Estimating the lifetime of black carbon in rural air

As part of its commitment to the European Monitoring and Evaluation Programme (EMEP) of the UNECE, Defra funds two ‘supersites’ in the UK for monitoring air composition. The site at Auchencorth (central Scotland, 15 km south of Edinburgh and 25 km SW of Cockenzie power station) has been operating since 2006, measuring black carbon using a Magee Aethalometer with an inlet cutoff at 2.5 μm (PM2.5). Five-minute data are aggregated to half-hour averages for reporting to EMEP. June 2006 to May 2010 median concentrations were 0.15 μg m-3 with peaks up to 5 μg m-3. Data are distributed log-normally. Even in well-mixed conditions (wind speed > 2 m s-1) concentrations are log-normal with geometric mean and standard deviation of 0.13 μg m-3 and 2.62, respectively. Volatile organic compounds (VOCs) are measured hourly at the site as part of the UK Hydrocarbons network. Based on analysis (Jobson et al., 1999) of the concentration distribution of VOCs measured at the site during well-mixed daytime conditions, black carbon has a lifetime similar to that of butane, i.e. between 20 and 50 days, depending on the average OH concentration (2.5 to 1 x 105 cm-3). Analysis by wind sector allows separation of potential local and long-range sources, which are presumably linked to particle sizes. This method makes several assumptions concerning the relative sources of black carbon and VOCs, but if both pollutant type have similar sources and transport pathways to the measurement site, then VOC distributions can be used to infer black carbon lifetimes. Jobson B.T., McKeenS.A., Parrish D.D., FehsenfeldF.C., Blake D.R., Goldstein A.H., SchaufflerS.M., Elkins J.C., 1999. Trace gas mixing ratio variability versus lifetime in the troposphere and stratosphere: Observations. Journal of Geophysical Research-Atmospheres 104, 16091-16113.