Potential environmental and health problems caused by sulfur dioxide (SO2) and sulfuric acid (H2O2) have long been known, for example since the 1970´s through topics such as “Waldsterben” or “Acid Rain” (e.g., acidification of soils and lakes; respiratory problems due to inhalation of sulfur – containing gases and particles). Sulfuric acid is highly soluble in water and also a strongly etching acid. It is formed both in the gas and aqueous phase by the oxidation of SO2. The major reactants are the OH-radical (gas phase) and dissolved H2O2 or ozone (aqueous phase). In addition to nitric acid atmospheric H2SO4 contributes significantly to the acidification (reduction of pH values) in cloud-, fog-, and rainwater as well as to corrosion of buildings and other materials. Consequent usage of modern filtering techniques can to a large extent reduce sulfur emissions and thereby prevent the formation of damaging concentrations of H2SO4 in the atmosphere.
As the figure indicated schematically acids are predominantly formed from reactions with peroxy radicals, especially OH with a variety of chemical compounds. Air pollution caused by humans can significantly affect the acid concentration and thus particle formation in the atmosphere. Particle formation may have a cooling effect on our atmosphere.
However, there is always a natural ” background ” of H2 SO4 in the atmosphere which may serve important functions such as, e.g., the formation of aerosol particles and clouds. Such processes ” naturally ” influence the global radiation budget and climate of the atmosphere. Gaseous sulfuric acid has a very low vapor pressure under atmospheric conditions and thus readily deposits on surfaces, for example on aerosol particles. In addition, it can also condense to molecular clusters in conjunction with other H2SO4 as well as H2O and possibly, ammonia molecules. By further condensation these clusters may grow to ” particles ” of a few 100 nanometers in diameter. These sub-micron sized sulfate particles possess two important properties: 1. they effectively backscatter solar UV radiation, and 2. they also serve as condensation nuclei for water vapor to condense on and form fog and cloud droplets. The resulting droplets also backscatter solar radiation depending on their respective size distribution. Thus sulfate particles tend to cool the atmosphere and counteract the well-known ” greenhouse effect” (“Anti-Greenhouse Effect”). Generally, one distinguishes between the “direct” and the “indirect” (via cloud formation) “climate forcing” of atmospheric sulfate particles.
Results
These natural processes may have been severely perturbed by the dramatically increasing anthropogenic sulfur emissions since the beginning of the industrial age. One of the troubling questions resulting from the recognition of this possibility is whether the build-up of the global greenhouse effect by concomitant anthropogenic emissions of CO2 and other gases has hitherto largely been obscured by the anti-greenhouse effect of sulfate aerosols. If so, then mankind will have to face the major dilemma that further global reduction of sulfur emissions (highly desirable for reasons of health and avoiding the damages of acid rain) could lead to a much more conspicuous warming of the earth´s atmosphere than we are already experiencing today.
Therefore, one of the most important goals in present atmospheric chemistry research is to elucidate the mechanism(s) of new particle formation in the atmosphere and to understand, in particular, the role of sulfuric acid in this process. In addition to its relevance for the hydrological cycle and the climate of the atmosphere, recent studies indicate that the atmospheric aerosol may yet play another potentially important role in the atmosphere, namely as a substrate for heterogeneous chemical reactions which could significantly influence the oxidant budget, and thus, the chemical self – purification power of the atmosphere.
Xem thêm : Zn + HNO3 → Zn(NO3)2 + NO2 + H2O
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Instrumentation
Atmospheric sulphuric acid (H2SO4) and hydroxyl-radicals (OH) are measured by selective Chemical-Ionisation Mass-Spectrometry (CIMS). A sketch of the system is shown in the following figure. The technique has been presented in detail elsewhere (Berresheim et al. Technical Paper on CIMS OH/H2SO4, 1999; PDF 530k) and only a short overview is given here. The sample gas (ambient air) flows at 16 L/min through an inlet system to an ion reaction zone. Here, a sheath gas of pre-filtered ambient air (SO2-free) with some HNO3 added is supplied circular around the sample gas. In this sheath gas, NO3- ions are generated from radioactive 241Americium which are subsequently focussed by electrical fields to the flow axis of the sample gas. The H2SO4 molecules in the sample gas react with the NO3- ions by charge-exchange reactions according to: H2SO4 + NO3- —> HSO4- + HNO3 (1) Since in atmosphere only few acids stronger than nitric acid exist, sulphuric acid is selectively ionized and can be measured by mass-spectrometry (see below). To enable measurement of OH, the sample gas is doped by SO2 from needle injectors to titrate the entire OH and transform it to H2SO4. Thus, a difference signal of ambient + titrated sulphuric acid minus the ambient sulphuric acid must be measured to determine OH. Often, an isotopically enriched mixture of 34SO2 (34S/32S>95%) is used instead of the naturally occurring isotope fractions (34S/32S=4%). Thus, the sulphuric acid from titration of OH has a much higher fraction of the 34S isotope than the background sulphuric acid and measurements are more sensitive. The ions formed in (1) are then transferred into a vacuum chamber through a pin-hole of 0.2 mm by electrical fields. On their way, they are separated form air by a counter-flow of inert gas. In the vacuum chamber, the ions are first separated from neutral cluster molecules in a collision-dissociation chamber, then they are separated in a quadrupole mass filter and detected by a secondary electron multiplier. The concentrations of H2SO4 and OH are calculated from the ratios of the signals of product ions relative to the NO3- reaction-ions, and a calibration-factor F: [H2SO4] = F(H32SO4-/NO3- ) [OH] = F(H34SO4-/NO3 -) (2) F is determined from OH-measurements with a calibration-unit placed in front of the titration zone. Here, filtered UV light (185 nm) is utilised to photolyse water molecules in the sample gas and thus generate OH at 106-108 molecules/cm3. From measurements of photon flux density, dew point and flow velocity, the concentration of OH is calculated. For 5 min integrated signals, the system achieves a precision of 13% and an uncertainty of 20% for OH and H2SO4 (both 1-sigma), and detection limits of 3 x 104 molecules/cm3 (H2SO4) and 7 x 104 molecules/cm3 (OH, 2-sigma). Different compounds have also been successfully measured with this technique, e.g. methanesulphonic-acid (MSA)
Xem thêm : Zn + HNO3 → Zn(NO3)2 + NO2 + H2O
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