SO₂ data and alert service

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Product information

Introduction

Slant column density vs.
vertical column density

Geographic regions

Data presentation
and delivery

Solar Zenith Angle

What is the Dobson Unit?

Slant column retrieval

Background correction

Reference spectrum

Cloud cover fraction

Near-real time service

Criteria for exceptional
SO2 concentrations

Air-mass factor using
look-up tables

Air-mass factor using a
chemistry transport model

SO2 column from OMI

Time period of
available data

Data format specification

Data and Service
version history

Validation of the
data products

South Atlantic Anomaly

Downloading
data & image files

Documentation

References

Acronyms

Acknowledgments

NOTE: This is the OLD product info. Some parts of it are no longer up to date, while other parts are missing -- see the remark on the main product info page.

What is the Dobson Unit (DU)?

The concentration of a certain trace gas, for example ozone and SO2, in a column of air in the earth's atmosphere is often given in Dobson Units.

The "Dobson Unit" is named after professor G.M.B. Dobson (1889 - 1976), who has from the 1920s onwards done research on the ozone layer. Around 1930 he built the first "Dobson spectrophotometer", with which reliable measurements of the ozone layer became possible.

The "Dobson Unit" indicates how much of a given trace gas there is in the air above a certain point on earth. A proper unit in the International System of units would thus be "kilogram per square meter".

The unit introduced by Dobson is defined as follows. Suppose that all the trace gas in question in the air would be in a (gas) layer just above the ground, at standard pressure (1013.25 hPa) and at standard temperature (0.0 Celsius). The amount of the trace gas, for example ozone, is then indicated by the thickness of this layer, expressed in 0.01 millimeter. (This is why the ozone layer is sometimes referred to as being "thick" or "thin".)

1 Dobson Unit (DU) is:

2.6867E+20 molecules per square meter
4.4615E-04 MOL per square meter
2.1415E-05 kilogram of ozone per square meter

Averaged over the entire world the ozone column has a value of about 300 DU. For the Netherlands this is an average of 280 DU in autumn and 380 DU in spring. During spring on the southern hemisphere, September-November, the so-called "ozone hole" develops, with ozone values (well) below 200 DU.

For SO2, the typical background level concentration (i.e. away from emissions related to pollution and volcanic eruptions) is much less than 1 DU. Emissions related to pollution and small volcanic eruptions are of the order of 1 DU or a few DU. Strong and explosive eruptions may lead to concentrations well above 10 DU, even as high as 100 DU.

Volume mixing ratio

Trace gas concentrations at a particular pressure level in the atmosphere are often given as a volume mixing ratio, or simply mixing ratio. This unit is defined as the ratio of the number density of the gas to the total number density of the atmosphere. In other words, the SO2 volume mixing ratio is the density of SO2 divided by the density of all constituents of the atmosphere in a unit volume (i.e. the number of molecules per unit volume).

Therefore, an SO2 mixing ratio of 10E-09 means that the number density of SO2 is 10E-09 times the total number density of air in a unit volume. Following the standard convention for the earth's troposphere and stratosphere, this mixing ratio equals 1 ppbv (parts per billion by volume).

For examples how to convert ozone volume mixing ratios into other units or vice versa, see this page elsewhere on the Web.

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SO2 data and alert service

What is the Dobson Unit (DU)?

Volume mixing ratio

SO₂ data and alert service