Annual NO\(_2\) as a Predictor of Hourly NO\(_2\) Variability: Do Defra UK’s Heuristics Make Sense?

Background: In the UK an hourly objective exists for NO2 concentrations and assessment against this objective is required for various administrative purposes. The vast majority of NO2 measurement in the UK is non-hourly however. Thus, Defra guidance provides a heuristic to estimate hourly objective exceedance likelihood from an annual average. Methods: We examine the performance of this heuristic using a Europe wide dataset containing over 20,000 site-years of data, and perform a sensitivity test to account for data uncertainty. Results: The heuristic misses 64% of sites that break the hourly objective. The heuristic is neither a necessary nor sufficient condition for predicting hourly objective breaches. The sensitivity test reveals that the heuristic is input-fragile. Conclusions: The heuristic performs poorly, is weakly coupled to medical evidence, and work is needed to develop new short term exposure limits for NO2.