Yinon Rudich

The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest

About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodele depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodele emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day

Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements. Part 2: Application to combustion-generated soot aerosols as a function of fuel equivalence ratio

Composition, shape factor, size, and fractal dimension of soot aerosol particles generated in a propane/O2, flame were determined as a function of the fuel equivalence ratio (φ). Soot particles were first size-selected by a differential mobility analyzer (DMA) and then analyzed by an Aerodyne aerosol mass spectrometer (AMS). The DMA provides particles of known mobility diameter (dm ). The AMS quantitatively measures the mass spectrum of the nonrefractory components of the particles and also provides the vacuum aerodynamic diam eter (dva ) corresponding to the particles of known mobility diameter. The measured dm, dva , and nonrefractory composition are used in a system of equations based on the formulation presented in the companion article to estimate the particle dynamic shape factor, total mass, and black carbon (BC) content. Fractal dimension was estimated based on the mass-mobility relationship. Two types of soot particles were observed depending on the fuel equivalence ratio. Type 1: for φ < 4 (lower propane/O2), dva ; was nearly constant and independent of dm . The value of dva increased with increasing φ. Analysis of the governing equations showed that these particles were highly irregular (likely fractal aggregates), with a dynamic shape factor that increased with dm and φ. The fractal dimension of these particles was approximately 1.7. These particles were composed mostly of BC, with the organic carbon content increasing as φ increased. At φ = 1.85, the particles were about 90% BC, 5% PAH, and 5% aliphatic hydrocarbon (particle density = 1.80 g/cm3). Type 2: for φ > 4 (high propane/O2), dva was linearly proportional to dm . Analysis of the governing equations showed that these particles were nearly spherical (likely compact aggregates), with a dynamic shape factor of 1.1 (versus 1 for a sphere) and a fr actal dimension of 2.95 (3 for a sphere). These particles were composed of about 50% PAH, 45% BC, and 5% aliphatic hydrocarbons (particle density = 1.50 g/cm3). These results help interpret some measurement s obtained in recent field studies.

Direct observation of completely processed calcium carbonate dust particles

This study presents, for the first time, field evidence of complete, irreversible processing of solid calcium carbonate (calcite)-containing particles and quantitative formation of liquid calcium nitrate particles apparently as a result of heterogeneous reaction of calcium carbonate-containing mineral dust particles with gaseous nitric acid. Formation of nitrates from individual calcite and sea salt particles was followed as a function of time in aerosol samples collected at Shoresh, Israel. Morphology and compositional changes of individual particles were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental scanning electron microscopy (ESEM) was utilized to determine and demonstrate the hygroscopic behavior of calcium nitrate particles found in some of the samples. Calcium nitrate particles are exceptionally hygroscopic and deliquesce even at very low relative humidity (RH) of 9-11% which is lower than typical atmospheric environments. Transformation of non-hygroscopic dry mineral dust particles into hygroscopic wet aerosol may have substantial impacts on light scattering properties, the ability to modify clouds and heterogeneous chemistry.

Overview of the inorganic and organic composition of size-segregated aerosol in Rondônia, Brazil, from the biomass-burning period to the onset of the wet season

The aerosol characterization experiment performed within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) field experiment carried out in Rondonia, Brazil, in the period from September to November 2002 provides a unique data set of size-resolved chemical composition of boundary layer aerosol over the Amazon Basin from the intense biomass-burning period to the onset of the wet season. Three main periods were clearly distinguished on the basis of the PM10 concentration trend during the experiment: (1) dry period, with average PM10 well above 50 mu g m(-3); (2) transition period, during which the 24-hour-averaged PM10 never exceeded 40 mu g m(-3) and never dropped below 10 mg m(-3); (3) and wet period, characterized by 48-hour-averaged concentrations of PM10 below 12 mu g m(-3) and sometimes as low as 2 mu g m(-3). The trend of PM10 reflects that of CO concentration and can be directly linked to the decreasing intensity of the biomass- burning activities from September through November, because of the progressive onset of the wet season. Two prominent aerosol modes, in the submicron and supermicron size ranges, were detected throughout the experiment. Dry period size distributions are dominated by the fine mode, while the fine and coarse modes show almost the same concentrations during the wet period. The supermicron fraction of the aerosol is composed mainly of primary particles of crustal or biological origin, whereas submicron particles are produced in high concentrations only during the biomass-burning periods and are mainly composed of organic material, mostly water-soluble, and similar to 10% of soluble inorganic salts, with sulphate as the major anion. Size-resolved average aerosol chemical compositions are reported for the dry, transition, and wet periods. However, significant variations in the aerosol composition and concentrations were observed within each period, which can be classified into two categories: (1) diurnal oscillations, caused by the diurnal cycle of the boundary layer and the different combustion phase active during day (flaming) or night (smouldering); and (2) day-to-day variations, due to alternating phases of relatively wet and dry conditions. In a second part of the study, three subperiods representative of the conditions occurring in the dry, transition, and wet periods were isolated to follow the evolution of the aerosol chemical composition as a function of changes in rainfall rate and in the strength of the sources of particulate matter. The chemical data set provided by the SMOCC field experiment will be useful to characterize the aerosol hygroscopic properties and the ability of the particles to act as cloud condensation nuclei.

Product studies of the OH‐ and ozone‐initiated oxidation of some monoterpenes

The OH- and O3-initiated oxidation of five monoterpenes (myrcene, terpinolene, Δ3-carene, α-pinene, and β-pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The OH-oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and ∼2%, for α- and β-pinene, respectively). Formaldehyde has been identified as a major product (yields of 20–40%) in the OH-initiated oxidation of all five species. Formic acid was also observed in the OH-initiated oxidation of all five monoterpenes, with yields of 2% from β-pinene and 5–9% from the other species studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an OH scavenger was measured. The yields of acetone for the O3 reactions were α-pinene, 0.03±0.01; β-pinene, 0.009±0.009; Δ3-carene, 0.10±0.015; myrcene, 0.25±0.06; and terpinolene, 0.50±0.06. The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made.

Reactive uptake of ozone by proxies for organic aerosols : Surface versus bulk processes

Uptake measurements of ozone were conducted with two types of proxies for atmospheric organic aerosols: organic liquids and self-assembled organic monolayers. Alkanes and terminal alkenes were used. The monolayer surface was characterized, prior to and after reaction, using IR spectroscopy. Uptake experiments were conducted using a flow tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficient, γ, is shown to be due to reaction with the double bond. For the monolayers, γ is composed solely of a surface reactive component and is smaller by at least an order of magnitude than values obtained for a liquid of the same chain length. Uptake by the liquids is higher due to solubility and reaction in the bulk. The phase of the atmospheric organic aerosol will determine the appropriate use of a bulk or surface uptake probability in atmospheric models. Since the aerosol surface is processed and sites are consumed, γ is time variant. We define a parameter γ as the surface uptake probability per reactive site and determine its value as 9×10−19 cm2 molecule−1. This enables the modeling of surface reactions as surface site concentrations diminish following interaction with the gaseous species.

Reactive uptake of NO3 on pure water and ionic solutions

The reactive uptake coefficients (γ) of NO 3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO 2 were measured using a wetted-wall flow-tube setup combined with a long-path absorption cell for the detection of NO 3 . The measured γ values were in the range 1.5 × 10 -4 - 6 × 10 -3 , depending on the salt concentration in the water. By measuring γ as a function of salt concentration, HD l 0.5 for NO 3 in water was determined to be (1.9±0.4)×10 -3 M atm -1 cm s -0.5 at 273 K, assuming that the rate coefficient for the reaction of NO 3 with Cl - is 2.76×10 6 M -1 s -1 at 273 K. The Henrys law coefficient for NO 3 in water is estimated to be 0.6±0.3 M atm -1 , assuming that the diffusion coefficient of NO 3 in water is D l = (1.0±0.5)×10 -5 cm 2 s -1 . Uptake of NO 3 on pure water is interpreted as due to reaction of NO 3(aq) with H 2 O (l) to produce HNO 3 and OH in the liquid phase. Implications of these findings to the chemistry of NO 3 in the troposphere are also discussed.

The role of BrNO3 in marine tropospheric chemistry: A model study

Laboratory studies have shown that bromine nitrate (BrNO3) reacts on sulfuric acid and on ice particles. Here we investigate the potential role of BrNO3 in the marine boundary layer (mbl) assuming that it reacts on sea-salt particles as well. Using the chemical box model MOCCA we find that heterogeneous reactions of BrNO3 on aerosol particles could affect the chemistry in four major ways: 1) They increase loss of NOx (=NO+NO2) from the gas phase; 2) They accelerate loss of bromide and chloride from sea-salt aerosols. This dehalogenation occurs without the consumption of aerosol acidity; 3) The resulting loss of NOx and the increase of gas-phase bromine species both lead to O3 destruction; 4) The resulting increase of reactive chlorine species affects gas-phase hydrocarbons as well as S(IV) oxidation by HOCl in sea-salt aerosols.

Effects of reversible adsorption and Langmuir–Hinshelwood surface reactions on gas uptake by atmospheric particles

An expression for the reactive uptake coefficient (γ) of gas phase species due to surface reaction with particle-bound reactants via a Langmuir–Hinshelwood mechanism is derived from first principles. The new parameterization separates the processes of adsorption and chemical reaction, and it implies that γ for a surface reaction limited system will depend on the gas-phase concentration of the reacting species and decrease with reaction time, unless the particle-bound surface reactants are replenished. It also implies that γ will scale linearly with the concentration of the particle-bound reactants, in contrast to the square root dependence typical of diffusion-limited reactive uptake by liquids. The presented formulae enable calculation of γ from basic physico-chemical parameters for relevant atmospheric conditions, and they extend the existing resistor models of gas uptake by particles. The implications of this modified parameterisation are discussed by numerically integrating a reactive system with parameters of relevance to the atmosphere and by addressing a few recent laboratory studies.

The NH4+‐NO3−‐Cl−‐SO42−‐H2O aerosol system and its gas phase precursors at a pasture site in the Amazon Basin: How relevant are mineral cations and soluble organic acids?

Real-time measurements of ammonia, nitric acid, hydrochloric acid, sulfur dioxide and the water-soluble inorganic aerosol species, ammonium, nitrate, chloride, and sulfate were performed at a pasture site in the Amazon Basin (Rondonia, Brazil). The measurements were made during the late dry season (biomass burning), the transition period, and the onset of the wet season (clean conditions) using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC). Measurements were conducted from 12 September to 14 November 2002 within the framework of LBA-SMOCC (Large-Scale Biosphere Atmosphere Experiment in Amazonia - Smoke Aerosols, Clouds, Rainfall, and Climate: Aerosols From Biomass Burning Perturb Global and Regional Climate). Real-time data were combined with measurements of sodium, potassium, calcium, magnesium, and low-molecular weight (LMW) polar organic acids determined on 12-, 24-, and 48-hours integrated filter samples. The contribution of inorganic species to the fine particulate mass (Dp = 2.5 µm) was frequently below 20% by mass, indicating the preponderance of organic matter. The measured concentration products of NH3 × HNO3 and NH3 × HCl persistently remained below the theoretical equilibrium dissociation constants of the NH3/HNO3/NH4NO3 and NH3/HCl/NH4Cl systems during daytime (RH 90%) fine-mode NH4NO3 and NH4Cl are predicted to be formed in the aqueous aerosol phase. Probably, Cl- was driven out of the aerosol phase largely by reaction of pyrogenic KCl with HNO3 and H2SO4. As shown by an updated version of the equilibrium simplified aerosol model (EQSAM2), which incorporates mineral aerosol species and lumped LMW polar organic acids, daytime aerosol NH4 + was mainly balanced by organic compounds.