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 20 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 over-predicted 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, 25 contained higher concentrations of VOC. 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 -15 ppbv. 30The model uncertainty under low NO conditions leads to more ozone production predicted using modelled peroxy radical concentrations (~3 ppbv hr -1 ) versus ozone production from peroxy radicals measured (~1 ppbv hr -1 ). Conversely, ozone 2 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 under-prediction of RO2 under these conditions.

Explicit diagnosis of the local ozone production rate and the ozone-NOx-VOC sensitivities

“…However, the positive dependence of P(O 3 ) on NO persists towards a higher NO regime because of elevated RO 2 concentrations at high NO x [14], which is consistent with the observation in Wangdu and also other polluted environments [31][32][33][34]. In other studies, the instantaneous ozone production calculated from the box model was smaller than that measured directly [21,22,32] or derived from measured peroxy radicals [31,[33][34][35][36], which showed large discrepancies for the high NO x conditions. The higher than expected ozone production rate for the high NO x conditions implies that ozone production would also be NO x -sensitive for more of the day than what the model predicts.…”

An increasing role for solvent emissions and implications for future measurements of volatile organic compounds

“…Ethyne is one of the most challenging VOCs to measure using thermal desorption techniques due to its very Table 3. Most abundant VOCs ranked by concentrations based on the recent research of wintertime city centre observations in London [26], Beijing [27] and Delhi all based on a common two-channel gas chromatography-flame ionization detection method. a Minor contribution to ambient concentrations possible from the oxidation of other VOCs.…”

Elevated levels of OH observed in haze events during wintertime in central Beijing

“…A general outline, specific set-up and the running conditions during APHH are described here. Further details on the methodology for sequential measurements of OH and HO2 that are made in the first fluorescence cell (HOx) and sequential measurements of HO2 * and RO2 using the ROxLIF method (described in detail below) in the second cell (ROx) can be found in Whalley et al (2018). HO2 * refers to the measurement of HO2 and complex RO2 species; complex RO2 145 12.3 km https://doi.org/10.5194/acp-2020-362 Preprint.…”