MAX-DOAS NO<sub>2</sub> observations over Guangzhou, China; ground-based and satellite comparisons

Abstract

Abstract. In this study, the tropospheric NO 2 vertical column density (VCD) over an urban site in Guangzhou megacity in China is investigated by means of MAX-DOAS measurements during a campaign from late March 2015 to mid-March 2016. A MAX-DOAS system was deployed at the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and operated there for about 1 year, during the spring and summer months. The tropospheric NO 2 VCDs retrieved by the MAX-DOAS are presented and compared with space-borne observations from GOME-2/MetOp-A, GOME-2/MetOp-B and OMI/Aura satellite sensors. The comparisons reveal good agreement between satellite and MAX-DOAS observations over Guangzhou, with correlation coefficients ranging between 0.795 for GOME-2B and 0.996 for OMI. However, the tropospheric NO 2 loadings are underestimated by the satellite sensors on average by 25.1, 10.3 and 5.7 %, respectively, for OMI, GOME-2A and GOME-2B. Our results indicate that GOME-2B retrievals are closer to those of the MAX-DOAS instrument due to the lower tropospheric NO 2 concentrations during the days with valid GOME-2B observations. In addition, the effect of the main coincidence criteria is investigated, namely the cloud fraction (CF), the distance (d) between the satellite pixel center and the ground-based measurement site, as well as the time period within which the MAX-DOAS data are averaged around the satellite overpass time. The effect of CF and time window criteria is more profound on the selection of OMI overpass data, probably due to its smaller pixel size. The available data pairs are reduced to half and about one-third for CF ≤ 0.3 and CF ≤ 0.2, respectively, while, compared to larger CF thresholds, the correlation coefficient is improved to 0.996 from about 0.86, the slope value is very close to unity (∼ 0.98) and the mean satellite underestimation is reduced to about half (from ∼ 7 to ∼ 3.5 × 10 15 molecules cm −2 ). On the other hand, the distance criterion affects mostly GOME-2B data selection, because GOME-2B pixels are quite evenly distributed among the different radii used in the sensitivity test. More specifically, the number of collocations is notably reduced when stricter radius limits are applied, the r value is improved from 0.795 (d ≤ 50 km) to 0.953 (d ≤ 20 km), and the absolute mean bias decreases about 6 times for d ≤ 30 km compared to the reference case (d ≤ 50 km).

An improved air mass factor calculation for NO<sub>2</sub> measurements from GOME-2

“…Urban stations are generally underestimated by GOME-2 data due to the averaging of a local source over a pixel size (Pinardi et al, 2015;Pinardi, in preparation). other satellites, and other NO 2 products (Celarier et al, 2008;Kramer et al, 2008;Chen et al, 2009;Irie et al, 2012;Ma et al, 2013;Wu et al, 2013;Kanaya et al, 2014;Wang et al, 2017;Drosoglou et al, 2017Drosoglou et al, , 2018. Similar figures for previous GDP products can be found in Liu et al (2019b) and AC-SAF validation website (http://cdop.aeronomie.be/validation/valid-results).…”

Inferring the anthropogenic NO<sub><i>x</i></sub> emission trend over the United States during 2003–2017 from satellite observations: was there a flattening of the emission trend after the Great Recession?

“…OMI-BEHR uses the tropospheric SCDs from OMI-NASA products but updates some inputs for the tropospheric AMF calculation (Laughner et al, 2018b). These product archives have been previously validated Drosoglou et al, , 2018Krotkov et al, 2017;Laughner et al, 2018b;Wang et al, 2017;Zara et al, 2018). Generally, the pixel-size uncertainties of these products are > 30 % over polluted regions under clear-sky conditions.…”

Inferring the anthropogenic NO<sub><i>x</i></sub> emission trend over the United States during 2003–2017 from satellite observations: Was there a flattening of the emission tend after the Great Recession?

“…OMI-BEHR uses the tropospheric SCDs from OMI-NASA products but updates some inputs for the tropospheric AMF calculation (Laughner et al, 2018). These product archives have been previously validated (Boersma et al, 2018;Drosoglou et al, 2017;Drosoglou et al, 2018;Krotkov et al, 2017;Laughner et al, 2018;Wang et al, 2017;Zara et al, 2018). Generally, the pixel-size uncertainties of these products are > 30% over polluted regions under clear-sky conditions.…”