75 research outputs found
Experimental study of H<sub>2</sub>SO<sub>4</sub> aerosol nucleation at high ionization levels
Abstract. One hundred and ten direct measurements of aerosol nucleation rate at high
ionization levels were performed in an 8 m3 reaction chamber. Neutral and
ion-induced particle formation from sulfuric acid (H2SO4) was
studied as a function of ionization and H2SO4 concentration. Other
species that could have participated in the nucleation, such as NH3
or organic compounds, were not measured but assumed constant, and the
concentration was estimated based on the parameterization by
Gordon et al. (2017). Our parameter space is thus [H2SO4] =4×106-3×107 cm−3, [NH3+ org] = 2.2 ppb, T=295 K,
RH = 38 %, and ion concentrations of 1700–19 000 cm−3. The
ion concentrations, which correspond to levels caused by a nearby supernova,
were achieved with gamma ray sources. Nucleation rates were directly measured
with a particle size magnifier (PSM Airmodus A10) at a size close to critical
cluster size (mobility diameter of ∼ 1.4 nm) and formation rates at a
mobility diameter of ∼ 4 nm were measured with a CPC (TSI model 3775).
The measurements show that nucleation increases by around an order of
magnitude when the ionization increases from background to supernova levels
under fixed gas conditions. The results expand the parameterization presented
in Dunne et al. (2016) and Gordon et al. (2017) (for [NH3+org] =
2.2 ppb and T=295 K) to lower sulfuric acid concentrations and higher
ion concentrations. The results make it possible to expand the
parameterization presented in Dunne et al. (2016) and Gordon et al. (2017) to
higher ionization levels.
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The role of atmospheric ions in aerosol nucleation:a review
Atmospheric aerosols affect climate and yet the reason for many observed events of new aerosol formation is not understood. One of the theories put forward to explain these events is that the presence of ions can enhance the formation of aerosols. The theory is called Ion Induced Nucleation and in this paper the state of observations, theory and experiments within the field will be reviewed. While evidence for Ion Induced Nucleation is accumulating the exact mechanism is still not known and more research is required to understand and quantify the effect
Model of optical response of marine aerosols to Forbush decreases
In order to elucidate the effect of galactic cosmic rays on cloud formation, we investigate the optical response of marine aerosols to Forbush decreases – abrupt decreases in galactic cosmic rays – by means of modeling. We vary the nucleation rate of new aerosols, in a sectional coagulation and condensation model, according to changes in ionization by the Forbush decrease. From the resulting size distribution we then calculate the aerosol optical thickness and Angstrom exponent, for the wavelength pairs 350, 450 nm and 550, 900 nm. In the cases where the output parameters from the model seem to compare best with atmospheric observations we observe, for the shorter wavelength pair, a change in Angstrom exponent, following the Forbush Decrease, of &minus;6 to +3%. In some cases we also observe a delay in the change of Angstrom exponent, compared to the maximum of the Forbush decrease, which is caused by different sensitivities of the probing wavelengths to changes in aerosol number concentration and size. For the long wavelengths these changes are generally smaller. The types and magnitude of change is investigated for a suite of nucleation rates, condensable gas production rates, and aerosol loss rates. Furthermore we compare the model output with observations of 5 of the largest Forbush decreases after year 2000. For the 350, 450 nm pair we use AERONET data and find a comparable change in signal while the Angstrom Exponent is lower in the model than in the data, due to AERONET being mainly sampled over land. For 550, 900 nm we compare with both AERONET and MODIS and find little to no response in both model and observations. In summary our study shows that the optical properties of aerosols show a distinct response to Forbush Decreases, assuming that the nucleation of fresh aerosols is driven by ions. Shorter wavelengths seem more favorable for observing these effects and great care should be taken when analyzing observations, in order to avoid the signal being drowned out by noise
The Ion and Charged Aerosol Growth Enhancement (ION-CAGE) code: A numerical model for the growth of charged and neutral aerosols
The presence of small ions influences the growth dynamics of a size
distribution of aerosols. Specifically the often neglected mass of small ions
influences the aerosol growth rate, which may be important for terrestrial
cloud formation. To this end, we develop a numerical model to calculate the
growth of a species of aerosols in the presence of charge, which explicitly
includes terms for ion-condensation. It is shown that a positive contribution
to aerosol growth rate is obtained by increasing the ion-pair concentration
through this effect, consistent with recent experimental findings. The
ion-condensation effect is then compared to aerosol growth from charged aerosol
coagulation, which is seen to be independent of ion-pair concentration. The
model source code is made available through a public repository.Comment: 29 pages, 12 figure
The response of clouds and aerosols to cosmic ray decreases
A method is developed to rank Forbush decreases (FDs) in the galactic cosmic ray radiation according to their expected impact on the ionization of the lower atmosphere. Then a Monte Carlo bootstrap-based statistical test is formulated to estimate the significance of the apparent response in physical and microphysical cloud parameters to FDs. The test is subsequently applied to one ground-based and three satellite-based data sets. Responses (>95%) to FDs are found in the following parameters of the analyzed data sets. AERONET: Ångström exponent (cloud condensation nuclei changes), SSM/I: liquid water content, International Satellite Cloud Climate Project (ISCCP): total, high, and middle, IR-detected clouds over the oceans, Moderate Resolution Imaging Spectroradiometer (MODIS): cloud effective emissivity, cloud optical thickness, liquid water, cloud fraction, liquid water path, and liquid cloud effective radius. Moreover, the responses in MODIS are found to correlate positively with the strength of the FDs, and the signs and magnitudes of the responses agree with model-based expectations. The effect is mainly seen in liquid clouds. An impact through changes in UV-driven photo chemistry is shown to be negligible and an impact via UV absorption in the stratosphere is found to have no effect on clouds. The total solar irradiance has a relative decrease in connection with FDs of the order of 10−3, which is too small to have a thermodynamic impact on timescales of a few days. The results demonstrate that there is a real influence of FDs on clouds probably through ions.</p
Measurement of the charging state of 4-70 nm aerosols
The charging state of aerosols in an 8 m3 reaction chamber was measured using an electrostatic classifier with a condensation particle counter at different levels of ionization in the chamber. By replacing the Kr-85 neutralizer in the classifier with a radioactively neutral dummy we were able to measure only the aerosols that were charged inside our reaction chamber. These measurements were then compared with measurements using the neutralizer to get the charging state of the aerosols, which refers to the charged fraction of the aerosols compared to an equilibrium charge distribution. Charging states were measured for both positively and negatively charged aerosols while the ionization in the chamber was varied using external gamma sources. We find that the negatively charged aerosols were overcharged (relative to the equilibrium) by up to about a factor of 10 below 10 nm and at 16±2% from 10 to 70 nm. At higher levels of radiation on the chamber the smaller aerosols were less overcharged while the large aerosols were more overcharged (23±2%). For the positively charged aerosols only the smallest aerosols were overcharged while those over 10 nm were undercharged (relative to the equilibrium) by 21±3%. Increasing the radiation on the chamber increased the undercharge above 10 nm to 25±2% while the overcharge below 10 nm disappeared. The split between positive and negative charges above 10 nm can be explained by differences in mobility of small negative and positive ions. The overcharge below 10 nm can be explained by ions participating in the formation of aerosols of both signs, while the reduction in this overcharge at higher levels of ionization may be explained by faster recombination
Response of Cloud Condensation Nuclei (> 50 nm) to changes in ion-nucleation
In experiments where ultraviolet light produces aerosols from trace amounts
of ozone, sulphur dioxide, and water vapour, the number of additional small
particles produced by ionization by gamma sources all grow up to diameters
larger than 50 nm, appropriate for cloud condensation nuclei. This result
contradicts both ion-free control experiments and also theoretical models that
predict a decline in the response of larger particles due to an insufficiency
of condensable gases (which leads to slower growth) and to larger losses by
coagulation between the particles. This unpredicted experimental finding points
to a process not included in current theoretical models, possibly an
ion-induced formation of sulphuric acid in small clusters.Comment: 4 pages, 3 figure
Response of cloud condensation nuclei (> 50 nm) to changes in ion-nucleation
In experiments where ultraviolet light produces aerosols from trace amounts of ozone, sulfur dioxide, and water vapor, the relative increase in aerosols produced by ionization by gamma sources is constant from nucleation to diameters larger than 50 nm, appropriate for cloud condensation nuclei. This result contradicts both ion-free control experiments and also theoretical models that predict a decline in the response at larger particle sizes. This unpredicted experimental finding points to a process not included in current theoretical models, possibly an ion-induced formation of sulfuric acid in small clusters
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