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

Abstract

Abstract. Measurements of OH, HO2, RO2i (alkene and aromatic-related RO2) and total RO2 radicals taken during the ClearfLo campaign in central London in the summer of 2012 are presented. A photostationary steady-state calculation of OH which considered measured OH reactivity as the OH sink term and the measured OH sources (of which HO2+ NO reaction and HONO photolysis dominated) compared well with the observed levels of OH. Comparison with calculations from a detailed box model utilising the Master Chemical Mechanism v3.2, however, highlighted a substantial discrepancy between radical observations under lower NOx conditions ([NO] < 1 ppbv), typically experienced during the afternoon hours, and indicated that the model was missing a significant peroxy radical sink; the model overpredicted HO2 by up to a factor of 10 at these times. Known radical termination steps, such as HO2 uptake on aerosols, were not sufficient to reconcile the model–measurement discrepancies alone, suggesting other missing termination processes. This missing sink was most evident when the air reaching the site had previously passed over central London to the east and when elevated temperatures were experienced and, hence, contained higher concentrations of VOCs. Uncertainties in the degradation mechanism at low NOx of complex biogenic and diesel related VOC species, which were particularly elevated and dominated OH reactivity under these easterly flows, may account for some of the model–measurement disagreement. Under higher [NO] (> 3 ppbv) the box model increasingly underpredicted total [RO2]. The modelled and observed HO2 were in agreement, however, under elevated NO concentrations ranging from 7 to 15 ppbv. The model uncertainty under low NO conditions leads to more ozone production predicted using modelled peroxy radical concentrations (∼ 3 ppbv h−1) versus ozone production from peroxy radicals measured (∼ 1 ppbv h−1). Conversely, ozone production derived from the predicted peroxy radicals is up to an order of magnitude lower than from the observed peroxy radicals as [NO] increases beyond 7 ppbv due to the model underprediction of RO2 under these conditions.

Anthropogenic Control Over Wintertime Oxidation of Atmospheric Pollutants

“…For example, HONO contributed a maximum of 8% to the projected daily radical budget on the flight shown in Figure 1, but over land HONO contributed 10-20% to the radical budget at the lowest altitudes surveyed, consistent with its main sources being tied to anthropogenic emissions. Somewhat surprisingly, our observations suggest a smaller role for HONO on a regional basis in the daily integrated radical budget than might be inferred from ground-based observations (Whalley et al, 2018). Near-source measurements in a poorly mixed nocturnal atmosphere may tend to overestimate the regional impact of this source (Febo et al, 1996;Stutz et al, 2002;Wong et al, 2012).…”

“…Other smaller summertime sources of hydrogen oxide radicals (HO x = OH + HO 2 ) can include formaldehyde (HCHO) and nitrous acid (HONO) photolysis (Michoud et al, 2012;Volkamer et al, 2010;Whalley et al, 2018;Young et al, 2014).…”

Daytime atmospheric oxidation capacity in four Chinese megacities during the photochemically polluted season: a case study based on box model simulation

“…The OH reactivity measurements performed in previous field campaigns in the NCP and the PRD showed large seasonal and spatial variations. In Beijing, k OH measurements were conducted at urban and suburban sites during summer Williams et al, 2016;Yang et al, 2017). The OH reactivity is on average in the range of 10 to 30 s −1 and shows a large daily variation due to meteorology changes.…”

Characterization of ozone production in San Antonio, Texas, using measurements of total peroxy radicals

“…CO and carbonyls were the other major contributors to OH reactivity at all locations, with CO being the dominant contributor at Floresville in the morning. Because one of the dominant contributors to HCHO production is isoprene (Wolfe et al, 2016a), it is likely that the biogenic contribution to OH reactivity is even higher than indicated here. Contributions from alkanes were unimportant at the UTSA site, 1 % or less during both morning and afternoon, and contributed only 4 %-5 % at Floresville.…”