Mobile Zenith DOAS Measurements of NO₂ and HCHO, and their Relationship to Collocated In-Situ measurements

Abstract. The TRACER-AQ (TRacking Aerosol Convection interactions ExpeRiment- Air Quality) field campaign, performed in Houston, Texas in the summer of 2021, presented opportunities to compare Mobile DOAS (Differential Optical Absorption Spectroscopy) and in-situ measurements of nitrogen dioxide (NO₂) and formaldehyde (HCHO), which can be used to obtain additional information about the vertical distribution of trace gas, without the need of a full sky scan.

During TRACER-AQ1, a Zenith DOAS instrument retrieved Tropospheric Vertical Columns of NO₂, with typical ranging from 1–3 × 10¹⁶ molecules cm⁻² as well as HCHO columns, which typically ranged from 2.5–3.5 × 10¹⁶ molecules cm⁻². The retrieved columns were then compared to in-situ measurements made on the same mobile platform in order to develop a column-to-surface ratio, which describes the vertical heterogeneity of the measured columns. The average NO₂ ratio ranged from 100 m to 3 km, and HCHO from 200 m to 5 km. In addition, the Column to Surface Ratio was used on two case days to determine the origin of a local ozone episodes observed by the mobile lab.