Sink Processes for l'etraalkyllead Compounds in the Atmosphere Roy M. Harrison* and Duncan P. H. Laxen Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, England
The kinetics and mechanism of the principal homogeneous reactions of tetramethyllead (TML) and tetraethyllead (TEL) in the atmosphere have been investigated. Rate constants have been determined for primary photolytic breakdown, and for reaction with ozone, O(3P), and OH. The reactions are principally those of photochemical oxidation, with attack by OH representing the main reaction pathway. Heterogeneous reaction pathways have also been investigated by exposing atmospheric particulates to tetraalkyllead vapors. The results are interpreted in terms of both a direct physical adsorption process and surface reaction in the presence of NO2 in dry air, neither process being of significance when extrapolated to conditions in an urban atmosphere. The overall rates of tetraalkyllead breakdown on a photochemically active sunny summer day are estimated to be up to 93% h-l for TEL and 28% h-l for TML. Both compounds are expected to be relatively stable a t nighttime. H
Two tetraalkyllead compounds, namely tetramethyl- and tetraethyllead (TML and TEL), are added to gasoline as antiknock agents. A substantial fraction of this organic lead is decomposed during combustion and exhausted as inorganic lead salts. A small proportion, however, escapes to the atmosphere unchanged. Additional contributions to tetraalkyllead in the atmosphere may also arise from crankcase blow-by gases in the absence of ventilation devices, as well as by evaporative loss from carburetors and gasoline tanks, and spillage a t gasoline stations. Tetraalkyllead emissions also arise during the processes of TML and TEL manufacture. To date most concern has been directed toward an understanding of the behavior and significance of the inorganic portion of lead in the atmosphere. In contrast, there is very little information about the organic portion of lead. The available data on the concentrations of vapor-phase tetraalkylleads, both absolute and relative to total lead, have recently been reviewed by Harrison and Perry (I).The concentrations measured in urban air, usually close to highways, are generally less than 10-5.ppm (350 kJ mol-') is well in excess of the mean lead to carbon bond dissociation energies of 153 kJ mol-' in TML and 129 kJ mol-I in TEL ( I O ) . Therefore, the first reaction to be investigated was that of photolysis in clean air. The loss of TML and TEL from purified dry air was monitored during irradiations in a static system (Figure 4). The data were highly repeatable and conformed to a firstorder reaction with photolytic rate constants of (3.4 f 0.34) X 10-4 min-1 for TML and (1.3 i 0.13) X min-' for TEL (after subtracting the respective rates for dark reaction). The reaction rate for TEL photolysis is thus approximately four times greater than that for TML. In order to relate these results more closely to conditions of natural sunlight it is necessary to determine the intensity of the UV lamps relative to sunlight over the spectral range (
