Sulfuric Acid Aerosol Particles Research Articles

High homogeneous freezing onsets of sulfuric acid aerosol at cirrus temperatures

Abstract. Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in the upper troposphere. There the air temperature is low, the ice supersaturation can be high and the concentration of ice-nucleating particles is too low to initiate and dominate cirrus cloud formation by heterogeneous ice nucleation processes. The most common description to quantify homogeneous freezing processes is based on the water activity criterion (WAC) as proposed by Koop et al. (2000). The WAC describes the homogeneous nucleation rate coefficients only as a function of the water activity, which makes this approach well applicable in numerical models. In this study, we investigate the homogeneous freezing behavior of aqueous sulfuric acid aerosol particles by means of a comprehensive collection of laboratory-based homogeneous freezing experiments conducted at the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud simulation chamber, which were conducted as part of 17 measurement campaigns since 2007. The most recent experiments were conducted during October 2020 with special emphasis on temperatures below 200 K. Aqueous sulfuric acid aerosol particles of high purity were generated by particle nucleation in a gas flow composed of clean synthetic air and sulfuric acid vapor, which was added to the AIDA chamber. The resulting chamber aerosol had number concentrations from 30 cm−3 up to several thousand per cubic centimeter with particle diameters ranging from about 30 nm to 1.1 µm. Homogeneous freezing of the aerosol particles was measured at simulated cirrus formation conditions in a wide range of temperatures between 185 and 230 K with a steady increase of relative humidity during each experiment. At temperatures between about 205 K and about 230 K, the AIDA results agree well with the WAC-based predictions of homogeneous freezing onsets. At lower temperatures, however, the AIDA results show an increasing deviation from the WAC-based predictions towards higher freezing onsets. For temperatures between 185 and 205 K, the WAC-based ice saturation ratios for homogeneous freezing onsets increase from about 1.6 to 1.7, whereas the AIDA measurements show an increase from about 1.7 to 2.0 in the same temperature range. Based on the experimental results of our direct measurements, we suggest a new fit line to formulate the onset conditions of homogeneous freezing of sulfuric acid aerosol particles as an isoline for nucleation rate coefficients between 5×108 and 1013 cm−3 s−1. The potential significant impacts of the higher homogeneous freezing thresholds, as directly observed in the AIDA experiments under simulated cirrus formation conditions, on the model prediction of cirrus cloud occurrence and related cloud radiative effects are discussed.

Model simulations of the chemical and aerosol microphysical evolution of the Sarychev Peak 2009 eruption cloud compared to in situ and satellite observations

Abstract. Volcanic eruptions impact climate through the injection of sulfur dioxide (SO2), which is oxidized to form sulfuric acid aerosol particles that can enhance the stratospheric aerosol optical depth (SAOD). Besides large-magnitude eruptions, moderate-magnitude eruptions such as Kasatochi in 2008 and Sarychev Peak in 2009 can have a significant impact on stratospheric aerosol and hence climate. However, uncertainties remain in quantifying the atmospheric and climatic impacts of the 2009 Sarychev Peak eruption due to limitations in previous model representations of volcanic aerosol microphysics and particle size, whilst biases have been identified in satellite estimates of post-eruption SAOD. In addition, the 2009 Sarychev Peak eruption co-injected hydrogen chloride (HCl) alongside SO2, whose potential stratospheric chemistry impacts have not been investigated to date. We present a study of the stratospheric SO2–particle–HCl processing and impacts following Sarychev Peak eruption, using the Community Earth System Model version 1.0 (CESM1) Whole Atmosphere Community Climate Model (WACCM) – Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics model (with no a priori assumption on particle size). The Sarychev Peak 2009 eruption injected 0.9 Tg of SO2 into the upper troposphere and lower stratosphere (UTLS), enhancing the aerosol load in the Northern Hemisphere. The post-eruption evolution of the volcanic SO2 in space and time are well reproduced by the model when compared to Infrared Atmospheric Sounding Interferometer (IASI) satellite data. Co-injection of 27 Gg HCl causes a lengthening of the SO2 lifetime and a slight delay in the formation of aerosols, and acts to enhance the destruction of stratospheric ozone and mono-nitrogen oxides (NOx) compared to the simulation with volcanic SO2 only. We therefore highlight the need to account for volcanic halogen chemistry when simulating the impact of eruptions such as Sarychev on stratospheric chemistry. The model-simulated evolution of effective radius (reff) reflects new particle formation followed by particle growth that enhances reff to reach up to 0.2 µm on zonal average. Comparisons of the model-simulated particle number and size distributions to balloon-borne in situ stratospheric observations over Kiruna, Sweden, in August and September 2009, and over Laramie, USA, in June and November 2009 show good agreement and quantitatively confirm the post-eruption particle enhancement. We show that the model-simulated SAOD is consistent with that derived from the Optical Spectrograph and InfraRed Imager System (OSIRIS) when both the saturation bias of OSIRIS and the fact that extinction profiles may terminate well above the tropopause are taken into account. Previous modelling studies (involving assumptions on particle size) that reported agreement with (biased) post-eruption estimates of SAOD derived from OSIRIS likely underestimated the climate impact of the 2009 Sarychev Peak eruption.

Long-term variations of the UV contrast on Venus observed by the Venus Monitoring Camera on board Venus Express

We analyze the Venus ultraviolet (UV) reflectivity as observed by the Venus Monitoring Camera on board Venus Express over 2000 orbits in the years 2006–2011. We compare several laws for the photometric correction of global images of Venus, and find that the combined law of Lambert and Lommel-Seeliger is most suitable for our study. Our analysis of the corrected images reveals strong fluctuations in the reflectivity contrast between low and high latitude regions of up to 40%, that follow variations of the SO2 abundance above the cloud top. Additionally, the phase angle dependence of the contrast gradually change from weak to strong, which may be related with the vertical distribution of the unknown UV absorber and the overlaying upper haze layer. We suggest that these variations result from a combination of two processes. One is the meridional transport of SO2, which forms sulfuric acid aerosol particles at high latitudes. The other is the presence of vertical winds near the cloud top level, which control the vertical mixing of the unknown UV absorber and the upper haze.

East Asian SO<sub>2</sub> pollution plume over Europe – Part 2: Evolution and potential impact

Abstract. We report on the first observation-based case study of an aged East Asian anthropogenic SO2 pollution plume over Europe. Our airborne measurements in that plume detected highly elevated SO2 mole fractions (up to 900 pmol/mol) between about 5000 and 7000 m altitude. Here, we focus on investigations of the origin, dispersion, evolution, conversion, and potential impact of the observed excess SO2. In particular, we investigate SO2 conversion to gas-phase sulfuric acid and sulfuric acid aerosols. Our FLEXPART and LAGRANTO model simulations, along with additional trace gas measurements, suggest that the plume originated from East Asian fossil fuel combustion sources and, 8–7 days prior to its arrival over Europe, ascended over the coast region of central East Asia to 9000 m altitude, probably in a cyclonic system with an associated warm conveyor belt. During this initial plume ascent a substantial fraction of the initially available SO2 must have escaped from removal by cloud processes. Hereafter, while mostly descending slowly, the plume experienced advection across the North Pacific, North America and the North Atlantic. During its upper troposphere travel, clouds were absent in and above the plume and OH-induced gas-phase conversion of SO2 to gas-phase sulfuric acid (GSA) was operative, followed by GSA nucleation and condensation leading to sulfuric acid aerosol formation and growth. Our AEROFOR model simulations indicate that numerous large sulfuric acid aerosol particles were formed, which at least temporarily, caused substantial horizontal visibility degradation, and which have the potential to act as water vapor condensation nuclei in liquid water cloud formation, already at water vapor supersaturations as low as about 0.1%. Our AEROFOR model simulations also indicate that those fossil fuel combustion generated soot particles, which have survived cloud induced removal during the initial plume ascent, have experienced extensive H2SO4/H2O-coating, during upper troposphere plume travel. This coating may have dramatically altered the morphology and markedly increased the light absorption efficiency of soot particles.

Initial results of a global circuit model with variable stratospheric and tropospheric aerosols

A new model of the global circuit has been constructed that treats more realistically than previous models the external influences due to varying charged particle fluxes from space, the internal variability due to the transport of radon by atmospheric dynamics, and the varying aerosol populations. An approximate treatment of the effects of explosive volcanic eruptions, that introduce SO2 and H2O into the stratosphere, involves the creation of ultrafine aerosol particles in the downward branch of the Brewer‐Dobson circulation from sulfuric acid aerosol particles vaporized in the upward branch. A large increase in stratospheric column resistance occurs at high latitudes, but for much of the time this column resistance is reduced by stratospheric ionizing radiation due to relativistic electron precipitation. Here we present results for the variability of the global circuit due to the variations of the aerosol populations in the stratosphere and troposphere, the variability of the cosmic ray flux, and the geographical and seasonal variability of the near‐surface radioactivity. We discuss inferred changes in the circuit parameters associated with climate change, on interannual to Milankovich timescales.

Effect of sulfuric acid coating on heterogeneous ice nucleation by soot aerosol particles

The low‐temperature aerosol and cloud chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe was used to investigate the effect of sulfuric acid coating on the ice nucleation efficiency of soot aerosol particles from a spark discharge generator. The uncoated (sulfuric acid–coated) soot aerosol showed a nearly lognormal size distribution with number concentrations of 300–5000 cm−3 (2500–56,000 cm−3), count median diameters of 70–140 nm (90–200 nm), and geometric standard deviation of 1.3–1.4 (1.5–1.6). The volume fraction of the sulfuric acid coating to the total aerosol volume concentration ranged from 21 to 81%. Ice activation was investigated in dynamic expansion experiments simulating cloud cooling rates between about −0.6 and −3.5 K min−1. At temperatures between 186 and ∼235 K, uncoated soot particles acted as deposition nuclei at very low ice saturation ratios between 1.1 and 1.3. Above 235 K, ice nucleation only occurred after approaching liquid saturation. Coating with sulfuric acid significantly increased the ice nucleation thresholds of soot aerosol to saturation ratios increasing from ∼1.3 at 230 K to ∼1.5 at 185 K. This immersion mode of freezing nucleates ice well below the thresholds for homogeneous freezing of pure sulfuric acid solution droplets measured in previous AIDA experiments. A case study indicated that in contrast to the homogeneous freezing the nucleation rate of the immersion freezing mechanism depends only weakly on relative humidity and thereby the solute concentration. These results show that it is important to know the mixing state of soot and sulfuric acid aerosol particles in order to properly assess their role in cirrus formation.

Influence of tropospheric SO2 emissions on particle formation and the stratospheric humidity

Stratospheric water vapor plays an important role in the chemistry and radiation budget of the stratosphere. Throughout the last decades stratospheric water vapor levels have increased and several processes have been suggested to contribute to this trend. Here we present a mechanism that would link increasing anthropogenic SO2 emissions in southern and eastern Asia with an increase in stratospheric water. Trajectory studies and model simulations suggest that the SO2 increase results in the formation of more sulfuric acid aerosol particles in the upper tropical troposphere. As a consequence, more ice crystals of smaller size are formed in the tropical tropopause, which are lifted into the stratosphere more readily. Our model calculations suggest that such a mechanism could increase the amount of water that entered the stratosphere in the condensed phase by up to 0.5 ppmv from 1950–2000.

Reactivity of BrONO2 and HOBr on sulfuric acid solutions at low temperatures

Uptake of BrONO2 onto bulk sulfuric acid solutions and submicron sulfuric acid aerosol particles was studied in laminar flow cylindrical reactors where a chemical ionization mass spectrometer was used to detect products and reactants. Results from measurements at low temperatures and for sulfuric acid content between 70 and 85 wt.% are presented. These measurements suggest that the reaction probability for BrONO2 on a given sulfuric acid solution does not depend strongly on temperature. The heterogeneous chemistry of HOBr, a product of the hydrolysis of BrONO2, was also studied on bulk sulfuric acid solutions (58–70 wt.%). Measurements of HOBr and HNO3 solubility and the reaction HOBr + HCl → BrCl + H2O are reported. Supplemented with previous measurements, a set of parameters to calculate the reaction probabilities appropriate to the stratosphere are deduced. These measurements confirm earlier assertions that the reaction can be a significant loss process for HCl when aerosol volume in the stratosphere is elevated.

Solubility of Methanol in Low-Temperature Aqueous Sulfuric Acid and Implications for Atmospheric Particle Composition

Using traditional Knudsen cell techniques, we find well-behaved Henry's law uptake of methanol in aqueous 45 - 70 wt% H2SO4 solutions at temperatures between 197 and 231 K. Solubility of methanol increases with decreasing temperature and increasing acidity, with an effective Henry's law coefficient ranging from 10(exp 5) - 10(exp 8) M/atm. Equilibrium uptake of methanol into sulfuric acid aerosol particles in the upper troposphere and lower stratosphere will not appreciably alter gas-phase concentrations of methanol. The observed room temperature reaction between methanol and sulfuric acid is too slow to provide a sink for gaseous methanol at the temperatures of the upper troposphere and lower stratosphere. It is also too slow to produce sufficient quantities of soluble reaction products to explain the large amount of unidentified organic material seen in particles of the upper troposphere.

Technical Note: Organics-Induced Fluorescence in Raman Studies of Sulfuric Acid Aerosols

Fluorescence is a troublesome side effect in laboratory Raman studies on sulfuric acid solutions and aerosol particles. We performed experiments showing that organic matter induces fluorescence in H 2 SO 4 /H 2 O solutions. The intensity of the fluorescence signal appears to be almost independent of the concentration of the organic substances, but depends strongly on the sulfuric acid concentration. The ubiquity of organic substances in the atmosphere, their relatively high abundance, and the insensitivity of the fluorescence with respect to their concentrations will render most acidic natural aerosols subject to absorption and fluorescence, possibly influencing climate forcing. We show that, while fluorescence may in the future become a valuable tool of aerosol diagnostics, the concurrent absorption is too small to significantly affect the atmosphere's radiative balance.

Ice Formation by Sulfate and Sulfuric Acid Aerosol Particles under Upper-Tropospheric Conditions

Ice formation in ammoniated sulfate and sulfuric acid aerosol particles under upper-tropospheric conditions was studied using a continuous flow thermal diffusion chamber. This technique allowed for particle exposure to controlled temperatures and relative humidities for known residence times. The phase states of (NH4)2SO4 and NH4HSO4 particles were found to have important impacts on their ice formation capabilities. Dry (NH4)2SO4 particles nucleated ice only at high relative humidity (RH ≥ 94%) with respect to water at temperatures between −40° and −60°C. This result suggested either an impedance or finite time dependence to deliquescence and subsequent homogeneous freezing nucleation. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze homogeneously at relative humidities that were 10% lower than where ice nucleated on initially dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH4HSO4 particles required higher relative humidities for ice nucleation than did initially liquid bisulfate particles. Liquid particles of size 0.2 μm composed of either ammonium sulfate or bisulfate froze at lower relative humidity at upper-tropospheric temperatures than did 0.05-μm sulfuric acid aerosol particles. Comparison of calculated homogeneous freezing point depressions suggest that size effects on freezing may be more important than the degree of ammoniation of the sulfate compound.

Hydrolysis of N2O5 on Submicron Sulfuric Acid Aerosols

The kinetics of reactive uptake of gaseous N2O5 on submicron sulfuric acid aerosol particles has been investigated using a laminar flow reactor coupled with a differential mobility analyzer (DMA) to characterize the aerosol. The particles were generated by homogeneous nucleation of SO3/H2O mixtures. In the H2SO4 concentration range 26.3−64.5 wt % the uptake coefficient was γ = 0.033 ± 0.004, independent of acid strength. For an acid strength of 45 wt % γ was found to decrease with increasing temperature over the range 263−298 K. From this, temperature dependence values of −115 ± 30 kJ/mol and −25.5 ± 8.4 J/K mol were determined for the changes in enthalpy and entropy of the uptake process, respectively. The results are consistent with a previous model of N2O5 hydrolysis involving both a direct and an acid catalyzed mechanism, with uptake under the experimental conditions limited by mass accommodation.

Solubilities and diffusion of trace gases in cold sulfuric acid films

The well-known technique of gas chromatography was used to determine solubilities and liquid-phase diffusion coefficients of trace gases in a thin sulfuric acid film deposited in a fused silica column. Solubilities of SO 2, NO 2, and N 2O 4 and the diffusion coefficient of SO 2 were measured in the temperature range 203–243 K using sulfuric acid concentrations in the range of 39–83 wt%. The results suggest that sulfuric acid aerosol particles are not a physical sink for these trace gases.

Investigations of gas-to-particle conversion in the atmosphere

Continental aerosol particle size distributions were measured as a function of time at the] background station Aisetas and in the suburb of Vilnius. Marine aerosol particle formation was studied in outdoor photochemical chamber experiments performed on the coast of the Baltic sea. The functional dependence of the particle diameter growth rate on the particle size was calculated from the aerosol particle size distribution data and was compared with the theoretical predictions for the particle growth due to vapor condensation or due to the adsorption of aerosol precursor gases on the particle surface or within an airborne droplet. It was shown that during summer months condensation of low pressure vapors formed by gas phase homogeneous chemical reactions was predominant mechanism of the continental aerosol particle growth. While in winter growth of a submicrometer continental aerosol particle was frequently governed by two competing processes: condensation of low pressure vapors and heterogeneous oxidation of aerosol precursor gases inside the liquid droplet. Ozone was found to be an active oxidant in the heterogeneous aqueous phase chemical reactions. The outdoor photochemical chamber experiments showed that aqueous phase chemical reactions were responsible for the formation of marine sulfuric acid aerosol particles from the dimethylsulfide. After these particles were partially neutralized by ammonia to the ammonium sulfate or bisulfate, their further growth was observed to be surface limited.

Gas Phase Reaction of Sulfur Trioxide with Water Vapor

Sulfur trioxide (SO3) has long been known to react with water to produce sulfuric acid (H2S04). It has been commonly assumed that the gas phase reaction in the Earth`s atmosphere between SO3 and water vapor to produce sulfuric acid vapor is an important step in the production of sulfuric acid aerosol particles. The kinetics of the gas phase reaction of SO3 with water vapor have previously been studied by Castleman and co-workers, Wang et al and Reiner and Arnold. Each of these studies was carried out in a flow reactor, with the first two studies performed at low pressure (1-10 Torr) and the latter from approx. 30 to 260 Torr. Each of these studies measured SO3 decays over a range of H2O vapor levels, obtaining data consistent with interpreting the reaction of gaseous SO3 and H2O as a bimolecular process. It is not clear why previous experimental studies failed to observe a nonlinear dependence of SO3 consumption on water vapor concentration. It is probable that sufficient water dimer exists in much of the Earth`s atmosphere to allow dimer reactions to participate in sulfuric acid vapor formation.

Heterogeneous Reactions of Chlorine Nitrate and Dinitrogen Pentoxide on Sulfuric Acid Surfaces Representative of Global Stratospheric Aerosol Particles

AbstractIncreasing evidence from field measurements, modeling studies, and laboratory experiments suggests that heterogeneous reactions on stratospheric sulfate aerosol particles can change the partitioning in the nitrogen and chlorine families and thereby affect global ozone levels. In this study, a Knudsen cell flow reactor was used to measure the uptake of ClONO2 and N2O5 by sulfuric acid solutions representative of background and volcanic stratospheric aerosol particles. The uptake coefficient (γ) of chlorine nitrate on 50–75 wt% H2SO4 at 223 K was found to be markedly dependent on the acid concentration, with γ ranging from about 1 × 10−2 to 1 × 10−4. These results are in good agreement with literature reports and the data fit the expression log γ= 1.87 – 0.074 × (wt% H2SO4). This reaction will thus have its largest impact when stratospheric temperatures are low and sulfuric acid aerosols are most dilute. Uptake of N2O5 was studied on solutions with compositions in the range 58–96 wt% H2SO4 at temperatures from 193 to 303 K. N2O5 reacted readily on sulfuric acid surfaces with uptake coefficients of about 0.06. The uptake coefficient was found to be independent of the sulfuric acid concentration and the solution temperature over the ranges studied. These results suggest that the reaction of N2O5 with H2O will occur readily on sulfuric acid aerosol particles for most stratospheric conditions.

Effects on stratospheric ozone from high‐speed civil transport: Sensitivity to stratospheric aerosol loading

The potential impact of high‐speed civil transport (HSCT) aircraft emissions on stratospheric ozone and the sensitivity of these results to changes in aerosol loading are examined with a two‐dimensional model. With aerosols fixed at background levels, calculated ozone changes due to HSCT aircraft emissions range from negligible up to 4–6% depletions in column ozone at northern high latitudes. The magnitude of the ozone change depends mainly on the NOx increase due to aircraft emissions, which depends on fleet size, cruise altitude, and engine design. The partitioning of the odd nitrogen species in the lower stratosphere among NO, NO2, N2O5, and HNO3 is strongly dependent on the concentration of sulfuric acid aerosol particles, and thus the sensitivity of O3 to NOx emissions changes when the stratospheric aerosol loading changes. Aerosol concentrations 4 times greater than background levels have not been unusual in the last 2 decades. Our model results show that a factor of 4 increase in aerosol loading would significantly reduce the calculated ozone depletion due to HSCT emissions. Because of the natural variability of stratospheric aerosols, the possible impact of HSCT emissions on ozone must be viewed as a range of possible results.

On the growth of nitric and sulfuric acid aerosol particles under stratospheric conditions

We present a theory for the formation of frozen aerosol particles in the Antarctic stratosphere, the coldest region of the Earth's stratosphere. The theory is applied specifically to the formation of polar stratospheric clouds. We suggest that the condensed ices are composed primarily of nitric acid and water with small admixtures of other compounds such as H2SO4 and HCl in solid solution. Our assumed particle formation mechanism is in agreement with the magnitude and seasonal behavior of the optical extinctions observed in the winter polar stratosphere. Physical chemistry and thermodynamic considerations suggest that at temperatures between about 200 and 185 K, stratospheric particulates are composed primarily of frozen nitric acid solutions with a composition near that of the trihydrate. Available data suggest the particles are amorphous solid solutions and not in the crystalline hydrate form. At lower temperatures (i.e., below the forst point of pure water) cirrus-like ice clouds can form.

Comments “On Sizing Impacted Sulfuric Acid Aerosol Particles”

Comments “On Sizing Impacted Sulfuric Acid Aerosol Particles”

On Sizing Impacted Sulfuric Acid Aerosol Particles

Abstract The method of sizing sulfuric acid aerosol particles collected by impaction is examined and a new model is proposed. It is found that the volume of a particle with the typical morphology of a central particle surrounded by numerous “droplets” can be determined by adding to the volume of the central particle the volume of an acid disc with a diameter equal to the droplet ring diameter and thickness proportional to this diameter. Effects of dehydration of the collected particle in the vacuum evaporation shadowing process and the uptake of ammonia from the laboratory air are found to be important in the determination of the original particle size.