I'm reading a paper about the atmospheric chemistry on Titan {1}. It quotes "column photochemical and loss rates" for various species in units of $\mathrm{cm^{-2}s^{-1}}$. These figures are the overall rates at which these species are generated photochemically according to a model, and I would like to convert them into units of $\mathrm{mol/s}$ or $\mathrm{mol/s/m^2}$.

However, I don't know how to interpret the given units of $\mathrm{cm^{-2}s^{-1}}$. It feels like it should be something like CCs of gas produced per $\mathrm{cm}^2$ of the planet's surface area - but that would be $\mathrm{cm\,s^{-1}}$ rather than $\mathrm{cm^{-2}s^{-1}}$. No matter how I look at it I can't see where the $\mathrm{cm^{-2}}$ comes from - is anyone able to explain this?

{1} P. P. Lavvas et al. (2008) Planetary and Space Science 56, pp. 67-99.

  • $\begingroup$ Please do not use markup in the title field, see here for details. $\endgroup$ Apr 14 '15 at 10:03

The "strange" unit describes the photon flux, which is the number of photons (of unspecified wavelength) that hit an area (thus $\mathrm{cm^{-2}}$) in a time interval (thus $\mathrm{s^{-1}}$).

It feels like it should be something like CCs of gas produced […]

Not really. Not every photon leads to an excited state, not every excited state triggers a chemical reaction.

  • $\begingroup$ Hmm, but the figures for "net production" of the various compounds are also quoted in the same unit. Surely net production must mean "amount produced", in some units, per unit volume or area? (It is the net production rates, in this sense, that I want to know.) $\endgroup$
    – Nathaniel
    Apr 14 '15 at 9:08
  • $\begingroup$ @Nathaniel I can't see the figure, there's a building between your desk and mine. In general, the $\mathrm{cm^{-2}\cdot s^{-1}}$ typically refers to a flux. $\endgroup$ Apr 14 '15 at 9:17
  • $\begingroup$ I agree that for the "amount of compounds" produced, a mass flux might be a more reasonable value. $\endgroup$ Apr 14 '15 at 9:27
  • 2
    $\begingroup$ I guess in this case it's molecules per cm^2 per second - I was just confused because I expected the flux to be in moles or volume, and because the numbers are a lot smaller than I thought, so I didn't suspect it to be in molecules. $\endgroup$
    – Nathaniel
    Apr 14 '15 at 11:22
  • $\begingroup$ @Nathaniel I'm not really familiar with the Titan's atmosphere and the (photochemical) reactions in it. But since neither mass, nor moles are given in the flux, number of molecules sound plausible. $\endgroup$ Apr 14 '15 at 11:27

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