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<p>Heatwaves are a substantial health threat in the UK, exacerbated by co-occurrence of ozone pollution episodes. Here we report on the first use of retrieved vertical profiles of nitrogen dioxide (<span class="inline-formula">NO₂</span>) and formaldehyde (<span class="inline-formula">HCHO</span>) over Central London from a newly installed multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument coincident with two of three heatwaves for the hottest summer on record. We evaluate space-based sensor observations routinely used to quantify temporal changes in air pollution and precursor emissions over London. Collocated daily mean tropospheric column densities from the high-spatial-resolution space-based TROPOspheric Monitoring Instrument (TROPOMI) and MAX-DOAS, after accounting for differences in vertical sensitivities, are temporally consistent for <span class="inline-formula">NO₂</span> and <span class="inline-formula">HCHO</span> (both <span class="inline-formula"><i>R</i></span> <span class="inline-formula">=</span> 0.71). TROPOMI <span class="inline-formula">NO₂</span> is 27 %–31 % less than MAX-DOAS <span class="inline-formula">NO₂</span>, as expected from horizontal dilution of <span class="inline-formula">NO₂</span> by TROPOMI pixels in polluted cities. TROPOMI <span class="inline-formula">HCHO</span> is 20 % more than MAX-DOAS <span class="inline-formula">HCHO</span>, greater than differences in past validation studies but within the range of systematic errors in the MAX-DOAS retrieval. The MAX-DOAS near-surface (0–110 <span class="inline-formula">m</span>) retrievals have similar day-to-day and hourly variability to the surface sites for comparison of <span class="inline-formula">NO₂</span> (<span class="inline-formula"><i>R</i></span> <span class="inline-formula">≥</span> 0.7) and for MAX-DOAS <span class="inline-formula">HCHO</span> versus surface site isoprene (<span class="inline-formula"><i>R</i></span> <span class="inline-formula">≥</span> 0.7) that oxidises to <span class="inline-formula">HCHO</span> in prompt and high yields. Daytime ozone production, diagnosed with MAX-DOAS <span class="inline-formula">HCHO</span>-to-<span class="inline-formula">NO₂</span> tropospheric vertical column ratios, is mostly limited by availability of volatile organic compounds (VOCs), except on heatwave days. Temperature-dependent biogenic VOC emissions of isoprene increase exponentially, resulting in ozone concentrations that exceed the regulatory standard for ozone and cause non-compliance at urban background sites in Central London. Locations in Central London heavily influenced by traffic remain in compliance, but this is likely to change with stricter controls on vehicle emissions of <span class="inline-formula">NO_<i>x</i></span> and higher likelihood of heatwave frequency, severity, and persistence due to anthropogenic climate change.</p>