The First Evaluation of Formaldehyde Column Observations by Pandora Spectrometers during the KORUS-AQ Field Study

Abstract

Diurnal variation in HCHO total column densities followed the same pattern at both sites, with the minimum daily values typically observed between 6:00-7:00 local time, gradually increasing to a maximum between 13:00 and 17:00 before decreasing into 15 the evening. Pandora vertical column densities were compared with those derived from the DC-8 HCHO in-situ measured profiles augmented with in-situ surface concentrations below the lowest altitude of the DC-8 in proximity to the ground sites. A comparison between 49 column densities measured by Pandora versus aircraft integrated in-situ data showed that Pandora values were larger by 16% (intercept = 0.22 DU, R 2 = 0.68). 20Pandora HCHO columns were also compared with columns calculated from the surface in-situ measurements over Olympic Park by assuming a well-mixed lower atmosphere up to a ceilometer measured mixed-layer height (MLH) and various assumptions about the small residual HCHO amounts in the free troposphere up to the tropopause. The best comparison (slope = 1.03 ± 0.03, intercept = 0.29 ± 0.02 DU and R 2 of 0.78±0.02) was 25 achieved assuming equal mixing within ceilometer measured MLH combined with an exponential profile shape. These results suggest that diurnal changes in HCHO surface concentrations can be reasonably estimated from the Pandora total column and information on the mixed-layer height. More work is needed to understand the bias in the intercept and the slope relative to columns derived from the in-situ aircraft and surface 30 measurements.Atmos. Meas. Tech. Discuss., https://doi

NO<sub>2</sub> and HCHO measurements in Korea from 2012 to 2016 from Pandora spectrometer instruments compared with OMI retrievals and with aircraft measurements during the KORUS-AQ campaign

“…Formaldehyde is derived from the same set of spectral measurements used for ozone (i.e., with a U340 blocking filter), but using the spectral range 332-359 nm. Sources of error in the C(HCHO) retrieval arise from the selection of the fitting window and the amount of C(HCHO) remaining in the reference spectrum after application of the modified Langley estimation (MLE) method of calibration (Herman et al, 2009;Spinei et al, 2018). The MLE extrapolation to zero C(HCHO) could have an offset error of 0.1 to 0.2 DU.…”

“…Absolute offset errors do not affect the retrieval precision (relative column amounts), which is approximately 0.1 DU. A detailed analysis of the algorithms and uncertainties is discussed by Spinei et al (2018). The Olympic Park area has much more vegetation than central Seoul for the production of isoprene (http://www.…”

“…A previous paper describes HCHO retrievals from a PSI located at Yonsei University in Seoul using a similar spectral fitting retrieval algorithm used in the current study (Park et al, 2018), but using a different wavelength fitting range, 335-358 nm instead of 332-359 nm used in this study. The choice of spectral fitting window is discussed in Spinei et al (2018).…”

“…NO 2 absorption cross sections were obtained from the laboratory measurements of Vandaele et al (1998), and HCHO cross sections from Meller and Moortgat (2000). For ozone, the PSI reference solar spectrum is constructed from a high resolution extraterrestrial spectrum (270 to 1000 nm) merged from different sources (Bernhard et al, 2004;Herman et al, 2015).…”

“…For ozone, the PSI reference solar spectrum is constructed from a high resolution extraterrestrial spectrum (270 to 1000 nm) merged from different sources (Bernhard et al, 2004;Herman et al, 2015). For NO 2 and HCHO, the reference spectrum is obtained from a Modified Langley Extrapolation method (Herman et al, 2009;Spinei et al, 2018). One of the advantages of using direct-sun observations is the accurate conversion to vertical column based on a geometric calculation of the slant path air mass factor AMF for a known solar zenith angle SZA with a slight correction to the function secant (SZA) (Herman et al, 2009, Eq.…”