Comparative assessment of TROPOMI and OMI formaldehyde observations and validation against MAX-DOAS network column measurements

Abstract

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI), launched in October 2017 on board the Sentinel-5 Precursor (S5P) satellite, monitors the composition of the Earth's atmosphere at an unprecedented horizontal resolution as fine as 3.5 × 5.5 km2. This paper assesses the performances of the TROPOMI formaldehyde (HCHO) operational product compared to its predecessor, the OMI (Ozone Monitoring Instrument) HCHO QA4ECV product, at different spatial and temporal scales. The parallel development of the two algorithms favoured the consistency of the products, which facilitates the production of long-term combined time series. The main difference between the two satellite products is related to the use of different cloud algorithms, leading to a positive bias of OMI compared to TROPOMI of up to 30 % in tropical regions. We show that after switching off the explicit correction for cloud effects, the two datasets come into an excellent agreement. For medium to large HCHO vertical columns (larger than 5 × 1015 molec. cm−2) the median bias between OMI and TROPOMI HCHO columns is not larger than 10 % (< 0.4 × 1015 molec. cm−2). For lower columns, OMI observations present a remaining positive bias of about 20 % (< 0.8 × 1015 molec. cm−2) compared to TROPOMI in midlatitude regions. Here, we also use a global network of 18 MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments to validate both satellite sensors for a large range of HCHO columns. This work complements the study by Vigouroux et al. (2020), where a global FTIR (Fourier transform infrared) network is used to validate the TROPOMI HCHO operational product. Consistent with the FTIR validation study, we find that for elevated HCHO columns, TROPOMI data are systematically low (−25 % for HCHO columns larger than 8 × 1015 molec. cm−2), while no significant bias is found for medium-range column values. We further show that OMI and TROPOMI data present equivalent biases for large HCHO levels. However, TROPOMI significantly improves the precision of the HCHO observations at short temporal scales and for low HCHO columns. We show that compared to OMI, the precision of the TROPOMI HCHO columns is improved by 25 % for individual pixels and by up to a factor of 3 when considering daily averages in 20 km radius circles. The validation precision obtained with daily TROPOMI observations is comparable to the one obtained with monthly OMI observations. To illustrate the improved performances of TROPOMI in capturing weak HCHO signals, we present clear detection of HCHO column enhancements related to shipping emissions in the Indian Ocean. This is achieved by averaging data over a much shorter period (3 months) than required with previous sensors (5 years) and opens new perspectives to study shipping emissions of VOCs (volatile organic compounds) and related atmospheric chemical interactions.

Source and variability of formaldehyde (HCHO) at northern high latitude: an integrated satellite, ground/aircraft, and model study

“…Wolfe et al (2019) finds a small bias in OMI HCHO when comparing to ATom-1 and ATom2 datasets. Using FTIR ground-based measurements, Vigouroux et al (2020) finds a positive bias of 25% in TROPOMI HCHO vertical column density in regions with low HCHO (<2.5×10 15 molecules cm -2 ) and a negative bias of 31% in regions with high HCHO (>8.0×10 15 molecules cm -2 ), consistent with a recent comparison between MAX-DOAS and TROPOMI (De Smedt et al, 2021). Zhu et al(2020) finds a similar bias for OMI HCHO product, with in-situ measurements from aircraft campaigns.…”

“…MAX-DOAS has been extensively used for boundary-layer species such as NO2 (Wagner et al, 2011), HCHO (Heckel et al, 2005;Peters et al, 2012; and BrO measurements (Simpson et al, 2017(Simpson et al, , 2018. Recently, De Smedt et al uses a global network of 18 MAX-DOAS instrument to validate large range HCHO columns measured by OMI and TROPOMI satellite sensors (De Smedt et al, 2021).…”

Spaceborne evidence for significant anthropogenic VOC trends in Asian cities over 2005–2019

“…For monthly mean OMI HCHO and NO 2 column data, the detection limit is estimated at 2.5 × 10 15 molec cm −2 (De Smedt et al 2021) and at 1.0 × 10 15 molec cm −2 (Compernolle et al 2020), respectively. The satellite products were recently validated against MAX-DOAS (Multiple AXis-Differential Optical Absorption Spectroscopy) ground-based observations (Wang et al 2019, Compernolle et al 2020, De Smedt et al 2021.…”

Observations by Ground-Based MAX-DOAS of the Vertical Characters of Winter Pollution and the Influencing Factors of HONO Generation in Shanghai, China

“…USTC-OMI measurements were obtained by spatially averaging the grid data with a radius of 20 km around the instrument location (30.8336 • N 121.5025 • E) considering the grid is >10 km [55][56][57][58]. It is necessary to exclude satellite data for accuracy with larger error (relative error > 100%) and cloud impacts (could fraction > 0.3).…”

“…is >10 km [55][56][57][58]. It is necessary to exclude satellite data for accuracy with larger error (relative error > 100%) and cloud impacts (could fraction > 0.3).…”