§ Mr. Atkins
I have been asked to reply.
Most benzene emissions are man made and it is widely distributed as a trace atmospheric constituent over urban and industrial regions of north-west Europe. Concentrations of benzene in the atmosphere show daily and seasonal cycles resulting from emission activities and chemistry occurring in the atmosphere.
Rural benzene measurements have been made in Oxfordshire at the Harwell laboratory since about 1986. Average concentrations at this rural site are about 0.8 ppb. Measurements made by the university of East Anglia at a rural site in Norfolk show a similar mean concentration to that observed at the Harwell site, whereas measurements at a remote rural site in Cumbria exhibited mean benzene concentrations of 0.4 ppb.
Measurements have been made in the remote northern hemisphere troposphere and in the southern hemisphere troposphere. Benzene concentrations were found to be 0.07 ppb in the northern hemisphere and less than 0.02 ppb in the southern hemisphere. Those figures, collected in the remote background atmosphere, give a sign of what may be described at the level of natural occurrence of benzene.