Another year, another winter. As winter arrives in Beijing and soon in Shanghai, I got to wondering: how much worse is winter air?
To get to the bottom of it, I analyzed the last eight years of US Embassy PM 2.5 data for Beijing and Shanghai. I found that the capital’s air has averaged 111 micrograms in the winter versus 92 micrograms for the rest of the year. Shanghai was a little better at 65 micrograms in winter versus 40 micrograms in summer.
Just how bad is that? The WHO 24-hour PM 2.5 limit is 25 micrograms (the year PM 2.5 limit is just 10 micrograms!). That means Beijing’s summers average three times the 24-hour limit, and winters average over four times the limit.
Don’t live in Beijing or Shanghai? Then don’t get complacent! This trend is the same across China:
When I started Smart Air in 2013, I wanted to buy a particle counter, and I had basically two options. I could buy a US$260 Dylos or spend thousands of dollars on the crazy expensive particle counters.
Since then, the market has exploded with new particle counters as cheap as 99 RMB. But are they any good?
Putting Particle Counters to the Test
To get to the bottom of it, we tested three popular particle counters on the market. We tested the Dylos DC1700, the Origins Laser Egg, and a new particle counter called the AirVisual Node.
The Laser Egg is the popular, more technologically savvy device.
And the Node is a fancier version, including a large screen, richer information, pollution forecasts, better user experience and even a CO2 monitor.
The Government Comparison
We placed the machines outside the Smart Air office on Dongzhimen Waidajie, about 1.3km away from the government PM2.5 monitor at the Agricultural Exhibition Center.
We ran the machines for six days. The Laser Egg and the Node give output in PM2.5 micrograms. The Dylos gives number of 0.5 micron particles, so we converted it to PM2.5 micrograms using the semi-official formula (0.5 microns – 2.5 microns)/100.
Here are the results for the first (72-hour) test outside our office in Beijing:
Next we tested on days with extraordinarily low PM2.5. That’s helpful because concentrations in homes—where most people use particle counters—are also typically low. So this data is good for testing how good the devices are at low concentration levels. We ran tests for 48 hours whilst the skies were clear.
Eyeballing both graphs, all three machines did a pretty good job of tracking the official numbers. Combining both tests, we found that both the AirVisual Node and the Laser Egg correlated r = 0.98 with the official PM2.5 numbers. For non-nerds, 0.98 is incredibly close to identical! The Dylos had the lowest correlation at r = 0.90, but still incredibly high (and similar to our previous test). These correlations are all extremely high and suggest that these particles counters are tracking government data well.
Another way to measure accuracy is to look at on average how far the numbers were from the government data. The Node was the closest: it was off from the official numbers by an average of 4.8µg/m3. The Laser Egg was consistently further than the government machine, with an average deviation of 6.5µg/m3. The Dylos was off by an average of 9.1µg/m3.
Perhaps one worry to note is that the Laser Egg was consistently under-estimating PM2.5 while air pollution was in the lower range. This means there could be a risk that the Laser Egg underestimates the real pollution levels in the home, giving a false sense of security. However, even these deviations were not large.
The Airpocalypse Test
To test accuracy at extremely high concentrations, we burned a cigarette in a closed 15m3 room. Our goal here was to see how well the particle counters were at reading concentration levels over a whole range of values, including toxic levels. With the help of cigarettes and a partner NGO in Beijing, we managed to get the concentration above 1,000µg/m3!
For this test we also has another machine (Sibata LD-6S) on hand as a reference. This is an industrial PM2.5 dust indicator, with an accuracy of ±10% and repeatability error of ±2%. Thus, we used the LD-6S as our baseline.
Looking at the data, it’s immediately clear that the Laser Egg and the Dylos had a hard time keeping up with these really high levels of concentration. In contrast, the Node and the LD-6S matched very closely, and were able to measure values over 1,000µg/m3. The chances of you needing to measure these values outside of experiments are very slim, but it shows that the Node is more accurate at these high levels.
Overall, the three particle counters were reasonably accurate compared to the government machines. In the estimation of the Smart Air team, all of them are suitable for giving an approximate AQI value in your home. Of all three, the Node scored the highest, with the lowest deviation from the government machines in both outdoor tests and the highest accuracy in the “crazy bad” test.
Since all three machines are reasonably accurate, the question then really comes down to: How easy it is to use the device? And what features do they have?
The Dylos (1800 RMB)
The Dylos easily loses this fight. It has no phone connectivity, and downloading the data is a terrible pain—and that’s if you have one of the old school pin connecter cables.
The Laser Egg (499RMB)
The Laser Egg is an entry-point particle counter. It gives reasonably accurate results with a simple interface. It’s not feature rich, but it does what it says on the box.
The AirVisual Node (988RMB)
To our eyes, the Node offers the best features. For starters, it can measure CO2, temperature, and humidity. That makes it more of an ‘environment monitor’ than just a particle monitor. CO2 is important if you have lots of people in a small space as it can give an indication of how confined the space is. If you have indoor sources of air pollution (VOCs) like new furniture or remodelling, high CO2 levels can mean that those indoor pollutants are building up. Its user design shows AQI and CO2 for the past 24 hours both indoors and outdoors, a forecast for the coming days, and suggestions to help you decide when to open your windows and wear a mask. We’ve found these features helpful in our office.
After passing our tests, we will start shipping the AirVisual Node through our Taobao shop and website. It’s a great option for anybody wanting a solid device for both home use and research (if you’re a nerd like us). Go take a look!
Over the next few months, we hope to get a larger pool of particle counters together and run more extensive tests. This is only the beginning! Once we’ve independently verified more devices, we may well be adding them to our shop as well.
Map of today’s pollution levels across China – 9th September 2016.
What a glorious day in Beijing! Right now, the US Embassy in Beijing is giving a PM2.5 value of 0. Is summer normally this good? And what’s the pollution like in other parts of China right now? (Short answer: not good! Long answer: read on!)
US Embassy Beijing’s Twitter account
A few months back we posted our analysis on the summer/winter variation in air pollution in Beijing. Using the US Embassy’s data for four more cities we’re able to paint a wider picture of the difference in summer and winter pollution levels across major cities in China.
This time around we’ve analyzed the US Embassy’s data for Shanghai, Chengdu, Guangzhou, and Shenyang. Using data from the past 7 years we have calculated each city’s pollution on a monthly and seasonal basis.
The result? Our analysis across these four cities confirmed the popular theory that summer air is better than winter air; PM 2.5 levels were on average 29% better in the summer across all cities.
It’s likely that during the winter months, air pollutants which would often disperse away from city centers remain locally confined due to inversion. Inversion is an atmospheric condition in which cold air is trapped beneath a layer of warm air close to the earth’s surface. Summer heat prevents this inversion.
Although summer pollution is “better” than winter, it’s doesn’t mean these levels are satisfactory or safe by WHO standards. The summer average across the Chinese cities we tested (60µg/m3) still exceeded the WHO yearly limit (10µg/m3) by 600%.
Of all the cities, the lowest summer pollution levels were seen in Shanghai and Guangzhou (49µg/m3, five times the WHO limit). The worst summer pollution levels (excluding Beijing) were seen in Chengdu. In fact, Chengdu’s winter average pollution levels are even worse than Beijing’s!
Pollution levels on a monthly basis:
We also plotted the average monthly pollution levels for all the cities with US Embassy data, these graphs can give a good idea of which cities have the worst pollution levels, and which months are the worst overall.
The above graphs show a clear annual trend in PM2.5 across each of the cities: pollution levels rise in “winter” months (October-March) and dip in“summer” months (April-September). July and August look to be the best months across most cities, although Beijing has a peculiar peak in air pollution levels in July – most likely due to the lack of wind to blow the pollution away. In fact, Beijing’s yearly variation in pollution is the smallest of all cities – it remains at a consistent average concentration above 80µg/m3.
December and January are consistently the worst months for pollution, which is most likely due to the burning more fossil fuels during winter for heating.
Smart Air Filters is a social enterprise that promotes DIY air filters as a low-cost solution to indoor particulate air pollution in China, India, Mongolia, and other countries where air pollution is causing health problems.