Why the air is dirtier at nine in the evening than at rush hour

Most winter evenings#

Over the five winters with hourly fine-particle measurements, from 2021–22 to 2025–26 (November to February), the median of the Bucharest and Ilfov stations was lowest at 3 p.m. (17.3 µg/m³) and highest at 9 p.m. (28.5 µg/m³). The morning rush hour barely shows in the particles: 23.1 at 8 a.m. Nitrogen dioxide, the traffic gas, has two peaks, morning and evening.

The weather makes the difference. On the 46 calm nights, with the air mixed through at most 60 m according to the model and wind of at most 1.5 m/s, the station median at 11 p.m. was 43.0 µg/m³, about twice the 2 p.m. median for the same nights. On the 82 nights when the model had the air mixed through at least 400 m, it was 11.5. Same hour, same city: 3.7 times as many particles on calm nights.

The lid, measured by balloon#

The Băneasa weather balloon is launched every night for the 2 a.m. observation and records the temperature as it rises. Of the 824 winter nights with a sounding, 77% had warmer air above than near the ground; on 51% the difference was at least 3 degrees. At midday a difference that large appeared on only 8% of days.

The warmer the lid, the dirtier the air under it. At 2 a.m. with no lid the station median was 10.9 µg/m³; under a lid of more than 8 degrees, 37.5 µg/m³, 3.4 times as much. The chart covers 404 nights, those on which at least five fine-particle stations were measuring.

The ERA5 model and the balloon do not always agree. The model shows a mixed layer of at most 60 m on 24% of nights; the balloon shows a lid of at least 3 degrees on 51%. The two thresholds give the same answer on 65% of nights. When the model sees the lid, the balloon confirms it 83% of the time. The model works in cells about 30 km across and does not treat the city apart from the fields around it, so its heights are an estimate.

The median mixed layer fell from 485 m at 2 p.m. to 110 m at 11 p.m., about four times lower. Over the same hours fine particles rose 1.53 times. The air is not a closed box: fewer cars run at night, and some of the smoke settles.

Where the evening smoke comes from#

Far fewer cars run on Sundays. To compare Sundays with weekdays in similar weather, the calculation sets side by side only hours that fall in the same ranges of mixed layer, wind and temperature. In similar weather, nitrogen dioxide is 39% lower on Sunday mornings and 18% lower on Sunday evenings. Evening fine particles show no clear difference: on Sundays they are 0.98 of a weekday's level, with a plausible range from 0.84 to 1.13. The exhaust gas falls on Sundays; the evening particles do not clearly follow. The calculation does not measure how much of the smoke comes from traffic.

The station map does not point to the boulevards either. In Bucharest and Ilfov, the median evening rise over the afternoon is 10.5 µg/m³ at the 14 background stations and 6.8 at the 11 traffic stations. The five stations with the largest rise in particle concentration are: B-27 — Primăria Voluntari; B-23 — Școala 161, Giulești; B-15 — Școala 39, Colentina; B-25 — Grădinița nr. 4, Sintești; and B-28 — Glina. Three of the five are in Ilfov, outside Bucharest.

"When there is burning in Sintești, the smoke cloud comes from the Berceni side," said the former head of Romania's Environmental Guard in 2021. Sintești is a village south of Bucharest known for illegal waste burning; Berceni is the southern district between it and the centre. The data here have no record of fires, so they cannot confirm or rule out any one episode. A comparison by wind direction between 8 p.m. and 2 a.m. shows that in the 66 hours with a southerly wind (0.8 to 4 m/s), the southern stations, including the one in Sintești, read 0.80 of the northern ones; in the 208 hours with a northerly wind, 0.97. Short bursts of smoke can be lost in an hourly average, but the data do not show the south wind bringing the dirtiest air.

The chimneys of the CET Vest and CET Sud heat-and-power plants are 150 to 180 m tall, higher than the level where the balloon found the warmest air that night (146 m). The data here do not show where their smoke went.

The same hour across the country#

Of the 112 stations in the country, the analysis uses PM10, particles up to 10 micrometres across, at 78; 34 have enough PM2.5 measurements. Each station has between two and five winters of hourly measurements. 98 of 112 stations measure their dirtiest winter air between 7 and 11 p.m., and 108 between 6 p.m. and midnight. Only four peak in the morning: two in Brașov (BV-1, BV-2) and two in Galați (GL-3, GL-4). During the day, 106 have their cleanest air between 1 and 4 p.m.

It would seem natural for evenings to be worst where many homes burn wood. The 2021 census gives, by county, the share of homes that report heating with wood, coal or other solid fuel (a home can report more than one kind): from 1.8% in Bucharest to over 60% in Suceava, Botoșani or Vrancea. Between that share and how much the particles rise in the evening there is no clear link (correlation -0.12 across 34 counties). The census counts homes across the whole county, though, not around the station.

The shape of the land is more closely linked to the rise. For each station, the height of the ground was read at 16 points on a circle 3 km around it. At the third of stations where that circle sits highest above the station (46 m, the group's median), evening particles are 1.6 times the afternoon level; at the lowest third, 1.4 times (correlation 0.42 across 112 stations). Cold air slides down the slopes and collects in the valleys.