As the lifetime of aerosol particles is relatively short, the atmospheric concentrations respond rapidly to changes in primary or aerosol precursor gas emissions. Recently, in some regions, such as Europe and North America, the atmosphere has experi-enced brightening while in some regions, for example India, dimming has increased (Cermak et al., 2010; Haywood et al., 2011). This means that the cooling effect of aerosols has also changed (globally and regionally). As the trend to decrease aerosol and aerosol precursor emissions still continues, one climatological issue raising from this is the aerosol cooling effect: how will it change? Since the different aerosol species have different climate impacts, the emission reductions should be studied also from species, or even sector point of view. In Paper V, the emission scenarios based on current legislation for BC, OC and SO2 were included. Additionally, one of the scenarios was targeted to reductions in short-lived climate forcers (in this case BC and OC) and one showed the maximum reduction potential of BC, OC and SO2. In this thesis, the results from the targeted and maximum potential scenarios are shown (henceforth called BCadd and MTFR, respectively). The MTFR scenario includes a portfolio of most important measures that could produce the largest re-ductions in their global radiative forcing. Measures with small increases in radiative forcing (or net impact) have been excluded (UNEP, 2011; Shindell et al., 2012).
The maximum reduction potential of anthropogenic aerosols (BC and OC) and SO2 with currently available technologies is also demonstrated. This scenario includes primarily end-of-pipe measures and excludes any further efficiency or fuel switching potential (Cofala et al., 2007; Klimont et al., 2009). These two chosen scenarios are represented so that the targeted reductions could be compared to the maximum potential of reductions.
The influence of the aerosol direct radiative effect for short-wave radiation (DRESW, cloud-free) and the cloud radiative effect for short-wave radiation (CRESW) at the top of the atmosphere (TOA) can be seen in Figure 4.7. The top panels show the absolute values from the reference simulation and the mid-dle and lower panels show the difference between the scenarios and the reference simulation.
Clearly, the MTFR simulation reduces the cooling effect of aerosols over the globe2. Over India, the BCadd simulation predicts increased DRESW cooling, which
2Because the absolute values are mainly negative, the difference plots show basically the dif-ference in cooling, thus a positive difdif-ference means that the cooling has decreased in the scenario simulation (and vice versa)
4. Modelling aerosols and climate 41
Figure 4.7: The yearly mean direct radiative effect for short-wave radiation (DRESW, cloud-free) and yearly mean cloud radiative effect for short-wave radia-tion (CRESW) at the top of the atmosphere (TOA) from the reference run and the difference between scenarios and the reference run. The result are based on the sim-ulations inPaper V.
comes partly from the reduced absorption component (BC), but mainly from the increased scattering component (sulphate). The cooling increases over India as the SO2 emission pathway follows one of the current legislation scenarios (and it is
esti-4. Modelling aerosols and climate 42
mated that SO2emission will increase over India, mainly coming from the industrial and energy sectors). If the emissions are reduced as much as it is technically feasible, the cooling over the globe would decrease significantly. Globally, the BCadd simula-tion predicts 0.06 W/m2 reduced cooling for DRESW and 0.38 W/m2 for CRESW, whereas the MTFR simulation predicts 0.4 W/m2 and 0.82 W/m2 cooling, respec-tively. This makes the targeted BC emission reductions clearly more beneficial for climate than the technically maximum reductions, which could probably be very harmful for climate practically in all continental regions.
The influence of BC upon snow albedo was not included in the simulations done inPaper V. Overall, the BC snow albedo effect is estimated to cause global warm-ing that can reach∼0.3 W/m2(Bond et al., 2013). One thing not shown inPaper V is how much the BC snow deposition decreases in different scenarios. For example, in the BC emission reduction scenario, the simulation predicts that the BC snow deposition decreases -60% globally and -37% over the Arctic area3 if compared to the reference year (2005) simulation. For the technically maximum reduction sce-nario, the simulation predicts -56% and -32% decrements, respectively. It should be mentioned that these values are not directly comparable, as the snow cover was changing according to the simulated climate (which explains why in the BC targeted simulation the change is slightly higher). Nevertheless, these values indicate that the BC snow forcing (warming) can be decreased by the targeted reductions as much as with the technical maximum reduction measures. The results above (and in Pa-per V) show that the technical maximum reduction measures can be quite harmful from the climate point of view. All of this implies that, with the BC targeted reduc-tions, the BC snow albedo effect can be reduced very efficiently without climatically harmful side effects. However, as the values here are based on an offline approach, they can only be used as rough estimates. Nevertheless, the potential for BC snow effect reductions can be seen and this is important especially over the Arctic area, where the air is very clean and small changes in BC concentrations/deposition can lead to significant changes in the overall forcing (Quinn et al., 2011).
3Definition used for the Arctic area: the area above the arctic circle; 66.55◦N
Chapter V
Review of the papers and the author’s contribution
Paper I describes the regional aerosol-climate model REMO-HAM and the evalua-tion of the new model version against measurements and the global aerosol-climate model ECHAM-HAMMOZ. REMO-HAM is able to reproduce measured aerosol concentrations and aerosol distributions, and was able to improve 2-m temperatures in Eastern Europe. I was the main developer of REMO-HAM and did all the simu-lations. I also wrote the paper and did all the data analysis and calcusimu-lations.
Paper II studies how well REMO-HAM can reproduce the measured black carbon (BC) concentrations over Finland using several statistical tools. The paper shows that the model underestimates the BC concentrations due to deficiencies in the emis-sions, mainly coming from domestic wood burning. The role of sinks, such as wet deposition, was also discussed because the model is known to precipitate too much over Finland. I did the simulations with REMO-HAM and helped write the model description part. Also, I did the map figures representing the REMO-HAM results and helped with the data analysis.
Paper III investigates aerosol forcing in India with temperature and precipitation responses. It shows that aerosol absorption can play a role in the monsoon precip-itation, but the solar dimming of aerosol can have an extinctive effect. I helped to create the emission files and helped with the simulations and analysis. Also, I wrote the emission description and helped with the model description part.
Paper IV describes the implementation of a measurement-based OH-proxy and shows that the new scheme improves simulated nucleation statistics of REMO-HAM for European boundary layer significantly. Together with Dr. Mikkonen, we designed the new approach of OH calculations, which I implemented into REMO-HAM and did all the simulations. I wrote most parts of the paper and did almost all the data analysis.
Paper Vstudies the impacts of global aerosol emissions reductions on aerosol
forc-43
5. Review of the papers and the author’s contribution 44
ing. Four different emissions scenarios were used to investigate the forcing changes in 2020 and 2030. The paper shows that targeted emission reductions for short-lived climate forcers can be very beneficial to the climate, even if compared against the technically maximum reductions scenario. I pre-processed the emission files for ECHAM-HAMMOZ and did the coding for the new and updated emission modules.
I did all the simulations and data analysis. In addition, I wrote most parts of the paper and prepared all the figures.
Chapter VI
Conclusions
The main focus of this thesis has been in increasing the understanding of aerosol-climate modelling and aerosol-aerosol-climate effects by developing and using both global and regional climate models. The main tool has been the regional aerosol-climate model REMO-HAM, which was introduced in Paper I and used to study black carbon over Finland in Paper II. In Paper IV, REMO-HAM was further modified for studying the European boundary layer nucleation. Additionally, the global aerosol-climate model, ECHAM-HAMMOZ, was used to study the Asian monsoon and future climate forcing of aerosols (Papers III and V). This was achieved by modifying the existing emission data used by the model.
Aerosol nucleation is a climatically important process that impacts the aerosol and CCN concentrations on regional and global scales (Laaksonen et al., 2005;
Spracklen et al., 2006). Paper I shows that both of the models used in this study have a tendency to overestimate sulphur dioxide concentrations, which leads to over-estimations in sulphuric acid concentrations and to too high nucleation rates. The reasons behind the overestimation were not fully resolved, but modifications to the chemistry improved the model performance. As shown in Paper IV, with the modifications, the European boundary layer nucleation can be modelled fairly ac-curately, although many error sources, starting from the underlying understanding of the nucleation process, still exists.
From all of the aerosol species, black carbon (BC) was studied most closely in this work. Paper IIdescribes how well REMO-HAM can reproduce the measured BC concentration over Finland. As is the case with ECHAM-HAMMOZ, REMO-HAM also underestimates the measured concentrations. Besides the error coming from, for example, too-efficient wet deposition, the role of an inaccurate emission database was discussed. It is known that domestic (residential) wood burning is a major source of BC in Finland and any inaccuracies in this sector will have a big impact upon the modelled results.
Using more recent and updated emission databases in the climate models is important. In Papers III and V, the anthropogenic emissions were updated and both Asian monsoon and future aerosol forcing were studied. Paper IIIshows that
45
6. Conclusions 46
aerosol absorption can play a role in monsoon rainfall, but also that decreased surface radiation and evaporation caused by solar dimming can decrease or even cancel out this effect. In Paper V, the near future (years 2020 and 2030) was studied with different emission scenarios. One of the main findings was that emission reductions of the short-lived climate forcers, such as BC, can be climatically beneficial. Even if the technologically maximum emissions reductions are used, the targeted actions provide a much better choice from the climate point of view.
The development of REMO-HAM was successfully carried out in this work. Some model components still need updating in order to make use of all the benefits of an online aerosol module. For example, an interactive radiation code is needed to con-nect the aerosol information to the radiative budget. This work will be probably done in the future and then, for example, the optical properties of aerosols can be studied extensively. The aerosol model itself has been updated since its implemen-tation (Zhang et al., 2012) and some of these updates, such as an organic aerosol module (O’Donnell et al., 2011), should also be implemented in the future. The condensation of organics, as an example, would improve the initial growth of the nucleated particles (Paper IV). The model development done for REMO-HAM in Paper IV (in terms of the OH-proxy) will be also used in ECHAM-HAMMOZ in the work by Bergman et al. (2015). Also, all the new and updated emissions used in Paper V are already included in REMO-HAM, although the evaluation is not yet finished.
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