Long-term MAX-DOAS measurements of formaldehyde in the suburban area of London

Multi-AXis (MAX)-Differential Optical Absorption Spectroscopy (DOAS) instruments record spectra of scattered sun light under different elevation angles. From such measurements tropospheric vertical column densities (VCDs) and vertical profiles of different atmospheric trace gases and aerosols can be determined for the lower troposphere. These measurements allow a simultaneous observation of multiple trace gases, e.g. formaldehyde (HCHO), glyoxal (CHOCHO) and nitrogen dioxide (NO₂), with the same measurement setup. Since November 2018, a MAX-DOAS instrument has been operating at Bayfordbury Observatory, which is located approximately 30 km north of London. This measurement site is operated by the University of Hertfordshire and equipped with an AERONET station, a LIDAR and multiple instruments to measure meteorological quantities and solar radiation. Depending on the prevailing wind direction the air masses at the measurement site can be dominated by the pollution of London (SE to SW winds) or rather pristine air (northerly winds).

First results already showed that the highest formaldehyde and glyoxal columns are observed for southerly to southeasterly winds indicating the influence of the anthropogenic emissions of London. However, the detailed patterns of the different trace gases were found to be more complex. Therefore, this measurement site is well suited to study the influence of anthropogenic pollution on the atmospheric composition and chemistry at a rather pristine location in the vicinity of London, a major European capital with about 10 million inhabitants and 4 major international airports.

In this study, trace gas and aerosol profiles are retrieved using the MAinz Profile Algorithm (MAPA) with a focus on tropospheric HCHO which plays an important role in tropospheric chemistry. The HCHO results are combined with the results of other trace species such as NO₂, CHOCHO and aerosols in order to identify pollution levels, emission sources and different chemical regimes.