MAX-DOAS measurements characterise severe ozone pollution in Central London during summer 2022 heatwaves

In the UK, the public health threat of heatwaves is exacerbated by the co-occurrence of ozone pollution episodes. Here we present retrieved vertical profiles of nitrogen dioxide (NO₂) and formaldehyde (HCHO) over Central London from a newly installed long-term Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument on a 60-m altitude rooftop site during two of three heatwaves in the hottest summer on record. We evaluate routine space-based sensor observations of air pollutant precursors over London and enhance the permanent air quality monitor network. Daily mean tropospheric column densities of NO₂ and HCHO from the TROPOspheric Monitoring Instrument (TROPOMI) are consistent with those from the MAX-DOAS (both R = 0.71) after accounting for different vertical sensitivities. As expected, TROPOMI NO₂ is 27-31% less than MAX-DOAS NO₂, due to horizontal dilution of local sources of road-traffic NO₂. TROPOMI HCHO is 20% more than MAX-DOAS HCHO; a larger difference than in past validation studies, but within the range of systematic retrieval errors. MAX-DOAS lowest layer (0-110 m) retrievals have analogous day-to-day variability to surface site NO₂ (R ≥ 0.7). Surface site measurements of isoprene, which rapidly oxidises to HCHO with a high yield, also have similar diurnal variations to MAX-DOAS HCHO (R > 0.6). Generally, daytime ozone production, which is diagnosed with MAX-DOAS HCHO:NO₂ tropospheric column ratios, is limited by the availability of volatile organic compounds. During heatwaves, ozone production shifts from NO_x-saturated to NO_x-limited as biogenic emissions of isoprene increase exponentially with temperature, leading to non-compliance with ozone regulatory standards. In ongoing work, we are assessing the ability to retrieve the NO_x (NO + NO₂) reservoir compound nitrous acid (HONO) to address uncertainties in HONO chemistry. This will aid understanding of the oxidation capacity of the atmosphere and ozone budget in centres of megacities such as London.