Weather Bee | The curious case of clean air after midnight

Delhi saw unusual air quality patterns in February as pollution improved at night but worsened before sunrise, pointing to meteorological factors

Residents of the Indo-Gangetic plains are used to an intra-day pattern in air quality on dry and cool days — pollution levels worsen after sunset and improve after sunrise. However, data reviewed by HT show that on at least a few days this February, this trend was reversed at least in Delhi. Air quality improved as the night progressed but began deteriorating as sunrise approached, with pollution levels continuing to worsen even after daybreak.

Air quality improved as night progressed and deteriorated as sunrise approached, the deterioration continuing beyond sunrise. (Unsplash)

This curious intra-day pattern cannot be explained by the pollution load in the capital or its surrounding areas this February. Most prominent sources of pollution remain active during the day throughout the year, and that rarely leads to days becoming more polluted than nights, at least while winter is not completely over.

This suggests that the culprit (or benefactor) in this whodunnit is likely to be a meteorological factor. An HT analysis indicates that this is indeed the case. Here is what the analysis found.

To solve this air quality mystery, one must first examine its pattern. For illustration, the hourly average PM2.5 concentration for Delhi is shown in the accompanying chart from 8 pm to 9 am on four days.

On two occasions — the intervening nights of February 3–4 and 10–11 — air quality either deteriorated almost continuously during the 13-hour period or worsened until midnight and then remained at roughly the same level until 9 am. On two other occasions — the intervening nights of February 7–8 and 11–12 — air quality improved from 9 pm to 2 am, remained around the same level until 5 am, and then began deteriorating.

gfx (HT)

To be sure, the latter two days were generally cleaner than the former two. However, that is a somewhat less interesting question, because inter-day changes in meteorological factors regularly improve or worsen air quality. The intra-day changes are more intriguing because cooler temperatures trap pollutants close to the ground, and it is very difficult for temperatures to rise as the night progresses.

The intra-day pattern of temperatures cooling as the night progresses is almost certain because the earth’s surface can only lose heat after sunset and gain heat from sunlight on a clear day. If that is not self-evident, the hourly temperature in Delhi is shown in the chart below.

Temperatures cooled continuously until 7:30 am — the hours are shifted by half an hour because the data were available in Greenwich Mean Time (GMT), which is five and a half hours behind Indian Standard Time (IST) — on all four days illustrated above, and rose thereafter. This is expected, as Delhi sees sunrise around 7 am at this time of February.

gfx (HT)

The chart above rules out temperature as the secret benefactor cleaning up Delhi’s air pollution at night. In fact, it also shows that temperature was only partly responsible for the inter-day patterns. One of the two polluted nights (February 10–11) was warmer than both of the relatively cleaner nights.

Since there has been no rain in Delhi other than 0.01 mm on February 1, the most probable benefactor is wind. Hourly wind speed patterns show that it indeed played a major role and explains much of the inter-day variation patterns. The cleaner nights were windier than the polluted nights.

Moreover, on the nights when air quality deteriorated at night as usual, wind speeds decreased substantially before increasing as sunrise approached. On both of the nights with cleaner air, Delhi was windier at night than in the morning.

gfx (HT)

To be sure, air quality sleuths will say that even wind is not a complete explanation. For example, the mornings of February 4 and 8 had nearly identical temperatures, and the wind speed on the morning of February 4 was between the speeds observed on the mornings of February 8 and February 12. Yet, air quality on the mornings of February 8 and February 12 was substantially better than on February 4.

Some of this could be due to the cleaning effect of wind on the preceding nights. However, it is worth examining whether humidity also played a role, since humid air is more capable of trapping pollutants. Data show that it did have an effect, with humidity on the morning of February 4 much higher than on the other two mornings. The humidity chart also adds another nuance to our understanding of why the two nights were cleaner than the following morning: higher humidity in the morning than at night.

gfx (HT)

Clearly, wind and humidity together produced the curious intra-day patterns we set out to explain. When considered alongside temperature, these two factors also better account for the inter-day patterns. This analysis highlights just how dependent the national capital is on weather for clean air. Consistently good air quality requires that we do not rely on mysterious meteorological benefactors, but instead focus on eliminating the sources of pollution.