How Does Yellow Sand and Air Pollution Travel from the Continent to Japan?

1. How Does Sand from Deserts Reach Japan’s Sky?

Some days, the view outside your window looks yellowish and hazy. This is called "Kosa" or yellow sand. The homeland of yellow sand is in dry regions inside the Eurasian continent, such as the Taklamakan Desert, the Gobi Desert, and the Loess Plateau [2]. Strong winds create dust storms that lift soil and mineral particles into the air, where they ride on upper-level winds. According to Ministry of the Environment materials, most yellow sand reaching Japan consists of particles about 4 μm (0.004 mm) in diameter. Small particles under about 10 μm are easily carried far away [2]. Some yellow sand born in Northeast Asia is said to cross the Pacific Ocean all the way to North America [2].

One carrier of this sand is the westerly wind. The Japan Meteorological Agency defines this as a global belt of wind blowing from west to east around the poles. Within this, there is a particularly strong band of wind called the jet stream near 10 km (about 6 miles) altitude. In cold seasons, the central wind speed is 50–100 m/s, which is 180–360 km/h (about 110–220 mph) [3]. However, how particles arrive is not decided by westerly winds alone; it also changes depending on the wind direction and pressure patterns at that time. So, is only sand carried over? No, other things come too.

2. Does Yellow Sand Carry Pollution from the Continent?

From yellow sand particles, scientists have found ammonium ions, sulfate ions, and nitrate ions. The Ministry of the Environment states that these particles may pick up air pollution substances produced by human activities during transport, changing their properties [2]. However, this is still at the stage of possibility.

PM2.5 refers to tiny particles with a diameter of 2.5 μm or less (0.0025 mm or less). Yellow sand also contains particles of this size [2]. In the winter of 2013, high values continued mainly in Western Japan. When the National Institute for Environmental Studies analyzed national data from January 1 to February 5, they found that on 16 days, at least one monitoring station exceeded the environmental standard of a daily average of 35 μg/m³ [4]. High concentrations were noticeable in Western Japan, such as northern Kyushu and the Seto Inland Sea, showing a pattern where pollution is higher in the west. Cross-border transport from the continent was considered a cause. It was also pointed out that in metropolitan areas, cross-border pollution might have overlapped with urban pollution [4]. In response, on February 27, 2013, the Ministry of the Environment issued a provisional guideline for alerting people at a daily average of 70 μg/m³. This is not a legal standard [5].

This raises a question: If particles coming from the west are noticeable, does that mean most PM2.5 in Japan’s air comes from the continent?

3. Where Does PM2.5 in Major Japanese Cities Mainly Come From?

There is a 2017 estimate calculated by researchers from Japan, China, and South Korea using their respective models. For major Japanese cities like Fukuoka, Osaka, and Tokyo, the estimated sources of PM2.5 were: 55.4% from domestic sources in Japan, 24.6% from China, and 8.2% from South Korea [1]. These add up to 88.2%, meaning about 12% comes from other sources.

Looking at the average for cities in each country, domestic sources contributed the most in every nation. The contribution rate of domestic sources was 91.0% for China, 51.2% for South Korea, and 55.4% for Japan [1].

It is important to note that this is an annual average estimate for cities in 2017. The calculations have some uncertainty, and the percentages change if the year, location, or season changes. It is not a fixed number that says "one quarter of Japan’s PM2.5 comes from China." So how do differences by location look? Comparing Western and Eastern Japan shows different pictures.

4. Why Is Cross-Border PM2.5 Pollution Larger in Western Japan?

According to a literature review compiled by the Ministry of the Environment, the contribution of cross-border pollution to PM2.5 is said to be larger in Western Japan and smaller in Eastern Japan (excluding Tohoku and Hokkaido) [6]. However, this varies depending on the calculation method, target year, and regional division used in each study.

The same material suggests that due to measures in China, the contribution from cross-border sources may have decreased, making the relative contribution of domestic sources increase [6]. Measures in China are also visible in the numbers. A release from Kyushu University and the National Institute for Environmental Studies states that China’s sulfur dioxide (SO2) emissions dropped by 63% between 2012 and 2017, and nitrogen oxides (NOx) dropped by 25%. The annual average PM2.5 in Beijing decreased from 102 μg/m³ in 2013 to 43 μg/m³ in 2019, a reduction of about 58%. The decrease in SO2 is considered the main factor in this drop [7].

Interestingly, not just the "amount" but the "contents" of PM2.5 may also change. Observations and numerical models from Fukue Island in Nagasaki Prefecture report that the main components of PM2.5 are shifting from sulfates to nitrates. Because SO2 and NOx decreased at different rates, the balance of ammonia and acids changed, leading to an increase in nitrates [8]. This does not mean PM2.5 levels increased overall.

5. How Do Japan, China, and South Korea Study Air Pollution Together?

Japan, China, and South Korea continue joint research to measure the atmosphere across borders. The Long-range Transboundary Air Pollution in Northeast Asia (LTP) project began in 2000 at South Korea’s proposal. Its fourth phase ran from 2013 to 2017. Japanese observation points were Rishiri and Oki, with a total of 12 sites across the three countries. The annual average values for SO2, NO2, PM2.5, and PM10 show a decreasing trend in recent years, with significant improvement seen especially in China [1].

For acid rain, the East Asian Acid Deposition Monitoring Network (EANET), proposed by Japan, began full operation in 2001. At the time of the Ministry of the Environment’s announcement, there were 13 participating countries [9].

Surprisingly, yellow sand observation was done by meteorological station staff looking at the sky with their eyes. If they judged that yellow sand particles were floating in the air, they recorded it. This was done at 11 domestic locations but ended on March 31, 2024, except for Tokyo and Osaka [10]. By combining human observation, machine measurements, and calculations between countries, a clearer overall picture is gradually emerging.

6. How Can You Check Where Today’s Air Is Coming From?

You can try this today: Look at the wind direction in the weather forecast and write down how the sky looks that day side-by-side. You can record wind direction and visibility of distant scenery for one month to compare them. However, because weather also affects visibility, you cannot decide the reason based on wind direction alone.

The Ministry of the Environment publishes information on yellow sand and PM2.5. Your local municipality may also provide measurements from nearby stations. Recording your area’s PM2.5 levels, yellow sand information, and wind direction on the same day can help find clues about changes.

All the numbers in this article are estimates that changed by year and location. When looking at percentages, checking "which year and region were studied, and who calculated it" leads to further questions.