Spatially Resolved MAX-DOAS Measurements of Formaldehyde at Torrejon De Ardoz (41 N).

The presence of formaldehyde (HCHO) in the atmosphere is an indicative of the existence of oxidation processes of volatile organic compounds (VOCs), and plays a big role in a number of tropospheric chemical processes, most importantly in the formation of tropospheric ozone. It can also be used as a proxy for local air quality and air pollution studies. In the framework of the FRM4DOAS project, a series of MAX-DOAS measurements has been carried out in an unpolluted environment situated at the north of the city of Torrejón de Ardoz (Madrid, Spain, 41° N), to determine the distribution of NO2 and HCHO in both urban and green areas, with special emphasis on the detection of the spatial heterogeneity of both species. The instrumentation’s operation site was situated in TOTEM, an observation station inside the premises of INTA, and on the top of a tower which height ensures that there are no immediate obstacles in the observations’ lines of sight.This work is focused on HCHO measurements. The series of measurements spans from 2019 to 2022, with some gaps in between due to instrumental problems, and 4 different azimuth angles that enable the study of airmasses around nearby urban areas, highways and airport landing strips, and also around nearby fields and mountains. Some measurements have also been filtered out with a HCHO detection limit criterion to ensure that the studied spectra properly detect this trace gas.The instrument involved in these measurements was a MAX-DOAS spectrometer observing in the UV region (320-415 nm), with a resolution of FHWM 0.55 nm, and equipped with a calibrated inclinometer to precisely adjust to the selected elevation angles. The instrument was configured to measure in the following elevation angles: 0°, 1°, 2°, 3°, 5°, 10°, 30°, 60° and zenith, and in 4 different azimuth angles (50°, 100°, 180° and 220° N, clockwise).The spectral analysis for the HCHO retrieval was made by a software developed at INTA. O4 retrieval was also performed, at a different spectral range than that used for HCHO but still in the UV region, to obtain information of the optical paths that the light followed before being measured by our instrument.A clear seasonality can be observed throughout the years, with a maximum peak around July- August, and a minimum in the months of February-March. This seasonal behaviour can be observed in all azimuthal directions, but it’s more prevalent in the 100° N azimuth direction, pointing to the city of Alcalá a few kilometers away.