How to understand the bizarre high level for both PM2.5 and Ozone

Question

Background

Eastern China now (2017.01) are suffering severe haze with striking concentration of PM2.5.
Haze, with low visibility and complex chemical components threatens billions of persons' health.

But when I look through the pollutants concentration all over the country, I found some phenomenon interesting.

General

Here is a snapshot of PM2.5 concentration distribution map of eastern china (clipped from aqistudy.cn).

The circle with red and purple color represent the high ground-level loading of PM2.5.

From the figure above, we can identify the locations with high PM2.5 conc.

Meanwhile, I also clipped the O3 concentration distribution at the same time.

From the figure above, we can observe that O3 pollution are not severe in haze zone.
Actually, the conc of Ozone are lower in the area which the PM2.5 conc are higher.

The real-time data are also presented here a supplement. I choose Beijing, Xi'an (two large city in China), Zhoushan(an island in Eastern China Sea), Sanya(nearby the southern China Sea)

City     | PM2.5 |   NO2  | SO2  | O3
-------------------------------------
Beijing  |  229  |  109   |  30  |  7  
Xi'an    |  381  |  147   |  34  |  12 
Zhoushan |  20   |  28    |  14  |  70 
Sanya    |  16   |   9    |   2  |  77  

 ###  units: ug/m3

IMHO, the high conc of PM2.5 and low conc of O3 can be explained as follows:

High loading of Aerosol -> strong extinction effect -> week solar radiation -> O3 formation slow down

Here is another figure I clipped from Cheng et al. Sci. Adv. 2016. It depicts the NO2 and O3 conc in different PM2.5 level.

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An Special case

Look at the city of Anqing (30.5319° N, 117.1151° E), its PM2.5 and O3 concentration has a strange phenomenon. I clipped some figure of its air quality status (2016-01-04 local 12:00:00)

Fig 1 Anqing City and Chizhou City

These two nearby cities are in the distance less than 60 Km, has the similar PM2.5 conc with different O3 conc

Fig 2 Anqing City and its surrounding area

What attract me is the high conc O3 of Anqing City comparing with its nearby area. And I found this feature is rare in China.

Anyone has some theory to explain that? Any discussion or advises would be great!

Answer 1

I think it is important to keep in mind the mechanisms that form PM$_{2.5}$ are different than the mechanisms that form O$_3$.

From the data your provided you can draw some simple conclusions:

• Increasing SO$_2$ increases PM$_{2.5}$
• Increasing NO$_2$ increases PM$_{2.5}$
• Increasing NO$_2$ doesn't necessarily increase O$_{3}$

The last point is what you seem to be struggling with. I think what you see here is 'the weekend effect' where lowering the NO$_2$ concentration increases O$_3$ concentration. In this effect, the generation of nitric acid and the destruction of O$_3$ by NO ($\ce{O_3 + NO → O_2 + NO_2}$) decreases the ozone, but increases NO$_2$. I think this explains the phenomenon seen.

Answer 2

Ozone is most closely correlated to temperature. Yes you need NOx and VOCs to create ozone, but you also need enough photolysis driven by sunlight in order for ozone to form. The day analyzed in the question is during the middle of winter, which generally does not have enough sunlight to allow ozone to build up in the atmosphere. Furthermore, when there are high NOx concentrations (and no sunlight), ozone will titrate out of the atmosphere. In contrast, PM2.5 generally reaches it's highest concentrations during the winter (or at night) when the mixing layer is shallow due to cold conditions, which traps particles close to the surface.

There are exceptions to these rules though they are rare. The exception for ozone is when you have very large VOC concentrations (e.g. near an oil field) over a snow layer, because sunlight reflects off the surface and can lead to twice the photolysis. The exception for PM2.5 is when there are very large sources (e.g. a wildfire) and you get directly within the path of the plume.