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. Measurements of OH, HO 2 , RO 2i (alkeneand aromatic-related RO 2) and total RO 2 radicals taken during theClearfLo campaign in central London in the summer of 2012 are presented. Aphotostationary steady-state calculation of OH which considered measured OHreactivity as the OH sink term and the measured OH sources (of whichHO 2+  NO reaction and HONO photolysis dominated) compared well with theobserved levels of OH. Comparison with calculations from a detailed box modelutilising the Master Chemical Mechanism v3.2, however, highlighted asubstantial discrepancy between radical observations under lower NO x conditions ([NO] < 1 ppbv), typically experienced during theafternoon hours, and indicated that the model was missing a significantperoxy radical sink; the model overpredicted HO 2 by up to a factor of10 at these times. Known radical termination steps, such as HO 2 uptakeon aerosols, were not sufficient to reconcile the model–measurementdiscrepancies alone, suggesting other missing termination processes. Thismissing sink was most evident when the air reaching the site had previouslypassed over central London to the east and when elevated temperatures wereexperienced and, hence, contained higher concentrations of VOCs. Uncertaintiesin the degradation mechanism at low NO x of complex biogenic and dieselrelated VOC species, which were particularly elevated and dominated OHreactivity under these easterly flows, may account for some of the model–measurement disagreement. Under higher [NO] (> 3 ppbv) the boxmodel increasingly underpredicted total [RO 2 ]. The modelled andobserved HO 2 were in agreement, however, under elevated NOconcentrations ranging from 7 to 15 ppbv. The model uncertainty under low NO conditions leads to more ozone productionpredicted using modelled peroxy radical concentrations( ∼  3 ppbv h −1) versus ozone production from peroxy radicalsmeasured ( ∼  1 ppbv h −1) . Conversely, ozone production derivedfrom the predicted peroxy radicals is up to an order of magnitude lower thanfrom the observed peroxy radicals as [NO] increases beyond 7 ppbv due to themodel underprediction of RO 2 under these conditions.

atmospheric chemistry and physics

Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)