Abstract
Abstract. Nine Pandora spectrometer instruments (PSI) were installed at eight sites in South Korea as part of the KORUS-AQ (Korea U.S.-Air Quality) field study integrating information from ground, aircraft, and satellite measurements for validation of remote sensing air-quality studies. The PSI made direct-sun measurements of total vertical column NO2, C(NO2), with high precision (0.05 DU, where 1 DU =2.69×1016 molecules cm−2) and accuracy (0.1 DU) that were retrieved using spectral fitting techniques. Retrieval of formaldehyde C(HCHO) total column amounts were also obtained at five sites using the recently improved PSI optics. The C(HCHO) retrievals have high precision, but possibly lower accuracy than for NO2 because of uncertainty about the optimum spectral window for all ground-based and satellite instruments. PSI direct-sun retrieved values for C(NO2) and C(HCHO) are always significantly larger than OMI (AURA satellite Ozone Monitoring Instrument) retrieved C(NO2) and C(HCHO) for the OMI overpass local times (KST=13.5±0.5 h). In urban areas, PSI C(NO2) 30-day running averages are at least a factor of two larger than OMI averages. Similar differences are seen for C(HCHO) in Seoul and nearby surrounding areas. Late afternoon values of C(HCHO) measured by PSI are even larger, implying that OMI early afternoon measurements underestimate the effect of poor air quality on human health. The primary cause of OMI underestimates is the large OMI field of view (FOV) that includes regions containing low values of pollutants. In relatively clean areas, PSI and OMI are more closely in agreement. C(HCHO) amounts were obtained for five sites, Yonsei University in Seoul, Olympic Park, Taehwa Mountain, Amnyeondo, and Yeoju. Of these, the largest amounts of C(HCHO) were observed at Olympic Park and Taehwa Mountain, surrounded by significant amounts of vegetation. Comparisons of PSI C(HCHO) results were made with the Compact Atmospheric Multispecies Spectrometer CAMS during overflights on the DC-8 aircraft for Taehwa Mountain and Olympic Park. In all cases, PSI measured substantially more C(HCHO) than obtained from integrating the CAMS altitude profiles. PSI C(HCHO) at Yonsei University in Seoul frequently reached 0.6 DU and occasionally exceeded 1.5 DU. The semi-rural site, Taehwa Mountain, frequently reached 0.9 DU and occasionally exceeded 1.5 DU. Even at the cleanest site, Amnyeondo, C(HCHO) occasionally exceeded 1 DU.
Highlights
The purpose of this paper is to present the retrieved total column amounts of nitrogen dioxide and formaldehyde, C(NO2) and C(HCHO), obtained from Pandora spectrometer instruments (PSI) direct-sun observations during the KORUS-AQ campaign (Korea US Air Quality: May–June 2016)
Contributions to the differences include the selection of the PSI wavelength window (332–359 nm) and possible interference from overlapping NO2 and O3 absorption that are not properly included, and, more likely, the use of Compact Atmospheric Multispecies Spectrometer (CAMS) measured volume mixing ratios at the lowest altitudes from the nearby Seoul airbase, 8.5 km from Olympic Park, where spatial variation may affect the calculation of C(HCHO)
The measurements made during the months of April to June by PSI showed very high amounts of urban pollution from NO2 and HCHO, and more moderate, but still high values in Mt Taewha and Yeogju, which are some distance from the major urban centers
Summary
The purpose of this paper is to present the retrieved total column amounts of nitrogen dioxide and formaldehyde, C(NO2) and C(HCHO), obtained from Pandora spectrometer instruments (PSI) direct-sun observations during the KORUS-AQ campaign (Korea US Air Quality: May–June 2016). Though Yeoju is a relatively clean site in Korea (located to the southeast of Seoul, 37.5644◦ N, 126.934◦ W), C(NO2) amounts frequently reach moderately high values (e.g., 1 DU on 3 June 2016), and occasionally even higher (2–3 DU). As the observations at both sites had an extended period of missing data, the slopes were estimated separately for each segment and for the combined time series As the NO2 concentrations represented by these large column amounts are probably in the boundary layer near the sources of NO2, there is a strong effect on local air quality
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