Atmos. Chem. Phys., 17, 8525–8552, 2017 https://doi.org/10.5194/acp-17-8525-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Uncertainties of fluxes and 13C = 12C ratios of atmospheric reactive-gas emissions Sergey Gromov1,2, Carl A. M. Brenninkmeijer1, and Patrick Jöckel3 1Max Planck Institute for Chemistry, Mainz, Germany 2Institute of Global Climate and Ecology (Roshydromet and RAS), Moscow, Russia 3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Weßling, Germany Correspondence to: Sergey Gromov ([email protected]) Received: 20 December 2016 – Discussion started: 10 January 2017 Revised: 6 May 2017 – Accepted: 1 June 2017 – Published: 13 July 2017 Abstract. We provide a comprehensive review of the proxy tion of the emission inventories, (ii) adequate approaches data on the 13C = 12C ratios and uncertainties of emissions of to special cases (e.g. boundary conditions for the long-lived reactive carbonaceous compounds into the atmosphere, with species) and, no less important, (iii) estimates of the perti- a focus on CO sources. Based on an evaluated set-up of the nent uncertainties. The latter, typically being largest in com- EMAC model, we derive the isotope-resolved data set of its parison to the other sources of error in the model (such as emission inventory for the 1997–2005 period. Additionally, for instance reaction rate coefficients), are often disregarded we revisit the calculus required for the correct derivation when the resulting simulated mixing ratios are reported. Of- of uncertainties associated with isotope ratios of emission ten the inferred variation (temporal or spatial) of the species’ fluxes. The resulting δ13C of overall surface CO emission in abundance is quoted, which, however, does not represent an 2000 of −(25:2 ± 0:7) ‰ is in line with previous bottom-up adequate uncertainty estimate. The situation becomes more estimates and is less uncertain by a factor of 2. In contrast complicated if the isotope-resolved emissions are to be used to this, we find that uncertainties of the respective inverse (i.e. fluxes separated using the information on the isotope ra- modelling estimates may be substantially larger due to the tios of the emitted compounds). For instance, which factors correlated nature of their derivation. We reckon the δ13C val- determine a particular emission source isotope ratio? How ues of surface emissions of higher hydrocarbons to be within do these (and their respective uncertainties) influence the un- −24 to −27 ‰ (uncertainty typically below ±1 ‰), with an certainties of the underlying fluxes? And finally, what is the exception of isoprene and methanol emissions being close contribution of the emission uncertainties to the overall un- to −30 and −60 ‰, respectively. The isotope signature of certainties of the simulated mixing/isotope ratios? ethane surface emission coincides with earlier estimates, but The above-mentioned issues and questions interested us integrates very different source inputs. δ13C values are re- in the course of the implementation of a fully 13C = 12C- ported relative to V-PDB. resolved comprehensive trace gas atmospheric chemistry study with the ECHAM/MESSy Atmospheric Chemistry (EMAC) model (Jöckel et al., 2006, 2010), particularly for the stable carbon isotope extension of its emission set-up, 1 Introduction which we communicate in this paper. The reader is re- ferred to the preceding phases of this model development, Next to the kinetic chemistry implementation, magnitude and viz. the isotope extension of the kinetic chemistry submodel distribution of emissions of airborne compounds constitute MECCA (Module Efficiently Calculating the Chemistry of perhaps the most crucial aspect of a modelling system deal- the Atmosphere) and its application in simulating the carbon ing with the chemical state of Earth’s atmosphere. A con- and oxygen isotope composition of gas-phase constituents sistent emission set-up, in turn, requires (i) a careful selec- Published by Copernicus Publications on behalf of the European Geosciences Union. 8526 S. Gromov et al.: 13C = 12C ratios of reactive-gas emissions within the CAABA (Chemistry As A Boxmodel Applica- parameters. The EVAL2 set-up includes the emissions from tion) atmospheric box model (Sander et al., 2011; Gro- data sets comprising the following categories: mov et al., 2010). Both EMAC (which embodies an atmo- spheric chemistry general circulation model, AC-GCM) and – anthropogenic emissions, based on the EDGAR emis- CAABA serve as base models within the Modular Earth Sub- sion inventory (detailed in Sect. 3.1), model System (MESSy, Jöckel et al., 2005) that we employ. The overarching aim of our studies is a consistent simulation – biomass burning emissions (GFED project database, of the isotopic composition of atmospheric carbon monox- second version; see Sect. 3.2), and ide (CO). A handful of modelling studies dedicated to CO isotopes exist to date (see the review by Brenninkmeijer et – biogenic emissions based on the OLSEN/GEIA al., 1999) and have proven to yield deeper insights into their databases (see Sect. 3.3). budget. However it leaves questions on missing atmospheric 13CO in models (see Sect. 4). We therefore attempt to revisit Various key assumptions determine the emission isotopic this issue in a detailed and more comprehensive framework signatures. Depending on the specific emission category, of the EMAC model, which we will communicate in subse- each of the data sets requires separate preprocessing for the quent papers. In addition to CO, the current study provides a isotopic extension. These are described in Sects. 3.1–3.5. bottom-up assessment of the emission 13C = 12C isotope ra- The online emissions, in contrast, are calculated during the tios for the suite of other carbonaceous compounds, informa- runtime and require some of the model variables (e.g. surface tion that we believe will be useful for other isotope-enabled temperature or precipitation) to calculate the resulting emis- (modelling) studies focussing on them. For further informa- sion flux at the given model time step. For example, online tion we refer to Brenninkmeijer et al. (2003), Goldstein and emission suits for parameterisation of the trace gas emissions Shaw (2003) and Gensch et al. (2014), who review the ben- related to the biosphere–atmosphere interaction processes. In efits of using stable isotope ratios in atmospheric trace gases particular, the EVAL2 set-up includes the online emissions considered in this work. of VOCs (isoprene/monoterpenes) from plants (see below, The paper consists of three main parts. In the first part Sect. 3.3.1), which were scaled to achieve net yearly emis- (Sect. 2), we briefly reiterate the implementation of the trace sions of 305–340Tg.C/ of isoprene (see Pozzer et al., 2007, gas emissions in the evaluation set-up of the EMAC model Supplement). With this adjustment, more realistic mixing ra- (MESSy Development Cycle2, Jöckel et al., 2010, referred tios of isoprene in the boundary layer are achieved in EMAC to hereafter as EVAL2) and supplement it with the formula- simulations. tion used to separate isotope emission fluxes. Furthermore, At last, the pseudo-emission approach (tracer nudging) is we derive some practical approaches with which to calcu- a technique that performs the relaxation of the mixing ra- late combined flux/isotope ratio uncertainties of emissions tios of sufficiently long-lived tracers towards prescribed (in in Sect. 2.2. The second part (Sect. 3) revisits proxies for space/time) fields. In the EVAL2 set-up, these are the zonal 13 12 signatures ( C = C isotope ratios) of particular emission averages of the observed mixing ratios of CH4, chlorinated sources for CO, non-methane hydrocarbons (NMHCs), bio- hydrocarbons (CH3CCl3, CCl4, CH3Cl) and CO2 which are genic volatile organic (VOCs) and other carbonaceous com- used as the lower boundary conditions (surface layer) in pounds represented by EMAC. Special focus is on CO (the the model. The isotopic separation of these pseudo-emission tracer of our primary interest) and its precursors. Finally, in fields is described below in Sect. 3.5. the last part (Sect. 4) we summarise the results and discuss Further details of the emission processes implementation our estimates in comparison with previous studies. We reca- in EMAC and the corresponding model parameterisations are pitulate our results in Sect. 5 with concluding remarks. given by Kerkweg et al. (2006), Jöckel et al. (2006, 2010), Pozzer et al. (2007, 2009). In the next sections we describe chiefly the choice of the isotope emission signatures for the 2 Emission processes in EMAC model set-ups including stable carbon isotopes. The emission of trace gases in EMAC is treated by the 2.1 Individual fluxes of isotopologues submodels OFFEMIS (formerly OFFLEM), ONEMIS (for- merly ONLEM) and TNUDGE, which embody offline and The isotopic extension procedure consists of the separation online emission processes, and a pseudo-emission approach of the total (i.e. sum of the abundant and rare isotope bear- (tracer nudging), as detailed by Kerkweg et al. (2006). The ing) species fluxes into the individual isotopologues fluxes, offline emission process embodies a prescribed (precalcu- accounting for the given isotopic ratio and thus the isotope lated) tracer flux into the atmospheric reservoir at the surface content of a given species. Additionally, the consistency be- layer(s) or, for instance for the emission from air transporta- tween the total flux and the sum of isotopically separated tion sector, at the respective altitudes. This type of emission fluxes is verified. The rare isotopologues fluxes are calcu- does not require a parameterisation dependent on the model lated by weighting the total species flux with the respective Atmos. Chem. Phys., 17, 8525–8552, 2017 www.atmos-chem-phys.net/17/8525/2017/ S. Gromov et al.: 13C = 12C ratios of reactive-gas emissions 8527 fractions rare; if according to elling results, particularly in view of comparison with ob- servational data.