Hello,
I am interested in how the concentration of O2 varies over time close to ground level. I have been informed that the concentration can drop when an inversion layer is present in the local area due to reduced atmospheric mixing and have seen this effect in data collected by my group.
What else can lead to O2 changes?
Also, what is the best way to identify when an inversion layer is present?
Background:
The reason for my asking is that my group use a constant value for oxygen concentration and use atmospheric air to calibrate our continuous gas oxygen analysers. These analysers are used for a clinical measurement called whole body room calorimetry, where we compare the changes in oxygen concentration of a room (nearly air tight) and air being pumped in to it. Further in the data processing, we calculate the amount of oxygen used by a person, which along with other gas exchange rates is used to calculate biological power output and fuel oxidation ratios.
Thanks for reading.
Dear Chris Georgiou, I am sorry but if you need to accurately calibrate your instrument, then you just cannot use the atmospheric air: there are too many atmospheric variables. I strongly recommend that you consider purchasing standard gas cylinders. Atmospheric air can be used for technical applications when you need a cheap source of oxygen with a concentration ca. 21% , but you cannot use it in analytical application as a reference standard for a concentration of e.g. 20.90%. Best regards.
Thank you for you response Valerio.
The type of analysis we typically perform results in the final output being relatively insensitive to initial calibration conditions. We correct for a range of atmospheric conditions such as humidity and pressure.
Some of the reasons why we use fresh air to perform the calibration is that the sample humidity in this calibration gas will be very close to that of our process gas after conditioning. We also like the long term stability, with measurements over many years having been compared.
I appreciate your concern about absolute calibration accuracy and that is 1 of the reasons why I am asking the question. Would you be able to expand on your answer, perhaps indicating some of the conditions you expect to change in more detail?
Dear Chris,
Thanks for you reply. Since you asked, I will expand my answer... despite I am afraid it would be quite useless to you, since "the final output [of your analysis is] relatively insensitive to initial calibration conditions" then you should not worry if O2 concentration is 20.89% or 20.91%.
An important factor is (A) reduced atmospheric mixing, as you correctly said. Three other major factors are: (B) local anthropogenic activities -especially fuel burning, waste incineration, road transportation, harbours...- occourring within few km from your observation point; (C) oxygen emission from large water bodies (see, large lakes or rivers) which are subject to daily and seasonal fluctuations related to changes in temperature and biological activities; and of course (D) photosynthesis especially if your observation point is close to a large forest. All these factors are then influenced by wind direction, atmospheric vertical mixing, and meteoclimatic conditions.
If you are interested, good references might be:
- C. Sirignano et al.: O2 and CO2 observations at Lutjewad and Mace Head 2000–2005 Atmos. Chem. Phys., 10, 1599–1615, 2010
Article Seasonal and short-term variations in atmospheric potential ...
- Keeling et al. GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 12, NO. 1, PAGES 141-163, MARCH 1998 Seasonal variations in the atmospheric O:/N: ratio in relation to the kinetics of air-sea gas exchange
- https://doi.org/10.1029/1999GB900054
Best regards
Valerio
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