Air Quality Index (AQI)

I received the feedback that my recent Blogpost called Smog as a Romamce  was too romantic and that I either protect the interest of the Automobile Industry or just don't know what I talk about. Well, I think I know about smog as well as about romance, but I prefer only to follow up on the science part of smog here.

Further, I found that there are very strong opinions on smog which reach from "we have to move out because my child has asthma" to "when I was a child it was even worse and see how strong I am today". Many of them are much less founded than my attempt to see smog romantically. I do not want to promote smog of course. Blue sky can be romantic too. And think how beautiful a clear spring day is. I am just making the best out of it, and I feel I personally do more against smog (not just in this city) than many others who complain a lot more than me.

In pubs and coffee shops I found that emotions can get very high on the Air Quality Index number published via various internet channels for Beijing. And no matter how strong the opinions are on the numbers published on Beijing's air quality, I found that many people do not know what the AQI (Air Quality Index) actually represents. It is a pleasure for me to spend my Saturday night, trying to bring a bit of light into this haze.

There are currently in Beijing two two public data sources: one is the Air Pollution Monitor in the American Embassy and the second is a network of measuring stations provided by the Beijing Environmental Department. Both are measuring about the same critical components in a slightly different manner and different locations. After a heated (and silly) debate whether particles should be measured according to 10 micrometers (PM 10) or the size which actually more likely to penetrate the deeper breathing system PM 2.5, now also the Beijing government measures PM 2.5.

The AQI (Air Quality Index) as defined by the USEPA (United States Environmental Protection Agency) is a composite index of five air pollutants: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

Ground level ozone, also called Tropospheric Ozone to distinguish it from the Stratospheric Ozone, is the product of photochemical reactions of NOX, CO or VOC (Volatile Organic Components). By the dependence on sunlight, it means that the concentrations are highest during daytime and in summer. As the process needs time, it also means that the high concentrations of ozone might be quite a distance downwind the actual emission source of NOX. This is why you often observe botanical damages caused by ozone not in the cities itself, but in the suburbs, where the air should be "cleaner".

The production of ozone includes two steps. First a peroxyl radical is formed by oxidizing carbon monoxide into carbon dioxide:

OH + CO → H + CO2
H + O2 → HO2


Then NO2 goes in photolysis to provide atomic oxygen for the formation of ozone:

HO2 + NO → OH + NO2
NO2 + hν → NO + O
O + O2 + N2 → O3 + N2


The resulting net reaction is: CO + 2O2 → CO2 + O3

Ozone is known positively for example as a disinfectant for drinking water, and for protecting us from harmful ultraviolet radiation by absorbing it in the Stratosphere. But in direct exposure of the respiration system and eyes it is causing irritations and also more serious long term health effects.

There are also other photochemical substances which are built by similar but more complex processes from primary pollutants. An example are the Peroxyacetylnitrates (PAN). They are more toxic than ozone, but hard to measure. However, as the process and the primary pollutants are similar, you can see ozone as a good tracer for the existence of PAN and its derivatives.

The next component which is contained in the AQI is Particulate Matter (PM). The kind of health effects which are caused by particles is mainly determined by their size and what they actually are. While most particles at around 10 micrometer diameter (PM 10) do not penetrate the lungs, particles smaller 2.5 micrometer (PM 2.5) might even cross into the blood stream via the alveoli, where they can cause cardiovascular diseases. Particles smaller than 0.1 micrometer even can penetrate cells, including those of the brain, where they cause damages leading to brain diseases similar to Alzheimer's Disease. A major source of such small particles are fumes and smokes, for example also emitted by low quality Diesel engines.

The next two components of the AQI we already met as primary pollutants: CO and NOX.

CO is toxic by itself and is the gas which you might know from being used to commit suicide in a garage where somebody keeps the engine running in an enclosed environment while writing a farewell letter - until he/she falls asleep and dies. Of course the concentrations we find in an open environment are much less and in open air CO coming from an exhaust pipe is unlikely to be lethal, but it may modify the oxygen household in the body. Eventually, CO turns into CO2 and by this is less a local problem but adds to the atmospheric concentration of greenhouse gasses which are driving climate change.

NOX again are a primary pollutant for photo-oxydants like Ozone or PAN but also in the end form gases which dissolve in water, similar to sulfur dioxide and have a negative impact on vegetation. Sulfur dioxide even forms acids in rain water which can dissolve limestone and damage buildings. This is less an issue in Beijing with modern buildings, as they are made of the good "old" brutalist concrete. But for some historical buildings this is still a disaster, not to talk about soil and vegetation.

So, these are the component for the AQI, which is an additive index of linear concentration functions. This means, if ozone is low at night and the AQI still high, you can imagine how bad particulate matter or sulfur dioxide is - specially in winter when cheap sulfur rich coal it doing its job to keep us warm. Isn't that romantic again? But this time I won't go down that road of argument.