Environ. Scl. Technol. 1986, 20, 797-802
Formation and Decomposition of Trialkyllead Compounds in the Atmosphere C. Nlchoias Hewitt*
Department of Environmental Science, University of Lancaster, Lancaster, LA1 4YQ U.K. Roy M. Harrison
Department of Chemistry, Universlty of Essex, Colchester, C04 3SQ U.K. The kinetics and mechanisms of the homogeneous reactions of tetramethyllead (TML) and tetraethyllead (TEL) with hydroxyl (HO) have been studied. A suite of lead products were found, including ionic trialkyllead (TriAL) and dialkyllead (DiAL) compounds and inorganic Pb. The ionic alkyllead products were found both in the gas phase and in the aerosol, as well as on the reaction chamber walls. The decomposition reactions of TML, TEL, and the ionic trimethyllead and triethyllead salts were found to proceed with pseudo-first-order kinetics, with the ionic TriAL compounds being about 3 times less reactive with HO than the corresponding tetraalkyllead (TAL) compounds. The ionic TriAL compounds are therefore quite persistent in the atmosphere, ionic TriML having a global mean half-life of about 5 days. The observed occurrence of ionic alkyllead compounds in the atmosphere is therefore explicable in terms of their atmospheric chemistry. Although legislative action has brought about a fall in their use in some countries, tetraalkyllead (TAL) compounds are still added to gasoline as antiknock agents in most countries of the world. Indeed, in some countries consumption of lead for this purpose continues to rise. In 1977 the amount of lead used as gasoline additives in the western world was 335 X lo3tons falling to 204 X lo3tons (or 5.4% of total lead consumption) in 1983 (I). In the US. 51 X lo3tons of lead were consumed as additives in 1983 (2),indicating that their use and effects still warrant detailed investigation. About 75% of the alkyllead additive in fuel is emitted from the exhaust as a complex mixture of inorganic lead salts (3)with the majority of the rest being deposited in the engine and exhaust system. However, about 1% of the ' alkyllead in the fuel is emitted unchanged in the exhaust with perhaps another 1% being lost to the atmosphere by evaporation from the fuel tank and carburetor and by fuel spillage during transport and handling. Such losses are difficult to quantify but overall -2% of the alkyllead in fuel may be released to the atmosphere (4). Until recently studies of these alkyllead species in the atmosphere were restricted to determining their concentrations in urban (and to a lesser extent, rural) air (5-7) and to investigations of their sink processes (8, 9). No information was available concerning the nature or atmospheric concentrations of the decomposition products of tetraalkyllead in the atmosphere; it was assumed that any intermediate compounds were insignificant compared with the ultimate product, inorganic lead. However, it has recently been shown that alkyllead compounds other than TAL are present in the atmosphere in the gas phase, with ionic trimethyllead salt being positively identified in both urban and rural air ( 1 0 , I I ) . Gas-phase ionic trimethyllead salt (TriML) accounted for about 10% of the total gasphase alkyllead in these samples. In contrast to this, determination of the alkyllead content of the atmospheric aerosol at both urban and rural sites has established that the concentrations of alkyllead, both TAL and TriAL, present in the solid phase are negligible compared with 0013-936X18610920-0797$01 .SO10
Table I. Composition of Smog Chamber Reaction Mixture for the Generation of Hydroxyl Radicals ( 8 ) compound
initial concn
compound
initial concn
methane ethane propane butane ethene
1.4 ppm 45 PPb 65 ppb 40 PPb 45 vvb
propene butene carbon monoxide nitrogen dioxide nitric oxide
40 PPb 45 PPb 7 ppm 0.06 ppm 0.4 vvm
~
those present in the gas phase (solid-phasealkyllead being
