In this study air masses are characterized in terms of their total OH reactivity which is a robust measure of the "reactive air pollutant loading". The measurements were per-formed during the DOMINO campaign (Diel Oxidant Mech-anisms In relation to Nitrogen Oxides) held from 21/11/2008 to 08/12/2008 at the Atmospheric Sounding Station – El Arenosillo (37.1 • N–6.7 • W, 40 m a.s.l.). The site was fre-quently impacted by marine air masses (arriving at the site from the southerly sector) and air masses from the cities of Huelva (located NW of the site), Seville and Madrid (located NNE of the site). OH reactivity values showed strong wind sector dependence. North eastern "continental" air masses were characterized by the highest OH reactivities (average: 31.4 ± 4.5 s −1 ; range of average diel values: 21.3–40.5 s −1), followed by north western "industrial" air masses (average: 13.8 ± 4.4 s −1 ; range of average diel values: 7–23.4 s −1) and marine air masses (average: 6.3 ± 6.6 s −1 ; range of average diel values: below detection limit −21.7 s −1), respectively. The average OH reactivity for the entire campaign period was ∼18 s −1 and no pronounced variation was discernible in the diel profiles with the exception of relatively high val-ues from 09:00 to 11:00 UTC on occasions when air masses arrived from the north western and southern wind sectors. The measured OH reactivity was used to constrain both diel instantaneous ozone production potential rates and regimes. Gross ozone production rates at the site were generally lim-ited by the availability of NO x with peak values of around 20 ppbV O 3 h −1 . Using the OH reactivity based approach, derived ozone production rates indicate that if NO x would no longer be the limiting factor in air masses arriving from the continental north eastern sector, peak ozone production rates could double. We suggest that the new combined ap-proach of in-situ fast measurements of OH reactivity, ni-trogen oxides and peroxy radicals for constraining instan-taneous ozone production rates, could significantly improve analyses of upwind point sources and their impact on re-gional ozone levels.