SO₂ data and alert service

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

Solar Zenith Angle (SZA)

Schematic illustration of the Solar Zenith Angle (SZA) and Viewing Zenith Angle (VZA) for observations from satellite-based instrument. [image taken from a NASA page with definitons]

 
The value of the Solar Zenith Angle depends on the position on the Earth and the local date and time. The following graphs shows the variation of the SZA for places at about the time SCIAMACHY passes directly overhead. The retrieval of SO2 data is not so reliable for very high SZA, say above 75 degrees, and becomes really unreliable at much higher SZA. That is why data presented in the maps and data files on this website is limited to SZA 85 degrees.

The time SCIAMACHY passes a place overhead is always the same local time, as the satellite orbits the Earth in a so-called sun-synchronus orbit. If a satellite in a polar orbit flies over the pole into the sunlight, the SZA can be well above 100 degrees. The minimum SZA along the centre of an orbit is about 24 degrees. While orbiting on the sun-lit side of the Earth from North to South, SCIAMACHY scans from East to West, with on the East (West) side an SZA that is a little smaller (larger) than in the centre of the orbit.

The Solar Zenith Angle as function of time in 2007 for several places at about the moment SCIAMACHY passes overhead.

place	latitude	longitude	time (UTC)
Reykjavik	64.15	-21.97	ca. 12.35
Rome	41.88	12.50	ca. 09.40
Mexico City	19.40	-99.15	ca. 16:55
Galapagos Ils.	-0.50	-91.20	ca. 16:00
Bulawayo	-20.17	28.72	ca. 07:50
Wellington	-41.28	174.78	ca. 21:55
South Orkney Ils.	-62.00	-45.00	ca. 12:00

Note that ENVISAT, the satellite that carries SCIAMACHY, passes the equator at 10:00 a.m. local solar time. For a given logitude the time in UTC is:

UTC_time = local_solar_time - longitude/15

 
The following table lists the time and direction of equator passes of the different polar-orbiting nadir-viewing satellite instruments measuring SO2. All these satellites have a sun-synchronus orbit at about 790 km above the surface.