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The mean of the daylight hours-averaged OH number densities is found to be 4.0 \u00d7 106 molecules cm\u20133, a value which is consistent with the OH concentrations measured at different polluted sites in different campaigns. A strong seasonal cycle of OH number density is observed with a peak in summer and a trough in winter. Although there are large changes in emissions from traffic and other sources and in ambient ozone levels in recent years, the nonlinear regression fitted seasonal cycle through the 24 years of daylight hours-averaged OH number densities exhibits a slight but highly statistically significant downward trend of (0.011 \u00b1 0.0026) \u00d7 106 molecules cm\u20133, (0.3 \u00b1 0.07) % per decade (where the quoted confidence intervals represent 1 \u2013 \u03c3 and the 2 \u2013 \u03c3 confidence limits do not encompass zero). The concept of OH collapses is introduced in the study and gives an idea about the impacts of air quality changes on the oxidizing capacity of the London atmosphere. The rise in the number of OH collapses during the early years of the study period shown is driven by the increased direct emissions of NO2 from diesel-engined vehicles. The implication of a collapse should be further researched; analysis could be undertaken on a shorter than hourly time scale to assess the impact on OH collapse on a scale more comparable to its lifetime. It should be noted that no long-term measurements of HCHO or HONO were available, two key components of the OH budget, and these have been estimated as described in the text.<\/jats:p>","DOI":"10.1021\/acs.jpca.6c02946","type":"journal-article","created":{"date-parts":[[2026,9,3]],"date-time":"2026-09-03T10:34:17Z","timestamp":1788431657000},"page":"7450-7459","source":"Crossref","is-referenced-by-count":0,"title":["Observations-Based\nApproach to the Estimation of the\nOxidizing Capacity of the Atmosphere of Central London over the 2000\u20132024\nPeriod"],"prefix":"10.1021","volume":"130","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-7836-3344","authenticated-orcid":true,"given":"M. Anwar H.","family":"Khan","sequence":"first","affiliation":[{"name":"University of Bristol , , Cantock\u2019s Close , ,","place":["Bristol, U.K., BS8 1TS"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Richard G.","family":"Derwent","sequence":"additional","affiliation":[{"name":"rdscientific , , ,","place":["Newbury, Berkshire, U.K., RG14 6LH"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2801-5581","authenticated-orcid":true,"given":"Frank A. 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