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 within ECHAM-HAMMOZ is evaluated
against observations of aerosol optical properties, aerosol mass, and size
distributions, comparing also to the skill of the M7 implementation. The
largest differences between the implementation of SALSA2.0 and M7 are in the
methods used for calculating microphysical processes, i.e., nucleation,
condensation, coagulation, and hydration. These differences in the
microphysics are reflected in the results so that the largest differences
between SALSA2.0 and M7 are evident 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 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 the volcano plume was improved by modifying the
mode widths, decreasing the standard deviations of the accumulation and coarse
modes from 1.59 and 2.0, respectively, to 1.2 similar to what was observed
after the Mt. Pinatubo eruption. Overall, SALSA2.0 shows promise in improving
the aerosol description of ECHAM-HAMMOZ and can be further improved by
implementing methods for aerosol processes that are more suitable for the
sectional method, e.g., size-dependent emissions for aerosol species and size-resolved wet deposition.
Long-term statistics of atmospheric aerosol and especially cloud scavenging
were studied at the Puijo measurement station in Kuopio, Finland, during
October 2010–November 2014. Aerosol size ...distributions, scattering
coefficients at three different wavelengths (450, 550, and 700 nm),
and absorption coefficient at wavelength 637 nm were measured with a special
inlet system to sample interstitial and total aerosol in clouds. On average,
accumulation mode particle concentration was found to be correlated with
temperature with the lowest average concentrations of 200 cm−3 around
0 ∘C increasing to 800 cm−3 at 20 ∘C. The
scavenging efficiencies of both scattering and absorbing material were
observed to have a slightly positive temperature correlation in in-cloud
measurements. At 0 ∘C, the scavenging efficiencies of
scattering and absorbing material were 0.85 and 0.55 with slopes of 0.005
and 0.003 ∘C−1, respectively.
Scavenging efficiencies were also studied as a function of the diameter at
which half of the particles are activated into cloud droplets. This analysis
indicated that there is a higher fraction of absorbing material, typically
black carbon, in smaller sizes so that at least 20 %–30 % of interstitial
particles within clouds consist of absorbing material. In addition, the
PM1 inlet revealed that approximately 20 % of absorbing material was
observed to reside in particles with ambient diameter larger than
∼ 1 µm at relative humidity below 90 %. Similarly,
40 % of scattering material was seen to be in particles larger than 1 µm. Altogether, this dataset provides information on the size-dependent aerosol composition and in-cloud scavenging of different types of
aerosol. The dataset can be useful in evaluating how well the size-dependent aerosol composition is simulated in global aerosol models and how
well these models capture the in-cloud scavenging of different types of
aerosol in stratus clouds.
We use the ECHAM-HAMMOZ aerosol-climate model to assess the effects of black carbon (BC) mitigation measures on Arctic climate. To this end we constructed several mitigation scenarios that implement ...all currently existing legislation and then implement further reductions of BC in a successively increasing global area, starting from the eight member states of the Arctic Council, expanding to its active observer states, then to all observer states, and finally to the entire globe. These scenarios also account for the reduction of the co-emitted organic carbon (OC) and sulfate (SU). We find that, even though the additional BC emission reductions in the member states of the Arctic Council are small, the resulting reductions in Arctic BC mass burdens can be substantial, especially in the lower troposphere close to the surface. This in turn means that reducing BC emissions only in the Arctic Council member states can reduce BC deposition in the Arctic by about 30 % compared to the current legislation, which is about 60 % of what could be achieved if emissions were reduced globally. Emission reductions further south affect Arctic BC concentrations at higher altitudes and thus only have small additional effects on BC deposition in the Arctic. The direct radiative forcing scales fairly well with the total amount of BC emission reduction, independent of the location of the emission source, with a maximum direct radiative forcing in the Arctic of about −0.4 W m−2 for a global BC emission reduction. On the other hand, the Arctic effective radiative forcing due to the BC emission reductions, which accounts for aerosol–cloud interactions, is small compared to the direct aerosol radiative forcing. This happens because BC- and OC-containing particles can act as cloud condensation nuclei, which affects cloud reflectivity and lifetime and counteracts the direct radiative forcing of BC. Additionally, the effective radiative forcing is accompanied by very large uncertainties that originate from the strong natural variability of meteorology, cloud cover, and surface albedo in the Arctic. We further used the TM5-FASST model to assess the benefits of the aerosol emission reductions for human health. We found that a full implementation in all Arctic Council member and observer states could reduce the annual global number of premature deaths by 329 000 by the year 2030, which amounts to 9 % of the total global premature deaths due to particulate matter.
Challenges in understanding the aerosol–cloud interactions and their impacts on global climate highlight the need for improved knowledge of the underlying physical processes and feedbacks as well as ...their interactions with cloud and boundary layer dynamics. To pursue this goal, increasingly sophisticated cloud-scale models are needed to complement the limited supply of observations of the interactions between aerosols and clouds. For this purpose, a new large-eddy simulation (LES) model, coupled with an interactive sectional description for aerosols and clouds, is introduced. The new model builds and extends upon the well-characterized UCLA Large-Eddy Simulation Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications (SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the aerosol, cloud and precipitation bin discretisation are presented. These enable tracking the effects of cloud processing and wet scavenging on the aerosol size distribution as accurately as possible, while keeping the computational cost of the model as low as possible. The model is tested with two different simulation set-ups: a marine stratocumulus case in the DYCOMS-II campaign and another case focusing on the formation and evolution of a nocturnal radiation fog. It is shown that, in both cases, the size-resolved interactions between aerosols and clouds have a critical influence on the dynamics of the boundary layer. The results demonstrate the importance of accurately representing the wet scavenging of aerosol in the model. Specifically, in a case with marine stratocumulus, precipitation and the subsequent removal of cloud activating particles lead to thinning of the cloud deck and the formation of a decoupled boundary layer structure. In radiation fog, the growth and sedimentation of droplets strongly affect their radiative properties, which in turn drive new droplet formation. The size-resolved diagnostics provided by the model enable investigations of these issues with high detail. It is also shown that the results remain consistent with UCLALES (without SALSA) in cases where the dominating physical processes remain well represented by both models.
Atmospheric new particle formation occurs frequently in the global atmosphere and may play a crucial role in climate by affecting cloud properties. The relevance of newly formed nanoparticles depends ...largely on the dynamics governing their initial formation and growth to sizes where they become important for cloud microphysics. One key to the proper understanding of nanoparticle effects on climate is therefore hidden in the growth mechanisms. In this study we have developed and successfully tested two independent methods based on the aerosol general dynamics equation, allowing detailed retrieval of time- and size-dependent nanoparticle growth rates. Both methods were used to analyze particle formation from two different biogenic precursor vapors in controlled chamber experiments. Our results suggest that growth rates below 10 nm show much more variation than is currently thought and pin down the decisive size range of growth at around 5 nm where in-depth studies of physical and chemical particle properties are needed.
Within the framework of the global chemistry climate model ECHAM–HAMMOZ, a novel explicit coupling between the sectional aerosol model HAM-SALSA and the chemistry model MOZ was established to form ...isoprene-derived secondary organic aerosol (iSOA). Isoprene oxidation in the chemistry model MOZ is described by a semi-explicit scheme consisting of 147 reactions embedded in a detailed atmospheric chemical mechanism with a total of 779 reactions. Semi-volatile and low-volatile compounds produced during isoprene photooxidation are identified and explicitly partitioned by HAM-SALSA. A group contribution method was used to estimate their evaporation enthalpies and corresponding saturation vapor pressures, which are used by HAM-SALSA to calculate the saturation concentration of each iSOA precursor. With this method, every single precursor is tracked in terms of condensation and evaporation in each aerosol size bin. This approach led to the identification of dihydroxy dihydroperoxide (ISOP(OOH)2) as a main contributor to iSOA formation. Further, the reactive uptake of isoprene epoxydiols (IEPOXs) and isoprene-derived glyoxal were included as iSOA sources. The parameterization of IEPOX reactive uptake includes a dependency on aerosol pH value. This model framework connecting semi-explicit isoprene oxidation with explicit treatment of aerosol tracers leads to a global annual average isoprene SOA yield of 15 % relative to the primary oxidation of isoprene by OH, NO3 and ozone. With 445.1 Tg (392.1 Tg C) isoprene emitted, an iSOA source of 138.5 Tg (56.7 Tg C) is simulated. The major part of iSOA in ECHAM–HAMMOZ is produced by IEPOX at 42.4 Tg (21.0 Tg C) and ISOP(OOH)2 at 78.0 Tg (27.9 Tg C). The main sink process is particle wet deposition, which removes 133.6 (54.7 Tg C). The average iSOA burden reaches 1.4 Tg (0.6 Tg C) in the year 2012.
The effect of nitric acid on the equilibrium size distributions of upper tropospheric aerosols is calculated as a function of relative humidity. It is shown that HNO3 concentrations above a few ...tenths of a ppb can cause substantial increases in haze mode particle concentrations at relative humidities at about 60% and above. The effect can be strongly magnified when letovicite particles are present in addition to sulfuric acid aerosols. Letovicite particles are less acidic than the sulfuric acid particles and so more nitric acid can be absorbed. This effect can be seen even at RH below 50% due to the lowering of the deliquescence RH of letovicite in the presence of gaseous nitric acid at low temperatures. We have also compared equilibrium calculations of the HNO3 effect with observations of increased haze mode concentrations at relative humidities above 50% (Petzold et al., 2000). Nitric acid mixing ratios on the order of 0.5-2ppb may explain the observed increase of haze mode particles at least partially.
The chemistry–climate model ECHAM-HAMMOZ contains a detailed representation
of tropospheric and stratospheric reactive chemistry and state-of-the-art
parameterizations of aerosols using either a ...modal scheme (M7) or a bin
scheme (SALSA). This article describes and evaluates the model version
ECHAM6.3-HAM2.3-MOZ1.0 with a focus on the tropospheric gas-phase chemistry.
A 10-year model simulation was performed to test the stability of the model
and provide data for its evaluation. The comparison to observations
concentrates on the year 2008 and includes total column observations of ozone
and CO from IASI and OMI, Aura MLS observations of temperature, HNO3,
ClO, and O3 for the evaluation of polar stratospheric
processes, an ozonesonde climatology, surface ozone observations from the
TOAR database, and surface CO data from the Global Atmosphere Watch network.
Global budgets of ozone, OH, NOx, aerosols, clouds, and radiation
are analyzed and compared to the literature. ECHAM-HAMMOZ performs well in
many aspects. However, in the base simulation, lightning NOx
emissions are very low, and the impact of the heterogeneous reaction of
HNO3 on dust and sea salt aerosol is too strong. Sensitivity
simulations with increased lightning NOx or modified heterogeneous
chemistry deteriorate the comparison with observations and yield excessively
large ozone budget terms and too much OH. We hypothesize that this is an
impact of potential issues with tropical convection in the ECHAM model.
Global models are widely used to simulate biomass burning aerosol (BBA). Exhaustive evaluations on model representation of aerosol distributions and properties are fundamental to assess health and ...climate impacts of BBA. Here we conducted a comprehensive comparison of Aerosol Comparisons between Observations and Models (AeroCom) project model simulations with satellite observations. A total of 59 runs by 18 models from three AeroCom Phase-III experiments (i.e., biomass burning emissions, CTRL16, and CTRL19) and 14 satellite products of aerosols were used in the study. Aerosol optical depth (AOD) at 550 nm was investigated during the fire season over three key fire regions reflecting different fire dynamics (i.e., deforestation-dominated Amazon, Southern Hemisphere Africa where savannas are the key source of emissions, and boreal forest burning in boreal North America). The 14 satellite products were first evaluated against AErosol RObotic NETwork (AERONET) observations, with large uncertainties found. But these uncertainties had small impacts on the model evaluation that was dominated by modeling bias. Through a comparison with Polarization and Directionality of the Earth’s Reflectances measurements with the Generalized Retrieval of Aerosol and Surface Properties algorithm (POLDER-GRASP), we found that the modeled AOD values were biased by −93 % to 152 %, with most models showing significant underestimations even for the state-of-the-art aerosol modeling techniques (i.e., CTRL19). By scaling up BBA emissions, the negative biases in modeled AOD were significantly mitigated, although it yielded only negligible improvements in the correlation between models and observations, and the spatial and temporal variations in AOD biases did not change much. For models in CTRL16 and CTRL19, the large diversity in modeled AOD was in almost equal measures caused by diversity in emissions, lifetime, and the mass extinction coefficient (MEC). We found that in the AeroCom ensemble, BBA lifetime correlated significantly with particle deposition (as expected) and in turn correlated strongly with precipitation. Additional analysis based on Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) aerosol profiles suggested that the altitude of the aerosol layer in the current models was generally too low, which also contributed to the bias in modeled lifetime. Modeled MECs exhibited significant correlations with the Ångström exponent (AE, an indicator of particle size). Comparisons with the POLDER-GRASP-observed AE suggested that the models tended to overestimate the AE (underestimated particle size), indicating a possible underestimation of MECs in models. The hygroscopic growth in most models generally agreed with observations and might not explain the overall underestimation of modeled AOD. Our results imply that current global models contain biases in important aerosol processes for BBA (e.g., emissions, removal, and optical properties) that remain to be addressed in future research.
Despite a large number of studies, out of all drivers of radiative forcing,
the effect of aerosols has the largest uncertainty in global climate model
radiative forcing estimates. There have been ...studies of aerosol optical
properties in climate models, but the effects of particle number size
distribution need a more thorough inspection. We investigated the trends and
seasonality of particle number concentrations in nucleation, Aitken, and
accumulation modes at 21 measurement sites in Europe and the Arctic. For 13
of those sites, with longer measurement time series, we compared the field
observations with the results from five climate models, namely EC-Earth3,
ECHAM-M7, ECHAM-SALSA, NorESM1.2, and UKESM1. This is the first extensive
comparison of detailed aerosol size distribution trends between in situ
observations from Europe and five earth system models (ESMs). We found that
the trends of particle number concentrations were mostly consistent and
decreasing in both measurements and models. However, for many sites,
climate models showed weaker decreasing trends than the measurements.
Seasonal variability in measured number concentrations, quantified by the
ratio between maximum and minimum monthly number concentration, was
typically stronger at northern measurement sites compared to other
locations. Models had large differences in their seasonal representation,
and they can be roughly divided into two categories: for EC-Earth and
NorESM, the seasonal cycle was relatively similar for all sites, and for
other models the pattern of seasonality varied between northern and southern
sites. In addition, the variability in concentrations across sites varied
between models, some having relatively similar concentrations for all sites,
whereas others showed clear differences in concentrations between remote and
urban sites. To conclude, although all of the model simulations had
identical input data to describe anthropogenic mass emissions, trends in
differently sized particles vary among the models due to assumptions in
emission sizes and differences in how models treat size-dependent aerosol
processes. The inter-model variability was largest in the accumulation mode,
i.e. sizes which have implications for aerosol–cloud interactions. Our
analysis also indicates that between models there is a large variation in
efficiency of long-range transportation of aerosols to remote locations. The differences in model results are most likely due to the more complex effect of different processes instead of one specific feature (e.g. the representation of aerosol or emission size distributions). Hence, a more detailed characterization of microphysical processes and deposition processes affecting the long-range transport is needed to understand the model variability.
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