09 Feb 2023
 | 09 Feb 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Measurement Report: MAX-DOAS measurements characterise Central London ozone pollution episodes during 2022 heatwaves

Robert G. Ryan, Eloise Ann Marais, Eleanor Gershenson-Smith, Robbie Ramsay, Jan-Peter Muller, Jan-Lukas Tirpitz, and Udo Frieß

Abstract. Heatwaves are a substantial health threat in the UK, exacerbated by co-occurrence of ozone pollution episodes. Here we report on first use of retrieved vertical profiles of nitrogen dioxide (NO2) and formaldehyde (HCHO) 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 NO2 and HCHO (both R = 0.71). TROPOMI NO2 is 27–31 % less than MAX-DOAS NO2, as expected from horizontal dilution of NO2 by TROPOMI pixels in polluted cities. TROPOMI HCHO is 20 % more than MAX-DOAS HCHO; greater than differences in past validation studies, but within the range of systematic errors in the MAX-DOAS retrieval. The MAX-DOAS lowest layer (~55 m altitude) retrievals have similar day-to-day and hourly variability to the surface sites for comparison of NO2 (R ≥ 0.7) and for MAX-DOAS HCHO versus surface site isoprene (R > 0.6) that oxidizes to HCHO in prompt and high yields. Daytime ozone production, diagnosed with MAX-DOAS HCHO-to-NO2 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 NOx and higher likelihood of heatwave frequency, severity and persistence due to anthropogenic climate change.

Robert G. Ryan et al.

Status: open (until 23 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-24', Anonymous Referee #1, 10 Mar 2023 reply
  • RC2: 'Comment on egusphere-2023-24', Anonymous Referee #2, 20 Mar 2023 reply

Robert G. Ryan et al.

Data sets

MAX-DOAS retrievals of formaldehyde (HCHO) and nitrogen dioxide (NO2) vertical profiles over Central London Eloise Marais, Robert Ryan, Jan-Lukas Tirpitz, Udo Frieß, and Eleanor Smith

Robert G. Ryan et al.


Total article views: 400 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
274 113 13 400 3 8
  • HTML: 274
  • PDF: 113
  • XML: 13
  • Total: 400
  • BibTeX: 3
  • EndNote: 8
Views and downloads (calculated since 09 Feb 2023)
Cumulative views and downloads (calculated since 09 Feb 2023)

Viewed (geographical distribution)

Total article views: 392 (including HTML, PDF, and XML) Thereof 392 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
Latest update: 22 Mar 2023
Short summary
We describe first data retrieval from a newly installed instrument for long-term measurement of vertical profiles of air pollution over Central London during heatwaves in summer 2022. We combine these observations with surface air quality network measurements to support interpretation that exponential increase in biogenic emissions of isoprene during heatwaves provides the limiting ingredient for severe ozone pollution leading to non-compliance with the national ozone air quality standard.