Observations-Based Approach to the Estimation of the Oxidizing Capacity of the Atmosphere of Central London over the 2000–2024 Period

Abstract

An observations-based approach has been applied to the air quality monitoring data at the London Marylebone Road over the 2000–2024 study period to estimate the oxidizing capacity of the atmosphere in Central London. The mean of the daylight hours-averaged OH number densities is found to be 4.0 × 10⁶ molecules cm^–3, 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 ± 0.0026) × 10⁶ molecules cm^–3, (0.3 ± 0.07) % per decade (where the quoted confidence intervals represent 1 – σ and the 2 – σ 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.