to revisit this topic, in order to determine a more quantitative picture of the fates of semivolatile organics in an urban setting.

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