Key chemical NO<sub>x</sub> sink uncertainties and how they influence top-down emissions of nitrogen oxides

Abstract

Abstract. Triggered by recent developments from laboratory and field studies regarding major NOx sink pathways in the troposphere, this study evaluates the influence of chemical uncertainties in NOx sinks for global NOx distributions calculated by the IMAGESv2 chemistry-transport model, and quantifies their significance for top-down NOx emission estimates. Our study focuses on five key chemical parameters believed to be of primary importance, more specifically, the rate of the reaction of NO2 with OH radicals, the newly identified HNO3-forming channel in the reaction of NO with HO2, the reactive uptake of N2O5 and HO2 by aerosols, and the regeneration of OH in the oxidation of isoprene. Sensitivity simulations are performed to estimate the impact of each source of uncertainty. The model calculations show that, although the NO2+OH reaction is the largest NOx sink globally accounting for ca. 60% of the total sink, the reactions contributing the most to the overall uncertainty are the formation of HNO3 in NO+HO2, leading to NOx column changes exceeding a factor of two over tropical regions, and the uptake of HO2 by aqueous aerosols, in particular over East and South Asia. Emission inversion experiments are carried out using model settings which either minimise (MINLOSS) or maximise (MAXLOSS) the total NOx sink, both constrained by one year of OMI NO2 column data from the DOMINO v2 KNMI algorithm. The choice of the model setup is found to have a major impact on the top-down flux estimates, with 75% higher emissions for MAXLOSS compared to the MINLOSS inversion globally. Even larger departures are found for soil NO (factor of 2) and lightning (1.8). The global anthropogenic source is better constrained (factor of 1.57) than the natural sources, except over South Asia where the combined uncertainty primarily associated to the NO+HO2 reaction in summer and HO2 uptake by aerosol in winter lead to top-down emission differences exceeding a factor of 2. Evaluation of the emission optimisation is performed against independent satellite observations from the SCIAMACHY sensor, with airborne NO2 measurements of the INTEX-A and INTEX-B campaigns, as well as with two new bottom-up inventories of anthropogenic emissions in Asia (REASv2) and China (MEIC). Neither the MINLOSS nor the MAXLOSS setup succeeds in providing the best possible match with all independent datasets. Whereas the minimum sink assumption leads to better agreement with aircraft NO2 profile measurements, consistent with the results of a previous analysis (Henderson et al., 2012), the same assumption leads to unrealistic features in the inferred distribution of emissions over China. Clearly, although our study addresses an important issue which was largely overlooked in previous inversion exercises, and demonstrates the strong influence of NOx loss uncertainties on top-down emission fluxes, additional processes need to be considered which could also influence the inferred source.

Updated SO<sub>2</sub> emission estimates over China using OMI/Aura observations

“…in Stavrakou et al, 2013) in estimating satellite-born NO x emissions, which encourages the usage of the mass balance technique when one cannot employ modelling results that calculate an adjoint matrix as well.…”

“…The obtained quantity is converted into a SO 2 VCD using an air mass factor, AMF, which accounts for changes in measurement sensitivity due to observation geometry, ozone column, clouds and surface reflectivity. The anthropogenic SO 2 profile required in the AMF calculation has been extracted from the Intermediate Model of the Global and Annual Evolution of Species, IMAGESv2, global tropospheric chemistry transport model (Stavrakou et al, 2013, and references therein) on a daily basis and for the overpass time of OMI. All details on the BIRA OMI SO 2 algorithm can be found in Theys et al (2015), updated recently in Theys et al (2017) in preparation for TROPOspheric Monitoring Instrument (TROPOMI) instrument.…”

Validation of OMI, GOME-2A and GOME-2B tropospheric NO<sub>2</sub>, SO<sub>2</sub> and HCHO products using MAX-DOAS observations from 2011 to 2014 in Wuxi, China: investigation of the effects of priori profiles and aerosols on the satellite products

“…The SO 2 SCDs are converted to VCDs using AMFs from a look-up table, which is generated using the Linearized Discrete Ordinate Radiative Transfer (LIDORT) version 3.3 RTM (Spurr et al, 2001(Spurr et al, , 2008. SFs for SO 2 are obtained from the IMAGES CTM (Müller and Brasseur, 1995) for individual measurements at a horizontal resolution of 2 • × 2.5 • and at 40 vertical unevenly distributed levels extending from the surface to the lower stratosphere (44 hPa) (Stavrakou et al, 2013. Like for the OMSO2 data set, the cloud information is obtained from the OMCLDO2 cloud product.…”

Global high-resolution simulations of tropospheric nitrogen dioxide using CHASER V4.0

“…Global chemical data assimilation (e.g., Inness et al, 2015;Miyazaki et al, 2015) and emission inversion (e.g., Stavrakou et al, 2013;Miyazaki et al, 2017) would also benefit from high-resolution global CTMs through improvements in model performance (e.g., Arellano Jr. et al, 2007) and reduced spatial representation gaps between observed and simulated fields. Several previous studies (Mijling and van der A, 2012;Ding et al, 2017b;Liu et al, 2017) demonstrated the importance of high-resolution modeling in detecting small-scale NO x emission sources such as urban, new power plant, and ship emissions.…”