H. Kokkola

Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds.

Observations indicate that the westerly jet in the Southern Hemisphere troposphere is accelerating. Using a global aerosol model we estimate that the increase in wind speed of 0.45 + /- 0.2 m s(-1) decade(-1) at 50-65 degrees S since the early 1980s caused a higher sea spray flux, resulting in an increase of cloud condensation nucleus concentrations of more than 85% in some regions, and of 22% on average between 50 and 65 degrees S. These fractional increases are similar in magnitude to the decreases over many northern hemisphere land areas due to changes in air pollution over the same period. The change in cloud drop concentrations causes an increase in cloud reflectivity and a summertime radiative forcing between at 50 and 65 degrees S comparable in magnitude but acting against that from greenhouse gas forcing over the same time period, and thus represents a substantial negative climate feedback. However, recovery of Antarctic ozone depletion in the next two decades will likely cause a fall in wind speeds, a decrease in cloud drop concentration and a correspondingly weaker cloud feedback.

Direct and indirect effects of sea spray geoengineering and the role of injected particle size

[1] Climate-aerosol model ECHAM5.5-HAM2 was used to investigate how geoengineering with artificial sea salt emissions would affect marine clouds and the Earth’s radiative balance. Prognostic cloud droplet number concentration and interaction of aerosol particles with clouds and radiation were calculated explicitly, thus making this the first time that aerosol direct effects of sea spray geoengineering are considered. When a wind speed dependent baseline geoengineering flux was applied over all oceans (total annual emissions 443.9 Tg), we predicted a radiative flux perturbation (RFP) of 5.1 W m , which is enough to counteract warming from doubled CO2 concentration. When the baseline flux was limited to three persistent stratocumulus regions (3.3% of Earth’s surface, total annual emissions 20.6 Tg), the RFP was 0.8 Wm 2 resulting mainly from a 74–80% increase in cloud droplet number concentration and a 2.5–4.4 percentage point increase in cloud cover. Multiplying the baseline mass flux by 5 or reducing the injected particle size from 250 to 100 nm had comparable effects on the geoengineering efficiency with RFPs 2.2 and 2.1 Wm , respectively. Within regions characterized with persistent stratocumulus decks, practically all of the radiative effect originated from aerosol indirect effects. However, when all oceanic regions were seeded, the direct effect with the baseline flux was globally about 29% of the total radiative effect. Together with previous studies, our results indicate that there are still large uncertainties associated with the sea spray geoengineering efficiency due to variations in e.g., background aerosol concentration, updraft velocity, cloud altitude and onset of precipitation.

Climate impacts of changing aerosol emissions since 1996

Increases in Asian aerosol emissions have been suggested as one possible reason for the hiatus in global temperature increase during the past 15 years. We study the effect of sulphur and black carbon (BC) emission changes between 1996 and 2010 on the global energy balance. We find that the increased Asian emissions have had very little regional or global effects, while the emission reductions in Europe and the U.S. have caused a positive radiative forcing. In our simulations, the global-mean aerosol direct radiative effect changes by 0.06 W/m2 during 1996 to 2010, while the effective radiative forcing (ERF) is 0.42 W/m2. The rather large ERF arises mainly from changes in cloudiness, especially in Europe. In Asia, the BC warming due to sunlight absorption has largely offset the cooling caused by sulphate aerosols. Asian BC concentrations have increased by a nearly constant fraction at all altitudes, and thus, they warm the atmosphere also in cloudy conditions.

Effect of aerosol size distribution changes on AOD, CCN and cloud droplet concentration: Case studies from Erfurt and Melpitz, Germany

[1] For the period of 1990 to 2000, atmospheric particulate mass concentrations have decreased in Central Europe. Simultaneously, the amount of shortwave radiation reaching the ground increased during clear sky conditions. The aerosol indirect effect has not been seen as clearly, as the radiation reaching the ground during overcast conditions has not increased as much as might be expected. Here we show that this may be caused by the condensation kinetics of water during cloud droplet formation. The decrease in the particulate mass led to a clear decrease in the number concentration of cloud condensation nuclei (CCN). However, in urban areas a relatively larger decrease in the number of particles in the upper end of the accumulation mode has led to slower condensation of water. As a result, a higher maximum supersaturation is reached during the cloud droplet formation. This compensates for the effect of decreased CCN concentrations. For example in Erfurt between 1991 and 1996, the aerosol properties changed so that aerosol optical depth decreased by 58% and CCN concentration decreased by 25 to 50%. These led to a 4 to 12% reduction in cloud droplet number concentration (CDNC) and less than a 2 Wm � 2 increase in shortwave radiation during overcast conditions. These results demonstrate that locally the aerosol direct effect can be much larger than the aerosol indirect effect. Furthermore, even though AOD appears to be a valid proxy for CCN, the correlation between AOD and CDNC is not straightforward and thus AOD cannot be used as a proxy for CDNC.

Stratospheric passenger flights are likely an inefficient geoengineering strategy

Solar radiation management with stratospheric sulfur aerosols has been proposed as a potential geoengineering strategy to reduce global warming. However, there has been very little investigation on the efficiency of specific injection methods suggested. Here, we show that using stratospheric passenger flights to inject sulfate aerosols would not cause significant forcing under realistic injection scenarios: even if all present-day intercontinental flights were lifted above the tropopause, we simulate global surface shortwave radiative forcings of 0:05 W m 2 and 0:10 W m 2 with current and five times enhanced fuel sulfur concentrations, respectively. In the highly unlikely scenario that fuel sulfur content is enhanced by a factor of 50 (i.e. ten times the current legal limit) the radiative forcing is 0:85 W m 2 . This is significantly lower than if the same amount of sulfur were injected over the tropics ( 1:32 W m 2 , for 3 Tg (S) yr 1 ) due to a faster loss rate and lower intensity of solar radiation in the northern midlatitudes where current flight paths are concentrated. We also predict lower global forcing in northern hemisphere winter than in summer due to the seasonalities of the solar radiation intensity at midlatitudes, the related OH chemistry that produces sulfate aerosol, and removal of particles.

SALSA2.0: The sectional aerosol module of theaerosol-chemistry-climate model ECHAM6.3.0-HAM2.3-MOZ1.0

In this paper, we present the implementation and evaluation of the aerosol microphysics module SALSA2.0 in the framework of the aerosol-chemistry-climate model ECHAM-HAMMOZ. It is an alternative microphysics module to the default modal microphysics scheme M7 in ECHAM-HAMMOZ. The SALSA2.0 implementation is evaluated against the observations of aerosol optical properties, aerosol mass, and size distributions. We also compare the skill of SALSA2.0 in reproducing the observed quantities to the skill of the M7 implementation. The largest differences between SALSA2.0 and M7 are evident 5 over regions where the aerosol size distribution is heavily modified by the microphysical processing of aerosol particles. Such regions are, for example, highly polluted regions and regions strongly affected by biomass burning. In addition, in a simulation of the 1991 Mt Pinatubo eruption in which a stratospheric sulfate plume was formed, the global burden and the effective radii of the stratospheric aerosol are very different in SALSA2.0 and M7. While SALSA2.0 was able to reproduce the observed time evolution of the global burden of sulfate and the effective radii of stratospheric aerosol, M7 strongly overestimates the removal 10 of coarse stratospheric particles and thus underestimates the effective radius of stratospheric aerosol. As the mode widths of M7 have been optimized for the troposphere and were not designed to represent stratospheric aerosol the ability of M7 to simulate 1 Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-47 Manuscript under review for journal Geosci. Model Dev. Discussion started: 3 April 2018 c

Modelling artificial sea salt emission in large eddy simulations

We study the dispersion of sea salt particles from artificially injected sea spray at a cloud-resolving scale. Understanding of how different aerosol processes affect particle dispersion is crucial when designing emission sources for marine cloud brightening. Compared with previous studies, we include for the first time an explicit treatment of aerosol water, which takes into account condensation, evaporation and their effect on ambient temperature. This enables us to capture the negative buoyancy caused by water evaporation from aerosols. Additionally, we use a higher model resolution to capture aerosol loss through coagulation near the source point. We find that, with a seawater flux of 15 kg s−1, the cooling due to evaporation can be as much as 1.4 K, causing a delay in particle dispersion of 10–20 min. This delay enhances particle scavenging by a factor of 1.14 compared with simulations without aerosol water. We further show that both cooling and particle dispersion depend on the model resolution, with a maximum particle scavenging efficiency of 20% within 5 h after emission at maximum resolution of 50 m. Based on these results, we suggest further regional high-resolution studies which model several injection periods over several weeks.

Correction of Model Reduction Errors in Simulations

In simulations of complex physical phenomena, model reductions are often required to decrease the computation time of the simulation model to a feasible level. Model reduction is often obtained by using a reduced model, which may be based on a reduced numerical approximation and simplifications of the underlying accurate model. The use of a reduced model, however, induces errors to the simulation results. In this paper, we describe and evaluate a novel approach for the correction of the approximation errors in reduced simulation models. The key idea is to model the approximation error between the accurate and reduced simulation model as an additive noise term to the reduced model and construct a low-cost predictor model for the approximation error based on statistical learning. In this paper, the approximation error approach is evaluated with the following problems: correction of spatial and temporal discretization errors in a time-varying heat equation--based evolution model, correction of spatial discre...

From nuclear power to coal power: Aerosol induced health and radiative effects

We have investigated what would be the climate and PM-induced air quality consequences if all nuclear reactors worldwide were closed down and replaced by coal combustion. In a way, this presents a “worst-case scenario” since less polluting energy sources are available. We studied simultaneously the radiative and health effects of coal power emissions using a global 3-D aerosol-climate model (ECHAM-HAMMOZ). This approach allowed us to estimate the effects of a major global energy production change from low carbon source to a high carbon one using detailed spatially resolved population density information. We included the radiative effects of both CO2 and PM2.5 but limited the study of health effects to PM2.5 only. Our results show that the replacement of nuclear power with coal power would have globally caused an average of 150,000 premature deaths per year during the period 2005–2009 with two thirds of them in Europe. For 37 years the aerosol emissions from the additional coal power plants would cool the climate but after that the accumulating CO2 emissions would accelerate the warming of the climate.

Effect of Nucleation and Secondary Organic Aerosol Formation on Cloud Droplet Number Concentrations

The global general circulation model ECHAM5 is used together with HAM aerosol module to investigate the effect of the nucleation scheme on cloud droplet number concentrations. It is shown that nucleation can have a significant role on indirect aerosol effect. Also an efficient SOA formation scheme is intro- duced, and results are compared with original ECHAM5-HAM.