Abstract
Abstract. 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 performed during the DOMINO campaign (Diel Oxidant Mechanisms 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 frequently 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 values 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 limited by the availability of NOx with peak values of around 20 ppbV O3 h−1. Using the OH reactivity based approach, derived ozone production rates indicate that if NOx 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 approach of in-situ fast measurements of OH reactivity, nitrogen oxides and peroxy radicals for constraining instantaneous ozone production rates, could significantly improve analyses of upwind point sources and their impact on regional ozone levels.
Highlights
The hydroxyl radical (OH.) plays a central role in the chemistry of the troposphere by controlling primary oxidation processes of volatile organic compounds (VOCs), daytime photochemical ozone production and formation of secondary organic aerosol
The OH reactivity ranged from below detection limit (∼3.5 s−1) to 84 s−1, the latter being observed between 12:00 UTC and 14:00 UTC on 22/11/2008 when air mass that had passed over Madrid and Seville region less than 13 hours earlier arrived at the site from the Continental sector
The likelihood of other reactive oxidation products being present at elevated levels, just like HCHO in air masses arriving in plume like fashion, is a likely explanation for the observed co-variations of HCHO with the measured OH reactivity
Summary
The hydroxyl radical (OH.) plays a central role in the chemistry of the troposphere by controlling primary oxidation processes of volatile organic compounds (VOCs), daytime photochemical ozone production and formation of secondary organic aerosol. While several previous field studies (Di Carlo et al, 2004; Hofzumahaus et al, 2010; Ingham et al, 2009; Kovacs et al, 2003; Ren et al, 2003; Sadanaga et al, 2004a; Sinha et al, 2010; Yoshino et al, 2006) have focussed on testing the completeness of the suite of measured OH reactants by comparing the directly measured OH reactivity with the summed up calculated OH reactivity due the measured reactants by applying Eq (1), in this study, we did not undertake this since the suite of VOCs measured during the campaign was not comprehensive enough to justify such an exercise. Employing insitu measurements of OH Reactivity, organic peroxy radicals (RO∗2), hydroxyl and hydroperoxy radicals (OH, HO2) and NOx (NO, NO2), we examine the diurnal instantaneous ozone formation regimes and rates of production potential in different air masses arriving at the site by a variety of approaches with implications for the future air quality of the region
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