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Climate Change classes 1
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Climate change 2007 IPCC special
UQ 1 Nov Dec 06 Particles in air
Preface
Introduction
F: Perception of air
F: Phase transitions
F: Polarity and ions
R: Particle Measurement
R: Formation of S Aerosol
A: Combustion of plants
C: Information for teachers
Special: Oct. 2006 Communication
Nr 10 Sept. 2006 Africa's emissions
Nr 9 July 06 Air traffic
Special: June 06 Climate summit
Nr 8 April 2006 Ozone & N2 cycle
Nr 7 March 2006 Climate modeling
Nr 6 Feb. 2006 acid rain
Nr 5 Jan. 2006 oceanic sulfur
Special: Nov 05 Ozzy Ozone
Nr 4 Oct. 2005 light/satellites
Special: Sept 05 Cyclones
Nr 3 Sept. 2005 methane/energy
Special: July 05 Greenhouse Earth
Nr 2 June 2005 forest/aerosols
No 1 May 2005 vegetation/CO2
     
Research

Research B: Formation of Aerosol - Role of sulfuric acid

 Amount of particles in air

We described in the research text part A how fine particles of a diameter of less than 10 µm are measured with a common particle analyser for ambient air control.
  

With the help of such analysers we can get an overview which amounts of particles are found in rural regions, in the environment of a city and directly in the city centre.
  

In average concentrations measured in the city centre (urban background) and immediately next to the street ( kerbside) are higher than concentrations in rural regions or in the wider environment around a city. However, the differences between a city and its environment are often less sharp if the whole region is strongly polluted.
  

PM 10 annual average

1. PM 10 annual average in different states in Europe. Grey lines connect rural, near-city, urban background and kerbside sites that belong to the same urban area. Source: Jean-Philippe Putaud, JRC, and EEA/AIRBASE.
Please click to enlarge!

 Sulfuric acid and particle formation

But also in remote uninhabitated regions there are always certain amounts of fine particles in the air. We assume two major sources for these particles: sulfuric acid and low volatile organic compounds in the air. On this page we discuss only sulfuric acid.
 

Schwefelsäure und Partikel

2. Time series of sulfuric acid (black line) with subsequent formation of new particles (dotted) at Idaho Hill, Colorado; 1993-09-21 (Weber et al., 1997).

The formation of sufate compounds from sulfuric acid in the air leads to very fine particles. During their formation their size is smaller than the detection limits of particle analysers. It has been observed during a field campaign in the Rocky Mountains that the formation of finest particles took place after a strong increase in the concentration of sulfuric acid (H2SO4) in the atmosphere.* This increase occured from morning to noon.
 

Particle formation can take place if a certain saturation with sulfuric acid in the air has been reached and if a certain humidity is given at the same time. The critical point for new particle formation is reached much earlier, if traces of ammonia (e.g. 1 ppt = part per trillion) are in the air. In this case sulfuric acid and ammonia can form ammonium sulfate, a very hygroscopic salt. Since water as the third partner plays still a role, we call this formation of new particles "ternary nucleation" (nucleation with three partners).

 What do we observe if new particles are formed?

In the following series of images we try to illustrate what is observed by a scientist during the formation of new particles and what is really taking place.
 
 

Nukleation - Illustration in Bildern

3. a-g) Different gas molecules (white, light green) float in the air but also tiny aggregations of several molecules, which are not in a gaseous state anymore (pink). We call them clusters.

Nukleation - Illustration in Bildern

From a certain source a gas consisting of relatively low voletile molecules is added to the air (claret pentagons).

Nukleation - Illustration in Bildern

An analytical instrument can detect certain gases in the atmosphere and for example observe: This new gas is sulfuric acid. Specific instruments often shield or ignore other compounds.
 

Nukleation - Illustration in Bildern

A particle analyser works in a different way. In its finest mode it can only see particles which are larger than a certain diameter (e.g. the dotted circle). The small pink clusters just pass the instrument like gases without being detected.
 

Nukleation - Illustration in Bildern

Suddenly the instrument sees a particle. Something has formed in the air which has not been there yet before. But the instrument detects only "something larger". It cannot tell us about the composition of the particle.
 

Nukleation - Illustration in Bildern

Since sulfuric acid has been measured before the scientists assume that the detected particles consist of sulfuric acid.

Nukleation - Illustration in Bildern

But we cannot be sure about this. It can also be that the sulfuric acid just sticked on the pre-exisiting clusters and maybe even mixed with other compounds in the particle.

Consequently, the moment of birth of a particle is still rather in the dark for us. Many very small particles which have been just forming stick to larger ones before they reach a size which is detectable for our instruments. The smaller the growth rates of particles are and the more particles have already been in the air, the more unlikely does it become to observe ultra-fine particles in the range between 3 and 20 nm. But since we can well generate ultra-fine particles in the laboratory from water and sulfuric acid we can assume that they form also in the atmosphere.
  

 Chemistry of sulfuric acid formation

We see in figure 2 that sulfuric acid is formed at a certain time of the day. This coherency is not surprising. Sulfuric acid is formed by oxidation of sulfur dioxide. The necessary oxygen compounds are primarily formed in chemical reactions, which are driven by the solar energy during the day. Sulfur dioxide has natural sources (emissions from algae and volcanoes) and since the industrialisation it is also released in larger amounts to the atmosphere by humans (combustion of coal and heavy oil as well as industrial processes).
  

4. In the atmosphere, sulfuric acid is formed on different rather complex ways, in the gas phase as well as in the liquid phase. The scheme on the right shows two important ways: the oxidation with hydroxy-radicals in the gas phase and the oxidation with ozone (or other soluble oxidants) in the liquid phase, i.e. in cloud or fog droplets. Also the reaction of sulfur trioxide with water is not a simple addition. Several water molecules are involved.
Scheme: Elmar Uherek; Please click to enlarge!
 

Bildung von Schwefelsäure


* It is a problem for aerosol researchers that ultra-fine particles are indeed detectable but their chemical composition cannot be determined, since the amounts are too small. Therefore, the theory of new particle formation from sulfuric acid is conclusive but not finally proven.
 

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last updated 18.01.2007 | © ACCENT - Atmospheric Composition Change 2013