External contribution to urban air pollution

Elevated particulate matter concentrations in urban locations have normally been associated with local traffic emissions. Recently it has been suggest ed that such episodes are influenced to a high degree by PM10 sources exter nal to urban areas. To further corroborate this hypothesis, linear regressi on was sought between PM10 concentrations measured at eight urban sites in the U.K., with particulate sulphate concentration measured at two rural sit es, for the years 1993-1997. Analysis of the slopes, intercepts and correla tion coefficients indicate a possible relationship between urban PM10 and r ural sulphate concentrations. The influences of wind direction and of the d istance of the urban from the rural sites on the values of the three statis tical parameters are also explored. The value of linear regression as an an alysis tool in such cases is discussed and it is shown that an analysis of the sign of the rate of change of the urban PM10 and rural sulphate concent rations provides a more realistic method of correlation. The results indica te a major influence on urban PM10 concentrations from the eastern side of the United Kingdom. Linear correlation was also sought using PM10 data from nine urban sites in London and nearby rural Rochester. Analysis of the mag nitude of the gradients and intercepts together with episode correlation an alysis between the two sites showed the effect of transported PM10 on the l ocal London concentrations. This article also presents methods to estimate the influence of rural and urban PM10 sources on urban PM10 concentrations and to obtain a rough estimate of the transboundary contribution to urban a ir pollution from the PM10 concentration data of the urban site.