{"id":27768,"date":"2026-01-20T21:52:07","date_gmt":"2026-01-20T13:52:07","guid":{"rendered":"https:\/\/smartairfilters.com\/?p=27768"},"modified":"2026-01-20T21:53:04","modified_gmt":"2026-01-20T13:53:04","slug":"why-air-pollution-is-oddly-worst-at-night","status":"publish","type":"post","link":"https:\/\/smartairfilters.com\/en\/blog\/why-air-pollution-is-oddly-worst-at-night\/","title":{"rendered":"Why Air Pollution is Oddly Worst at Night"},"content":{"rendered":"

It is common to think that nighttime should have lower PM2.5 since there are fewer car and industrial emissions in general. However, this is not the case.<\/p>\n\n\n\n

There\u2019s a great source of data to help answer this question: the US Embassy. The US Embassy (and its consulates in four cities around China) publish hourly PM2.5 readings. They also make all of their historical data freely available.<\/p>\n\n\n

\n
\"US<\/figure><\/div>\n\n\n

Why is Air Pollution Worst at Night?<\/h3>\n\n\n\n

I\u2019ve analyzed that data<\/a> to find patterns in pollution, such as the finding that PM2.5 in Beijing tends to spike at night:<\/p>\n\n\n

\n
\"Beijing<\/figure><\/div>\n\n\n

That\u2019s the opposite of many people\u2019s intuition (including mine). I asked 139 people to guess when PM2.5 is the lowest on average, and the most popular answer was nighttime. So the data doesn’t agree with most people\u2019s intuition.<\/p>\n\n\n

\n
\"\"<\/figure><\/div>\n\n\n

Is Winter Air Pollution Actually Worse?<\/h3>\n\n\n\n

But this question is about different months of the year, so let\u2019s see that data. I analyzed data from 2008\u20132015 for different months of the year, and here\u2019s what I found:<\/p>\n\n\n

\n
\"monthly<\/figure><\/div>\n\n\n

On average, PM2.5 is the worst in February and other winter months. Here it is all averaged out from winter and summer:<\/p>\n\n\n

\n
\"PM2.5<\/figure><\/div>\n\n\n

Is this just a Beijing phenomenon? The data from Chengdu shows a similar pattern:<\/p>\n\n\n

\n
\"Chengdu<\/figure><\/div>\n\n\n

Why is Winter Air Pollution Worse?<\/h3>\n\n\n\n

I said this data could give us a clue as to why winter air is worse. But isn\u2019t it just showing that winter air is worse? Here\u2019s the key:<\/p>\n\n\n

\n
\"Harmonic<\/figure><\/div>\n\n\n

This is the mixing height of air in the day time (black) and night time (gray) throughout the year. This data is for Delhi, but it should be roughly similar in most other places in the northern hemisphere.<\/p>\n\n\n\n

When air mixes over a larger area, those pollutants get dispersed. But when air is stuck close to the ground, those pollutants are thicker. That low mixing height can be particularly obvious in extreme cases of \u201cinversions,\u201d where cold, heavy air sits on top of warmer air near the surface:<\/p>\n\n\n

\n
\"mixing<\/figure><\/div>\n\n\n

Mixing height is lower in the winter and at night, when air is cooler and winds are calmer on average.<\/p>\n\n\n\n

Of course mixing height is only one factor. In the winter, people burn fuels to stay warm. I\u2019ve talked to construction workers next to my apartment in Beijing who told me they had to make a lot of noise after midnight because that\u2019s when they could get their big trucks past the police, who aren\u2019t supposed to let unpermitted heavy trucks into the city. But mixing height plays an important role.<\/p>\n\n\n

Bottom Line<\/div>
Bottom Line: Why Air Pollution is Oddly Worst at Night<strong><\/strong><\/div>
<\/p>\n

This mixing height could at least partly explain why PM2.5 in Beijing is the worst at night and in the winter.<\/div>

Smart Air<\/div><\/div>","protected":false},"excerpt":{"rendered":"

It is common to think that nighttime should have lower PM2.5 since there are fewer car and industrial emissions in general. However, this is not the case. There\u2019s a great source of data to help answer this question: the US Embassy. The US Embassy (and its consulates in four cities … <\/p>\n

Read more<\/a><\/p>\n","protected":false},"author":1035,"featured_media":27770,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_relevanssi_hide_post":"","_relevanssi_hide_content":"","_relevanssi_pin_for_all":"","_relevanssi_pin_keywords":"","_relevanssi_unpin_keywords":"","_relevanssi_related_keywords":"","_relevanssi_related_include_ids":"","_relevanssi_related_exclude_ids":"","_relevanssi_related_no_append":"","_relevanssi_related_not_related":"","_relevanssi_related_posts":"6631,13492,6248,22029,4921,28722","_relevanssi_noindex_reason":"","footnotes":""},"categories":[17,452,257],"tags":[272,278,74],"class_list":["post-27768","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-air-pollution","category-air-quality","category-pm2-5","tag-air-pollution","tag-air-quality","tag-pm2-5","resize-featured-image"],"acf":[],"_links":{"self":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/27768","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/users\/1035"}],"replies":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/comments?post=27768"}],"version-history":[{"count":5,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/27768\/revisions"}],"predecessor-version":[{"id":40306,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/posts\/27768\/revisions\/40306"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media\/27770"}],"wp:attachment":[{"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/media?parent=27768"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/categories?post=27768"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/smartairfilters.com\/en\/wp-json\/wp\/v2\/tags?post=27768"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}