Poor air quality – what's the cause?

Here's a graph from the government that gave me a shock. It shows that the pollution we generate in Hong Kong fell significantly between 1990 (on the left) and 2007 [1]:

How can that be, when we all know that air quality has got much worse?

Compare it with the Batgung Smog Index[2] for the same years:

In the early 1990s, as we produced more pollutants the amount of smoggy weather (the red line) also increased. But in the mid 1990's there was an important change: we cut our emissions of pollutants, yet the amount of smog just kept on rising.

So since the mid-1990s, we've lost control of the smog. It now depends on levels of pollutants that are produced outside Hong Kong, ie in southern China.

But we still see claims that local emissions are the source of our air-quality problems:

  • HK Magazine, June 12, 2009
    At the beginning of this month, levels recorded at all 11 general monitoring stations were above World Health Organisation (WHO) guidelines. This despite the low-wind [sic] speeds recorded by the Observatory, which Greenpeace campaigner Prentice Woo Wai-Muk said revealed that it was surely being produced locally.

  • Albert Cheng in his SCMP column, 15th Aug 2009
    Hongkongers have for many years blamed the city's poor air quality on the polluting factories on the other side of the border. Indeed, many of these factories have suspended operation since the economic downturn began last year. But, despite this, our air quality remains atrocious, which has unequivocally proved that the pollution is local.

I think we're disagreeing because we're really talking about two different things. Let's clear that up first, by turning to the government for their description of the problem [3]:

The two greatest challenges are local street-level pollution and regional smog. Diesel vehicles, particularly trucks, buses and light buses, are the main source of street-level pollution. Smog is caused by a combination of pollutants mainly from motor vehicles, industry and power plants in Hong Kong and the Pearl River Delta.

Instead of talking about 'Hong Kong's poor air quality', let's be clear we're really talking about two different issues:

  • Local street-level pollution
  • Regional smog

If we can agree on that, we can move past the argument about whether our air-quality problems are caused by local, or regional pollution: Street-level pollution is a result of local, diesel vehicles, while the smog is mainly caused by pollution from outside Hong Kong.

Then on to the important question, which of those two should we fix first?


  1. These are the government's estimates of Sulphur Dioxide (SO2) produced in Hong Kong over the years 1990-2007. The graphs for other pollutants vary slightly in shape, but all end at much lower values in 2007 than in 1990. See them all here. 
  2. This line shows the Batgung Smog Index, derived from the Observatory's 'number of hours of reduced visibility' figures. Full details
  3. GovHK: Air Quality in Hong Kong


blow away the smog...

Hi!  Seems that instead of putting in a windfarm out in the ocean, HK can put in giant fans to blow away the pollution from across the border.  ;)  (Sorry, I couldn't resist the joke).  But, to be serious, it seems that the priority should be for a local cleanup (no matter where you are in the world) since it seems intuitively to be easier to start at home and provide an example for the neighbours, who hopefully will take note.  Sometimes externally-produced pollution just can't be controlled (as from natural fires) or other-jurisdiction-produced smog.  Hopefully, by the example of conservation and implementation of alternative power producing methods, others would see that they would not be denying themselves of a "modern" lifestyle but rather they would be embracing an even more modern lifestyle that is on the cutting edge in terms of energy production, consumption and mindset...

re: blow away the smog

Hi Vinnie,

The 'windmill-as-fan' scheme had crossed my mind too!

it seems that the priority should be for a local cleanup

Usually I'd agree - fix our own problems first, before telling others to fix theirs. But in this case I'm not so sure.

The graphs above show the results of over ten years local cleanup: lower emissions, but much worse smog. Would another ten years of local cleanup, and say a further halving of our emissions make any difference to the smog?

The other worrying point I get from those graphs is that our local emissions are now only a small part of the pollutants causing the smog. That even if we could cut our local emissions to zero, it would only cause a small reduction in smog.

Regards, MrB

Focusing on the 'local'

Thanks again, MrB, for another excellent air pollution story. I wish the SCMP would contact you instead of relying on someone who's as painfully clueless as Mr Cheng to comment on this issue.

I'm also disappointed by how uninformed the statement from Greenpeace appears to be. 'Low-wind' 'surely' reveals that HK's pollution is produced locally? How naive is that? Does Prentice Woo have even the most basic idea how air masses behave, and how the weather affects air pollution here? It seems not.

Sometimes I wonder, though, if it's naivete at all. I despair at environmental organizations that resort to half- or untruths aimed at shaming people into altering their behaviors, even if those alterations are helpful. The ends cannot justify the means; to agree to this is to approve of propaganda.

How much more helpful might it be to tell Hong Kong people the full, direct truth for a change? That our efforts have made a real difference over the years?

If you keep piling on the doom and gloom and denying that any progress has been made, many, many people simply say 'to hell with the whole thing'. I am a conservationist at heart, but I find myself deeply frustrated with many aspects of environmentalism, often for this very reason.

Air Pollution

Many of the factories in the mainland generating the "imported" smog are owned by Hong Kong companies. - chickens coming home to roost?

re: Air Pollution

Yes that's part of the problem - that when we cut our local emissions of pollutants, in some cases we just moved them across the border. And a wind from the north brings them right back to us again!

RE: Air Pollution

It also means that the HK locals (who own the factories) have a vested economic interest in not curbing the pollution.

You can argue that they should have a vested interest in their (and our) health, but we all know the reality is that money is the deciding factor here.

And it's not all HKer's. There are a lot of Taiwanese owned factories there too.

mountains trap photochemical smog

I agree...cutting down on emissions (via regulations on auto industry, switching from high sulfur coal to low sulfur coal, adding 'scrubbers') won't get rid of the smog.  (And of course, blowing it away is out of the question!)

As Hong Kong is mountaineous, "thermal inversions" occur: cool air is trapped below a layer of warm air, and cannot be released back into the upper atmosphere... instead it just accumulates.

Develop our way to cleaner air?

The standard environmentalist argument is that population growth and economic development means more factories, more cars, more electricity generated by burning coal, etc. This is absolutely true.

The next step in the argument is to assume that this development means that pollution must inevitably increase along with the population and economic growth. This is absolutely not true.

Take a look at the following two charts, which come courtesy of the USA's Environment Protection Agency, the government department charged with monitoring and regulating air pollution in my ancestral homeland. The charts come from the EPA's latest annual report (hat tip to the Hot Air blog), which was released recently without much fanfare. The first chart shows how rapidly the USA's GDP, population, car miles driven, energy use, and other signs of economic development have gone up in the past 40 years -- while at the same time, its air quality has gotten far better. Check out the green line at the bottom of the graph that traces out the levels of six major air pollutants (you can click on the chart for a close look if you like):







The next chart zeroes in on the past 20 years; the improvements are ongoing and still quite sharp:







The point here is obviously not to say that growth will simply result in cleaner air. A lot of careful monitoring, regulation and especially technological development of cleaner-burning engines and power plants have gone into this improvement. But quality of life factors such as air pollution are only taken seriously when a country is rich enough to take them on. 

I do hope China is reaching that point. 

Mr Tall

Export our way to cleaner air?

Mr Tall,

Have you seen any figures that would give an idea how much of the pollution has been moved overseas? eg as manufacturing has moved from the US & Europe to China, its messy by-products have moved here too.

I wonder if the big importers will ever apply pollution guidelines to their suppliers?

They pay attention to the finished products, trying to avoid health problems for their customers (think lead paint on toys). And they pay attention to their suppliers' working conditions (eg no child labour). So how about paying attention to the effect suppliers' pollution has on their workers? (And on people that live nearby, like us!)


Where does Hong Kong's air come from?

You know how your mother used to say (or maybe you say yourself now to your own kids): 'Put down that filthy thing you found in the street! You don't know where it's been!'

Well, speaking of filthy things, there's Hong Kong's air on many days. The HK Observatory now has an extremely helpful little feature that shows us just where Hong Kong's air has been before it reaches us. It's a series of maps that tracks the trajectory of air masses in the three-day period before they arrive in Hong Kong.

You can find the daily map here, and combined monthly maps here. The HKO also provide a nice little tutorial explaining how these maps relate to Hong Kong's weather.

These maps should be very useful indeed in finding correlations between Hong Kong's wind directions (which control where our air's coming from) and the city's pollution levels.