Thursday, June 28, 2007

NOx

Hans Erren was kind enought to point to an interesting site in the Netherlands with a lot of data on trace gas concentrations from various satellites. He said:

If you really want to get an impression of the distribution of antropogenic CO2 emissions you can use the spectral NOx proxy.
Eli took a look at the June 2003 (that's what came up on entry) NO2 map (yes, I know NOx is NO2 + NO + PAN) from the GOME satellite instrument and replied

NOx is a measure of high temperature combustion without trapping or catalytic conversion. It arises in situations where the temperature is high enough that N2 thermally dissociates to N atoms which then react with O2 to form NOx.

NOx is the principle precursor to tropospheric ozone. It will be reduced relative to CO2 in any location where there are legislated ozone limitations. Thus California, the US and Europe have relatively low NO2/CO2 ratios and the industrial areas of China are huge.


Rabett Run had just blogged on CO2 distributions using a direct global mapping so this did not appear that interesting. However, idle time being the curse of long ears, we googled back to the site that Hans had pointed to, and looked at the January distribution (actually a couple of years of monthly distributions). It turns out that the tropsphereic NO2 is much higher in the winter than in the summer. Why?

Well one of the first things you can think of is that there is almost twice as many hours of sunlight in the summer as in the winter (in these parts). That means that the first step in the cycle that creates tropospheric ozone

NO2 + hv --> NO + O(3P)

is much more likely to occur which depletes NO2. BTW, this requires light at wavelengths shorter than 400 nm, e.g. UV-A and UV-B light and there is more of that in the summer too. We discussed this with our friend who does air pollution modelling and that seemed like a good place to start, but, of course there is the problem that the rest of the cycle goes
  1. NO2 + hv --> NO + O(3P)
  2. O + O2 + M --> O3 + M
  3. OH + CO --> H + CO2
  4. H + O2 + M --> HO2 + M
  5. HO2 + NO --> OH + NO2
----------------------------
CO + 2O2 + hv --> CO2 + O3
where the last line shows the net reaction. The net is why CO is often taken as a proxy for tropospheric ozone, e.g you can only complete the catlytic cycle if you convert CO to CO2 by reaction with OH radicals! Otherwise you only have a single step and very little ozone generated smog. Step 5 regenerates NO2, so we don't quite have the answer.

There is one other thing we need to add, termination
OH + NO2 --> HNO3 (nitric acid which gets washed out.)
OH is produced by O3 + hv (<310> O(1D) + O2 followed by abstraction of an H atom from water vapor O(1D) + H2O --> OH + OH. The O atom must be in the first electronically excited state. There is much less OH in the atmosphere in the winter because there are fewer daylight hours and much less UV. That means less H atom generation (reaction 3) thus less HO2 (reaction 4), and a slower recycling of NO back to NO2 (reaction 5), but most importantly, in the winter there will be less conversion of NO2 to HNO3

This was such a pleasing discussion and change of subject (they are changing the accounting system at work, and we are all shell shocked) that we celebrated by going out and raising a glass of carrot juice to Hans.

1 comment:

Anonymous said...

It would appear that forest fires also produce NOx".