Hannele Korhonen

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.

Emission Rates Due to Indoor Activities: Indoor Aerosol Model Development, Evaluation, and Applications

This study presents an indoor aerosol model based on size-resolved and multi-compartment approach. The current indoor aerosol model is also developed with a semi-empirical technique to estimate the emission rates due to indoor sources of aerosol particles. We present in this study a methodology to predict and estimate the best-fit input parameters for the current indoor aerosol model. The performance of the current indoor aerosol model in its single-compartment form was evaluated against previously measured indoor-outdoor aerosol data sets from an office room with mechanical ventilation and a family house with natural ventilation. The indoor aerosol model simulations show that the current methodology used to predict the best-fit input parameters to the indoor aerosol model is efficient. As expected, the penetration factor, aerosol particle deposition, and ventilation rate are the most important parameters in the indoor-outdoor relationship of aerosol particles transport. The emission rate analysis showed that fine aerosol particles production was as high as 26 particle/cm 3 s during wood burning in a fireplace. The emission rate was about eight times this value during grilling in a fireplace and sauna heating. Indoor activities take place in another room may significantly increase the aerosol particle concentrations in other rooms in the building. Therefore, it is recommended to use extra air cleaners in houses to reduce the number concentrations of emitted aerosol particles. The quantitative and qualitative results obtained by the current indoor aerosol model in this study are building and condition specific. Applying the current model to a broad range of conditions and previously measured indoor-outdoor aerosol data sets provides better understanding of aerosol particle characteristics indoors, especially regarding the aerosol particles produced during different indoor activities.

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.

Simulation of atmospheric nucleation mode: A comparison of nucleation models and size distribution representations

[1] Atmospheric particle formation and growth were investigated using different nucleation models and size distribution representations. Nucleation was modeled using recently developed parameterizations for binary nucleation of water and sulphuric acid and ternary nucleation of water, sulphuric acid, and ammonia. A comparison with older nucleation parameterizations, combined with full aerosol dynamics, demonstrated that the difference in nucleation rate (1-2 orders of magnitude) is clearly reflected in the resulting total particle concentration. A comparison of binary and ternary nucleation schemes showed that above 240 K the ternary nucleation rate exceeds the binary by over 10 orders of magnitude, indicating that in most cases, at lower tropospheric conditions, only ternary nucleation can be relevant. In addition, the performance of aerosol dynamics models applying either a multimodal monodisperse or a fixed sectional size distribution representation was evaluated against a molecular resolution model, which follows the changes in the nucleation mode particle size distribution molecule by molecule. Regarding total number concentration, the sectional method converged to the molecular resolution approach when increasing the number of size sections. With strong condensational growth, however, numerical diffusion problems were evident. Overall, the performance of the sectional method with low number of sections was not satisfactory. The monodisperse method gave very good results, at least in terms of total number, when the background modes were set to match the condensation sinks of respective lognormal modes. On the basis of our study the multimodal monodisperse method seems to be a possible candidate when selecting the size distribution approach for large-scale atmospheric models.

Is nucleation capable of explaining observed aerosol integral number increase during southerly transport over Scandinavia

Using a pseudo-Lagrangian approach, changes in aerosol size distribution was investigated during southerly transport under clear sky conditions from Finnish Lapland to Hyytiälä. Seventy-nine individual transport cases were considered. The mean transport distance was 700 km and mean transport time 66 h. On average, a sevenfold increase in Aitken mode number concentration could be observed. An increase in number concentration was observed in virtually all the cases. Several of the studied cases were associated with indications of nucleation at the receptor site. Six of the cases were simulated in detail utilizing a box-model approach. Aerosol dynamics was evaluated using the University of Helsinki Multi-component Aerosol model. Particle formation was assumed to be controlled by a kinetic nucleation mechanism. Growth of particles was suggested to be controlled by, except water and ammonia, sulphuric acid, and some unknown species with saturation vapour pressure of 3 × 106 cm-3. This product was supposed to derive from terpene oxidation by hydroxyl radical, ozone, and nitrate radical. The investigation strongly suggests nucleation events occurring over large scales to be responsible for the observed number increase during transport under modelled conditions. Using a simplified two layer structure of the lowermost troposphere, we highlight the role of vertical exchange. Modelled growth rates were found to be in agreement with observational data, in the order of 1–2 nm h–1. In order to reproduce the observed growth rates, a molar yield of condensable products from terpene oxidation of 10% was required. Concentration of sulphuric acid and condensable organic vapours were on average 3 × 106 and 1.5 × 107 cm-3, respectively.

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.

Heterogeneous nucleation as a potential sulphate‐coating mechanism of atmospheric mineral dust particles and implications of coated dust on new particle formation

[1] The plausibility of heterogeneous conucleation of water, sulphuric acid, and ammonia as a pathway leading to soluble coating of atmospheric mineral dust is investigated. In addition, the effect of such sulphate-coated dust on the formation and growth of atmospheric aerosol particles is addressed. The simulated new particle formation mechanism is ternary nucleation of water, sulphuric acid, and ammonia vapors, while in the condensational growth process the effect of condensable organic vapor is also studied. The results indicate that soluble coating of dust by heterogeneous nucleation can occur at atmospheric sulphuric acid concentrations. In addition, the simulations show that homogeneous ternary nucleation and subsequent growth are decoupled. Although observed (or even higher) dust concentrations are unable to inhibit new particle formation, coated dust particles acting as condensation and coagulation sinks can prevent the growth of newly formed particles to detectable sizes. This is particularly true in desert areas, where organic vapor concentrations are low. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 3210 Mathematical Geophysics: Modeling; KEYWORDS: aerosol, mineral dust, soluble coating, heterogeneous nucleation, particle formation and growth, condensation sink

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