Improving algorithms and uncertainty estimates for satellite NO<sub>2</sub> retrievals: results from the quality assurance for the essential climate variables (QA4ECV) project

Abstract

Abstract. Global observations of tropospheric nitrogen dioxide (NO2) columns have been shown to be feasible from space, but consistent multi-sensor records do not yet exist, nor are they covered by planned activities at the international level. Harmonised, multi-decadal records of NO2 columns and their associated uncertainties can provide crucial information on how the emissions and concentrations of nitrogen oxides evolve over time. Here we describe the development of a new, community best-practice NO2 retrieval algorithm based on a synthesis of existing approaches. Detailed comparisons of these approaches led us to implement an enhanced spectral fitting method for NO2, a 1° × 1° TM5-MP data assimilation scheme to estimate the stratospheric background and improve air mass factor calculations. Guided by the needs expressed by data users, producers, and WMO GCOS guidelines, we incorporated detailed per-pixel uncertainty information in the data product, along with easily traceable information on the relevant quality aspects of the retrieval. We applied the improved QA4ECV NO2 algorithm to the most current level-1 data sets to produce a complete 22-year data record that includes GOME (1995–2003), SCIAMACHY (2002–2012), GOME-2(A) (2007 onwards) and OMI (2004 onwards). The QA4ECV NO2 spectral fitting recommendations and TM5-MP stratospheric column and air mass factor approach are currently also applied to S5P-TROPOMI. The uncertainties in the QA4ECV tropospheric NO2 columns amount to typically 40 % over polluted scenes. The first validation results of the QA4ECV OMI NO2 columns and their uncertainties over Tai'an, China, in June 2006 suggest a small bias (−2 %) and better precision than suggested by uncertainty propagation. We conclude that our improved QA4ECV NO2 long-term data record is providing valuable information to quantitatively constrain emissions, deposition, and trends in nitrogen oxides on a global scale.

Effect of changing NO<sub><i>x</i></sub> lifetime on the seasonality and long-term trends of satellite-observed tropospheric NO<sub>2</sub> columns over China

“…Data availability. The QA4ECV data are available at https://doi.org/10.21944/qa4ecv-no2-omi-v1.1 (last access: 15 January 2019; Boersma et al, 2019), POMINO v2 at https://www.amazon.com/clouddrive/share/ zyC4mNEyRfRk0IX114sR51lWTMpcP1d4SwLVrW55iFG/ folder/S7IR7WSLSPikdLT_jsNX8g (last access: 23 January 2019; ACM group, 2019), MEE surface data at http://beijingair.sinaapp.com (last access: 10 March 2019; China National Environmental Monitoring Center, 2019), and MEIC data at http://www.meicmodel.org (last access: 30 June 2018; MEIC team, 2018). To access the MEIC data, contact Qiang Zhang (qiangzhang@tsinghua.edu.cn).…”

Daily Satellite Observations of Methane from Oil and Gas Production Regions in the United States

“…The NO 2 processing system uses a DOAS (Differential Optical Absorption Spectroscopy) fit on the visible spectra. The tropospheric NO 2 column is determined by subtracting the stratospheric NO 2 column amount and corrections for the effects of surface reflectance and clouds 36 . We use the NO 2 retrievals with a quality value larger than 0.75.…”

Evaluating the impact of spatial resolution on tropospheric NO<sub>2</sub> column comparisons within urban areas using high-resolution airborne data

“…Stratospheric profiles are estimated using the PRATMO chemical box model of stratospheric NO 2 profile climatology (Prather, 1992;McLinden et al, 2000), which estimates NO 2 between approximately 10 and 60 km as a function of month, latitude, and solar zenith angle. This stratospheric model has been used in previous GeoTASO retrievals and has an estimated uncertainty of ∼ 30 % in the stratospheric column (Bourassa et al, 2011;Nowlan et al, 2016). Temperature and pressure profiles from the NAM-CMAQ in the troposphere and the Realtime Air Quality Modeling System (RAQMS; Pierce et al, 2009) up to 60 km are extracted and merged, and ozone profiles are extracted from the NAM-CMAQ analysis for the troposphere and OMI gridded monthly climatology in the stratosphere for May and June 2017.…”