Evaluation of Pandora HCHO and NO 2 with airborne in situ observations

Abstract. The Pandora Global Network (PGN) is a system of ground-based spectrometers reporting continuous daytime column HCHO and NO2. While the Direct Sun (DS) NO2 product has been well studied, the Multi Axis Differential Optical Absorption Spectroscopy (MAX DOAS) products are largely unvalidated. Using the Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) airborne campaign in the summer of 2023, we evaluate the performance of select Pandora monitors relative to in situ airborne observations. A case study over a Pandora in the California desert shows MAX DOAS HCHO captures the total tropospheric column (within 4 % of the integrated in situ column) but does not match the vertical shape of the HCHO profile where the Pandora is biased high near the surface and low near the top of the boundary layer. The MAX DOAS NO2 is 80 % lower for the entire profile and is particularly sensitive to the viewing angle of the Pandora due to the spatial heterogeneity of NO2. Nine Pandoras located in the New York City (NYC) domain capture the day to day variability of HCHO as well as spatial gradients from New Jersey to NYC to Long Island. The mean NYC Pandora HCHO correlates well with mean Tropospheric Emissions Monitoring of Pollution (TEMPO) HCHO columns of a similar domain on clear sky days. On those days, MAX DOAS columns exhibit a lower slope (slope = 0.78, y-intercept=1.08 x 1015 molec/cm2; R2 = 0.62) while DS columns show a higher offset (slope = 0.90, y-intercept=2.83 x 1015 molec/cm2; R2 = 0.63). These results demonstrate the value of Pandora HCHO products while highlighting the need for improved uncertainty quantification.