Development and back-extrapolation of NO2 land use regression models for historic exposure assessment in Great Britain

Modelling historic air pollution exposures is often restricted by availability of monitored concentration data. We evaluated back-extrapolation of LUR models for annual mean NO2 concentrations in Great Britain for up to 18 years earlier. LUR variables were created in a GIS using land cover and road network data summarized within buffers, site coordinates and altitude. Four models were developed for 2009 and for 2001 using 75% of monitoring sites (in different groupings) and evaluated on the remaining 25%. Variables selected were generally stable between models. Within year, hold-out validation yielded MSE-R2 (i.e. fit around the 1:1 line) values of 0.25~0.63 and 0.51~0.65 for 2001 and 2009, respectively. Back-extrapolation was conducted for 2009 and 2001 models to 1991 and for 2009 models to 2001, adjusting to year using two background NO2 monitoring sites. Evaluation of back-extrapolated predictions used 100% of sites from an historic national NO2 diffusion tube network (n=451) for 1991, and 70 independent sites from automatic monitoring in 2001. Values of MSE-R2 for back-extrapolation to 1991 were 0.42~0.45 and 0.52~0.55 for 2001 and 2009 models, respectively, but model performance varied by region. Back-extrapolation of LUR models appears valid for exposure assessment for NO2 back to 1991 for Great Britain.