Topic 8

Question 1

What was the original gravimetric method?

Answer 1

The original gravimetric methods demonstrated the bulk composition of the atmosphere (i.e. inert gases llike NO2 and O2).

These were in-situ methods as they required air samples. Materials like copper would be left in air over time yielding compounds like copper oxide.

These had limited sensitivy and were not helpful in understanding atmospheric chemistry as an understanding of trace gases is needed.

Question 2

Describe the sample and return method.

Answer 2

The sample and return method is an in-situ method where clean evacuated stainless steel containers are filled with air at the desired atmospheric location They are then returned to the lab for analysis with mass spec. They are good at detecting stable VOCs such as propane/propene but GC-MS is not good with radical species.

Question 3

Explain the role of the trap in the sample and return method.

Answer 3

In the sample and return method, the trap condesnses most trace species but doesn’t trap the bulk gases (N2, O2)

Question 4

Give an example of an electrochemical method.

Answer 4

An example of an electrochemical method is ozonesonde - a balloon borne instrument that comprises an ElectroChem sensor that is used to meausre profiles of O3 at numerous sites across the globe - in situ.
Air is pumped through an aq sln of KI. As O3 can oxidise iodide ions to form I2.

Question 5

What is the Beer Lambert Law?

Answer 5

The Beer Lambert Law is Absorption = path length * absorption cross section * concentration

Question 6

What determines sensitivity in absorption spectroscopic methods?

Answer 6

Sensitivity in absorption spectroscopic methods is determined by the absorption cross section (cm2 molecule-1) or pathlength, (should be in cm)

Question 7

In what region are measurements made in absorption spectroscopic methods?

Answer 7

In absorption spectroscopic methods, measurements are made in the UV/VIs * IR regions.

Question 8

What are the two basic types of ASM measurements?

Answer 8

The two basic types of ASM measurements are single frequency measurements (using a single wavelength to establish a baseline by removing material of interest from air stream) and DOAS (differential optical AS) where measurements are made at two or more wavelengths and a baseline is established by comparing on and off resonance signals.

Question 9

What is an example of a single frequency measurement?

Answer 9

An example of a single frequency measurement is Cavity Ring-down Spectroscopy. This is a highly sensitive IN SITU method.