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.
Highlights
Nitrogen oxides (NOx = NO + NO2) in the atmosphere have far-reaching effects on the Earth system
Our SCD uncertainty evaluation is described in detail in QA4ECV Deliverable 5.5 (Boersma et al, 2017a) and in Zara et al (2018) for OMI and GOME-2A, and we summarise the results here
We found that the improved QA4ECV OMI NO2 retrieval shows smaller uncertainties than other OMI algorithms and good agreement between the DOAS and statistical SCD uncertainties
Summary
Nitrogen oxides (NOx = NO + NO2) in the atmosphere have far-reaching effects on the Earth system. Nitrogen oxides contribute to aerosol formation, and they are linked to the oxidising efficiency of the troposphere via ozone, which plays an important role in the formation of the hydroxyl radical (OH). NO2 itself is only a weak greenhouse gas (Solomon et al, 1999) but has considerable relevance for radiative forcing because nitrogen oxides are important precursors of tropospheric ozone, aerosols, and OH. The net effect of nitrogen oxides on climate forcing is modelled to be negative or cooling, with NOx-driven aerosol screening dominating over tropospheric ozone warming (Shindell et al, 2009). Apart from its relevance to climate change, atmospheric nitrogen oxides are important for the health of ecosystems and humans. The World Health Organization stated that it is reasonable to infer that NO2 has direct short-term health effects, such as airway inflammation and reductions in lung function (WHO, 2013), and a literature review of epidemiological studies over a wide geographic area by Hoek et al (2013) showed that human mortality was significantly associated with long-term exposure to NO2
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