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New studies may help to explain the high concentration of sulfuricacid in the atmosphere. The research could also have implications forglobal climate modelling, enabling scientists to reduce uncertaintiesrelated to the effects of aerosols in their predictions.

Scientistshave struggled for years to reconcile atmospheric concentrations ofsulfuric acid with the results of laboratory experiments on particleformation rates. According to Mikko Sipilä at the Universityof Helsinki in Finland, this is down to the inadequacy of particledetectors in previous experiments - the best could only detectparticles of 3nm and above. But now Sipilä and a team of internationalresearchers have developed methods to detect particles barely biggerthan a single nanometre.

Shedding light on sulfuric acid in the atmosphere

As Sipilä explains, at concentrations below 108 molecules per cubic centimetre, as in the atmosphere, sulfuric acid particles forming by condensation from H­­2SO4gas grow very slowly. 'This means that within the residence times usedin previous studies - some ten seconds typically - the particles cannotgrow above the detection limit of the particle counters they have beenusing,' he says.

Using their improved detection methods, theresearchers show that the discrepancy of several orders of magnitudebetween observed and theoretical growth rates does not exist. While thegrowth rate they have arrived at does not exactly match predictionsfrom theory, the agreement, they say, is 'good'.


In nucleationtheory, there is a critical threshold at which particles like sulfuricacid become stable as they are condensing. The researchers suggest thata critical nucleus in this case contains one to two sulfuric acidmolecules. But as Renyi Zhang, an expert in atmospheric sciencesat Texas A&M University in the US, says, 'it is still difficult toexplain how one to two sulfuric acid molecules, along with severalwater molecules, can make up a critical nucleus, from a thermodynamicpoint of view.' He adds that the results still need to be reproduced byother groups.

If the results are reproduced, however, they couldhave important implications for climate science. Sipilä notes that theindirect effects of aerosols are some of the least well understood inclimate models. 'Currently I think that in those models that are usedfor the [Intergovernmental Panel on Climate Change] report, thenucleation is either neglected completely or it's somehow based onambient observations,' says Sipilä. 'If the detailed molecular stepsare not known it creates lots of uncertainty in these models. That'swhy it's very important to understand the detailed steps as this wouldthen enhance the accuracy of global climate predictions.'

Hayley Birch

dauswq

sangat menakutkan biler difikirkan kepekatan sulfurik asid meningkat mendadak
hujan asid pun makin ligat...

Gas phase chemistry
In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction :
SO2 + OH → HOSO2.which is followed by:
HOSO2. + O2 → HO2. + SO3In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid:
SO3 (g) + H2O (l) → H2SO4 (l)

Chemistry in cloud droplets
When clouds are present, the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets.
HydrolysisSulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
SO2 (g) + H2O SO2·H2OSO2·H2O H+ + HSO3−HSO3− H+ + SO32−