Preliminary estimation of the rainfall chemical composition evaluated through the scavenging modeling for north-eastern Amazonian region (Amapá state,Brazil)

Numerical modeling of scavenging processes has been compared with data obtained for rainwater and aerosol chemistry at Serra do Navio, in the state of Amapá in the Brazilian Amazon region. Sulfate, nitrate and ammonium concentrations were determined in rainwater samples collected from May 1995 until June 1997. The levels of these same chemicals were also determined in aerosols for the same period and region. Scavenging processes have been evaluated on a rainfall event basis, via numerical modeling, in order to simulate the rainwater concentrations and compare them with the observed data. RAMS (Regional Atmospheric Modeling System) was used to simulate cloud structures. A model of below-cloud scavenging was evaluated, as well. The determinations made from the results of the scavenging model are the following: a) aerosol vertical profiles are quite important to rainwater concentrations; b) modeled sulfate in rainwater is a better fit to the observed data values than ammonium and nitrate; c) the obtained sulfate aerosol concentrations samples are similar to ones found in the literature, although the sulfate concentrations in rainwater are much lower than other studies in the literature; d) the in-cloud scavenging process dominates, e) our modeled results, using an input gas vertical profile extracted from the ABLE2B experimental data set, present a smaller ratio between gas and aerosol scavenging than found in other studies in the literature, other studies may have had larger rainfall times, which increase the importance of gas phase scavenging.     

A model study of smoke-haze influence on clouds and warm precipitation formation in Indonesia 1997/1998

In the last few decades, fire and smoke-haze occurrence increased in Indonesia by intentionally set land clearing fires and higher fire susceptibility of disturbed forests. Particularly, during El Niño years with prolonged droughts in Indonesia, land clearing fires become uncontrolled wildfires and produce large amounts of gaseous and particulate emissions. This paper investigates the influence of smoke-haze aerosols from such fires on clouds and precipitation over Indonesia during the El Niño event 1997/1998 by numerical modelling. Warm precipitation formation in both layered and convective clouds is calculated dependent on the atmospheric aerosol concentration. In the smoke-haze affected regions of Indonesia, aerosol–cloud interactions induce events with both precipitation suppression and increase compared to a reference simulation without aerosol–cloud interactions. The effect of precipitation suppression is found to dominate with about 2/3 of all precipitation modification events pointing to a prolongation of smoke-haze episodes. The corresponding convective cloud top height of shallow clouds is increased whereas distinct lower deep convective cloud top heights are found. The remaining about 1/3 events are characterised by increased precipitation and cloud liquid water content, accompanied by lower convective cloud top heights of shallow clouds and higher deep convective clouds.     

Black carbon (BC) in alpine aerosols and cloud water—concentrations and scavenging efficiencies

During April 1999 and March 2000, intensive field campaigns were performed on a mid-level mountain (Rax, 1644 m a.s.l.) in Central Europe both under out-of-cloud and in-cloud conditions. The black carbon (BC) content of both aerosol and cloud water as well as BC scavenging efficiencies of Rax clouds were measured. As a tracer for the non-carbonaceous aerosol, sulfate was used. Although BC concentrations on Rax were low (April 1999 out-of-cloud average: 0.43 μg/m³, March 2000: 0.72 μg/m³), the BC mass fraction of the aerosol was fairly high (1999: 3.5%, 2000: 6.4%). Average BC concentrations in cloud water were 1.09 μg/ml (1999) and 1.4 μg/ml (2000). These values are far higher than literature values, but comparable to those found in an earlier study (J. Geophys. Res. 105 (D20) (2000) 24637) at a high-level mountain (Sonnblick, 3106 m a.s.l.) some 200 km distant from Rax. The average BC scavenging efficiency of the Rax clouds in March 2000 was 0.54. The increase of scavenging efficiency with increasing liquid water content of the clouds found earlier on Sonnblick for sulfate and aerosol carbon (J. Atmos. Chem 35 (2000) 33), organic carbon (J. Geophys. Res. 105 (2000) 19857), and BC (J. Geophys. Res. 105 (D20) (2000) 24637) was also confirmed on Rax.     

Cloud physics and cloud water sampler comparison during FEBUKO

Optical methods for counting and sizing cloud droplets and a wide range of cloud water sampling methods were used to characterize the atmospheric liquid phase during the FEBUKO cloud experiments. Results near cloud base as well as more than 300 m inside the hill cap clouds are presented, reflecting their inhomogeneous nature. The cloud droplet number varies from 50 to 1000 cm⁻³ and drop sizes between 1 and 20 μm diameter are most frequent. Variations in the liquid water content (LWC) and in the total ion content (TIC) are much smaller when the measurement position is deeper in the cloud. Near cloud base variability in updraft strength and, near cloud top, entrainment processes (droplet evaporation by mixing with drier air, aerosol and gas scavenging) disturb the adiabatic conditions and produce large variations in LWC and chemical composition. Six different active cloud water collectors and impactors were running side by side; they differ in the principle of sampling, in the throughput of cloudy air per unit time and in the calculated 50% cutoff diameter, which influence also their sampling efficiency. Two of them are designed to collect cloud water in two droplet size fractions. Three cloud events were selected by the FEBUKO team for detailed cloud physical and chemical analyses because they serve best the modelling demands concerning connected flow between the upwind, summit and downwind sites for process studies. Frequency distributions of the LWC and, also of the cloud base height are given as statistical parameters for both FEBUKO experiments.     

Non-dissipative cloud transport in Eulerian grid models by the volume-of-fluid (VOF) method

The formation of clouds is coupled to the vapour saturation condition. Cloud modelling is therefore dramatically disturbed by dilution processes, which are induced by recurrent interpolations on the fixed (Eulerian) grid. The numerical diffusion gives rise to degeneration and premature disappearance of the modelled clouds. The difficulties increase, if sectional mass representation in the drop microphysics and aerosol chemistry is considered. To tackle this problem, stringently defined and tracked phase boundaries are required.The numerical diffusion of clouds can be totally suppressed by the volume-of-fluid (VOF) method, which is applied here in connection with an atmospheric model. The cloud phase is distinguished by prognosing the partial cloud volume in all grid cells near the cloud boundary. Adopting elementary geometrical forms for the intracellular cloud volume and simple diagnostic rules of their alignment, the standard transport fluxes can be used in the new equation. Separate variables for the cloud and environmental phase complete the transport scheme.The VOF method and its realisation are described in detail. Advection, condensation, evaporation, and turbulent diffusion are considered within the VOF framework. The variation of the grid resolution and turbulence conditions for a rising thermal leads to striking arguments in favour of the VOF method, resulting in higher intensity, lifting, and lifetime as well as clear boundaries of the simulated clouds (even for low grid resolution).     

Halogen production from aqueous tropospheric particles

Box model studies have been performed to study the role of aqueous phase chemistry with regard to halogen activation for marine and urban clouds and the marine aerosol as well. Different chemical pathways leading to halogen activation in diluted cloud droplets and highly concentrated sea salt aerosol particles are investigated. The concentration of halides in cloud droplets is significantly smaller than in sea-salt particles, and hence different reaction sequences control the overall chemical conversions. In diluted droplets radical chemistry involving OH, NO(3), Cl/Cl(2)(-)/ClOH(-), and Br/Br(2)(-)/BrOH(-) gains in importance and pH independent pathways lead to the release of halogens from the particle phase whereas the chemistry in aerosol particles with high electrolyte concentrations is controlled by non-radical reactions at high ionic strengths and relatively low pH values.For the simulation of halogen activation in tropospheric clouds and aqueous aerosol particles in different environments a halogen module was developed including both gas and aqueous phase processes of halogen containing species. This module is coupled to a base mechanism consisting of RACM (Regional Atmospheric Chemistry Mechanism) and the Chemical Aqueous Phase Radical Mechanism CAPRAM 2.4 (MODAC-mechanism). Phase exchange is described by the resistance model by Chemistry of Multiphase Atmospheric Systems, NATO ASI Series, 1986.It can be shown that under cloud conditions the bromine atom is mainly produced by OH initiated reactions, i.e. its concentration maximum is reached at noon. In contrast, the concentration level of chlorine atoms is linked to NO(3) radical chemistry leading to a smaller amplitude between day and night time concentrations.The contribution of radical processes to halogen atom formation in the particle phase is evident, e.g. by halogen atoms which undergo direct phase transfer. Furthermore, the application of the multiphase model for initial concentrations for sea-salt aerosols shows that the particle phase can act as a main source of halogen containing molecules (Cl(2), BrCl, Br(2)) which are photolysed in the gas phase to yield halogen atoms (about 70% of all Cl sources and more than 99% for Br).     

Personal reactive clouds: Introducing the concept of near-head chemistry

The “personal cloud” effect and its impact on human exposure to airborne pollutants are well documented. A great deal is also known regarding indoor air chemistry, particularly as related to ozone reactions with mono-terpenes. In this paper we hypothesize the presence of personal reactive clouds that result from ozone reactions with terpenes and terpenoids emitted from personal care products. A proof of concept assessment was completed based on reaction rates between ozone and five reactive organic compounds that are found in personal care products. Screening experiments were also completed with three perfumes and two hairsprays to determine the extent of secondary organic aerosol formation in the breathing zone of a subject who had applied these products. The results of screening calculations and preliminary experiments confirm that chemistry occurs in the near-head region of individuals who apply scented personal care products to their hair or facial skin. Additional research is needed to characterize reaction products and health consequences associated with near-head chemistry and associated personal reactive clouds.     

Nomogram for Relating Particle and Particle Cloud Parameters

Three concentration units, i.e., mass density, number density, and surface area, often are alternatively used to describe an aerosol cloud. While most aerosol clouds are composed of poly-disperse particles, the nomogram based upon monodisperse spherical particles (Figure 1) has been found useful for relating these different concentration values.     

A model of wet deposition to complex terrain

A two dimensional model of the seeder-feeder mechanism of orographic rainfall enhancement has been developed. The model has been extended to include the deposition of aerosol material incorporated into the orographic feeder cloud by nucleation scavenging. Parameterizations of any changes in the concentration of SO₄²⁻ in the cloud due to chemical reactions have also been included. The model is used to predict the rainfall enhancement and SO₄²⁻ deposition over terrain consisting of two parallel ridges oriented perpendicular to the wind. A wide range of spatial scales has been used of up to 150 km. It is found that the patterns of rainfall enhancement and deposition are strongly dependent on the spatial scales, the atmospheric structure and the cloud chemistry.     

SOA from methylglyoxal in clouds and wet aerosols: Measurement and prediction of key products

Aqueous OH radical oxidation of methylglyoxal in clouds and wet aerosols is a potentially important global and regional source of secondary organic aerosol (SOA). We quantify organic acid products of the aqueous reaction of methylglyoxal (30–3000 μM) and OH radical (approx. 4 × 10^?12 M), model their formation in the reaction vessel and investigate how the starting concentrations of precursors and the presence of acidic sulfate (0–840 μM) affect product formation. Predicted products were observed. The predicted temporal evolution of oxalic acid, pyruvic acid and total organic carbon matched observations at cloud relevant concentrations (30 μM), validating this methylglyoxal cloud chemistry, which is currently being implemented in some atmospheric models of SOA formation. The addition of sulfuric acid at cloud relevant concentrations had little effect on oxalic acid yields. At higher concentrations (3000 μM), predictions deviate from observations. Larger carboxylic acids (≥C₄) and other high molecular weight products become increasingly important as concentration increases, suggesting that small carboxylic acids are the major products in clouds while larger carboxylic acids and oligomers are important products in wet aerosols.     

A computer modelling study of the chemistry occurring during cloud formation over hills

A model has been developed which describes the chemical processes occurring during the formation of hill cloud. This model has been applied to the study of four cases which should be typical of the chemical conditions which can exist during the formation of such cloud over hills in the N of England.The results show that a wide range of chemical behaviour is possible in cloud droplets, depending on the history of the airmass. The composition of the water-soluble aerosol is of importance in determining the chemical composition of cloud droplets, with most of the sulphate and large proportions of the other ions being derived from this source. Gas-phase chemistry is also important, not only because it dictates the initial concentrations of oxidants such as H₂O₂ and O₃ and of acidic gases such as HNO₃ at the onset of condensation, but also because of the chemical processes which occur concurrently with those in clouds. These are important sources of radicals, as well as of nitric acid, which continue to dissolve in solution. Gasphase HCl, HNO₃ and NH₃ are all of primary importance in describing the acidity of cloud droplets. The various oxidation mechanisms for SO₂ can all be important, with their relative contributions varying from case to case. H₂O₂ does not invariably dominate this process, especially in winter when its concentration may be low.The chemistry of hill clouds is far from trivial and oversimplification can result in a misunderstanding of the behaviour which occurs. There is a need, rather, to consider a wide range of reactions if the nature of the overall processes and the impact of manmade emissions on the environment via this route are to be understood.     

A scheme for black carbon and sulphate aerosols tested in a hemispheric scale,Eulerian dispersion model

Production, transport and deposition of sulphate and black carbon (BC) are simulated separating aerosol modes by production mechanisms, thus facilitating calculation of aerosol physical properties. The scheme uses explicit sulphur chemistry with prescribed oxidants and aerosol transformation based on Brownian coagulation rates, and is implemented in a 3-D, hemispheric-scale transport model with off-line meteorology from ECMWF. The components are DMS, SO₂, sulphate and BC. Simulations are made for the year 1988. Predicted results for sulphate agree well with the measurements, except for considerable underestimations in the Arctic. In Europe SO₂ is slightly overestimated and sulphate is slightly underestimated in cold seasons, whilst trends are less clear in North America. Compared to many other models, we estimate a smaller effective oxidation rate for SO₂ due to reduced rates in cold clouds; a shorter turnover time for sulphate (3.7 d) due to a probably too large below-cloud scavenging ratio; and slightly smaller sulphate column burdens. Our BC results are similar to Liousse et al.’s (1996, J. Geophys. Res. 101, 19.411–19.432) except in USA, whilst the concentrations in remote areas and the turnover time (3.7 d) are considerably smaller than Cooke and Wilson’s (1996, J. Geophys. Res. 101, 190.395–19.409). Agreements with available measurements are quite close to large anthropogenic emissions (including USA), but they are considerably underestimated in Arctic winter. Transition from hydrophobic to hydrophilic BC due to coagulation is swift (6% h^-1). Sensitivity tests emphasize that sub-cloud scavenging coefficients rely on careful assumptions about size distributions, and that more research is needed on sulphate production in ice-clouds. Emphasis should also be put on production of sulphate and hydrophilic BC boundary-layer clouds; inclusion of hydrophobic accumulation mode BC and sub-grid transition to hydrophilic BC in emitting grid squares.     

Application of the SO4(2-)/Se tracer technique to study SO2 oxidation in cloud and fog on a time scale of minutes

We have demonstrated the use of Se as a tracer to quantitatively determine in situ SO4(2-) production from SO2 oxidation in clouds and fogs. Until now, it has not been possible to study the kinetics of SO2 oxidation because the aerosol sampling interval for Se determination was limited to 2 h or longer. Here we report results of 5-min aerosol measurements carried out at Lahore, Pakistan, during January 9-11, 2001, using new methodology for Se analysis coupled with hydride generation and ICP-MS detection. These improvements will enable the tracer technique to determine in situ SO4(2-) production in clouds and fogs on a time scale of several minutes and possibly 1 min. The method may prove useful for kinetic studies of in-cloud SO2 oxidation and in the study of other phenomena such as atmospheric mixing, cloud drop lifetimes, and aerosol formation that occur on the time scale of a few minutes.     

Scavenging of electrified radioactive aerosol

Scavenging by water droplets is a mechanism for aerosol removal near clouds. Numerical methods are developed to quantify the removal of charged radioactive aerosols, including the electrical image force's contribution, attractive at small separations. Charging of radioactive aerosols is found to have significant effects on their collision efficiency and scavenging coefficient. The effect depends on the aerosol charge, and therefore, on the radioactive aerosol's decay rate and number concentration, but it does not depend significantly on the charge carried by the water drops. Scavenging coefficients are calculated for radioactive aerosols. For small particles at low aerosol concentrations (Z∼10–100 cm⁻³), charging can increase the scavenging coefficients by up to an order of magnitude. Electrification will, therefore, encourage the removal of small radioactive aerosols from the atmosphere, more rapidly than equivalent non-radioactive aerosols. The increase in removal at low radioactive-aerosol concentration may account for underpredictions of surface concentrations and will contribute to spatial variations in aerosol removal.     

Two case studies of sulfate scavenging processes in the Amazon region (Rondônia)

The scavenging processes of chemical species have been previously studied with numerical modeling, in order to understand the gas and particulate matter intra-reservoir transferences. In this study, the atmospheric (RAMS) and scavenging (B.V.2) models were used, in order to simulate sulfate concentrations in rainwater using scavenging processes as well as the local atmospheric conditions obtained within the LBA Project in the State of Rondonia, during a dry-to-wet transition season. Two case studies were conducted. The RAMS atmospheric simulation of these events presented satisfactory results, showing the detailed microphysical processes of clouds in the Amazonian region. On the other hand, with cloud entrainments, observed values have been overestimated. Modeled sulfate rainwater concentration, using exponential decay and cloud heights of 16 km and no entrainments, presented the best results, reaching 97% of the observed value. The results, using shape parameter 5, are the best, improving the overall result.     

Mercury and photochemistry in the marine boundary layer-modelling studies suggest the in situ production of reactive gas phase mercury

Following a modelling investigation of the role of the ambient aerosol in the cycling—that is the transport, transformation and deposition—of mercury in the atmosphere, the precise part played by the sea salt component of the marine aerosol in the remote marine boundary layer has been studied using a combination of models to describe the photolytic, gas phase and aqueous phase and heterogeneous chemistry of the marine boundary layer, in conjunction with inter phase mass transport and mercury chemistry. The role of the ocean in the emission of elemental mercury is, as yet, not entirely understood, but certainly the speciation of mercury deposited to the ocean surface is important as regards its re-emission. Models of mercury chemistry to date have tended to focus on cloud chemistry, and with good reason, as precipitation accounts for a large part of the global mercury deposition pattern; however, the composition of the marine aerosol is entirely different from that of cloud or fog droplets and the modelling studies here show that it plays a more local role being partially responsible for the gas phase speciation of mercury. The role of photochemical processes is investigated and particular attention is paid to halogen chemistry, as the chloride ion has been shown previously to have a notable effect on the concentration of oxidised mercury associated with particles, or better, solution droplets. The role of the sea salt component of the marine aerosol in the production of gas phase oxidised mercury species is described qualitatively and quantitatively.     

Sources of hydrogen peroxide in cloudwater

Results of a theoretical investigation of H₂O₂ formation in cloud droplets arising from gaseous HO₂ radical scavenging are presented. It is shown that this process is pH dependent with the maximum rate of H₂O₂ production occurring below pH 3. This dependence arises as a result of the dissociation of HO₂ in water (pK_a = 4.9) and the subsequent disproportionation reaction of HO₂ and O₂⁻ to form hydrogen peroxide. O₂⁻ is also removed by reaction with O₃ to produce OH radicals and this process becomes more competitive as both the pH and $\text{O}{}_2{}^{\mathrm{-}}\text{HO}{}_2$ ratio increase. The presence of soluble organic species, such as aldehydes, in cloudwater counteracts the effect of ozone by converting OH back to HO₂. For low pHs (< 3) the net contribution of organic solutes of H₂O₂ production is predicted to be relatively small, being limited by the availability of OH radicals scavenged from the gas phase. Existing cloud chemistry models may overestimate the rate of aqueous oxidation of formaldehyde by OH radicals.Under conditions where scavenging of gas-phase free radicals by cloud droplets is efficient, uptake of HO₂ radicals may be reversible. The aqueous concentration of OH is unlikely to approach thermodynamic equilibrium with the gas phase (H ∼-30 M atm⁻¹ and can be treated as irreversible. In clouds with a small mean droplet radius, efficient scavenging of precursor OH radicals should result in a decrease in gas-phase HO₂ production with a reduction in the yield of aqueous H₂O₂, although this is offset by the presence of soluble organic species. A similar effect is predicted for clouds with a high liquid water content.The supply of HO₂ and OH radicals to cloud droplets is controlled by gas-phase ozone chemistry which is in turn dependent on the solar u.v. radiation intensity. The u.v. density in clouds may be higher than in clear air when the solar zenith angle is small, thus enhancing H₂O₂ production, but falls off markedly as the solar zenith angle becomes larger. Predicted rates of H₂O₂ formation in clouds based on midday conditions are likely to be considerably higher than the average daytime value, particularly in summer. Diurnal and seasonal effects on H₂O₂ generation are expected to be more marked in clouds than in clear air.     

Fate of products of degradation processes: consequences for climatic change

The end products of atmospheric degradation are not only CO2 and H2O but also sulfate and nitrate depending on the chemical composition of the substances which are subject to degradation processes. Atmospheric degradation has thus a direct influence on the radiative balance of the earth not only due to formation of greenhouse gases but also of aerosols. Aerosols of a diameter of 0.1 to 2 micrometer, reflect short wave sunlight very efficiently leading to a radiative forcing which is estimated to be about -0.8 watt per m2 by IPCC. Aerosols also influence the radiative balance by way of cloud formation. If more aerosols are present, clouds are formed with more and smaller droplets and these clouds have a higher albedo and are more stable compared to clouds with larger droplets. Not only sulfate, but also nitrate and polar organic compounds, formed as intermediates in degradation processes, contribute to this direct and indirect aerosol effect. Estimates for the Netherlands indicate a direct effect of -4 watt m-2 and an indirect effect of as large as -5 watt m-2. About one third is caused by sulfates, one third by nitrates and last third by polar organic compounds. This large radiative forcing is obviously non-uniform and depends on local conditions.