A grey sky, and a longer question.

Singapore had been looking grey when I brought up the air quality in October. It also made me realise how long it had been since I had stopped to think about Beijing’s history with haze.¶

I wanted to go beyond remembering the bad years. What changed, how much changed, and how should we judge the result? I asked about the history, the policies, the longer-term figures and how the experience had been discussed elsewhere.¶

One useful place to start is the measure itself. NEA publishes one-hour PM₂.₅ concentrations alongside the 24-hour PSI. One is a concentration, the other an index; a single hour and an annual average answer different questions.¶

The Beijing figures below give that longer view a concrete starting point: 89.5 micrograms per cubic metre in 2013 and 30.5 in 2024, a reduction of 65.9%. That is a substantial change in the annual mean. It is also a reason to keep the year, the unit and the original source attached to the number.¶

There is another distinction I want to keep visible as this site grows: a model forecast and a monitoring-station observation are different kinds of evidence. A model can help compare the same hour across cities, but its grid covers an area. It cannot tell me exactly what a particular street looked or felt like.¶

That is the starting point for the city observatory linked below. I can put Singapore beside Beijing, or look at cities connected to my studies and work, and move through a shared timeline. The source, retrieval time and local time stay beside the chart. The historical annual figures remain here, answering their own longer-term question.¶

I also wanted to understand how Beijing’s experience had been discussed elsewhere. A percentage change can start that conversation, but the policies, the period covered and the local context still deserve a closer look. The next useful question is often about how the result was reached.¶

This is one of the things I enjoy about following a question: an ordinary observation can turn into a much bigger piece of learning. The grey sky was the beginning of the conversation. Understanding what the numbers mean is where the investigation starts to become useful.¶