A comparison of the time series of high-frequency PM2.5 counts shows that hazardous air quality levels were recorded within the Kathmandu Valley in Bhaisepati, Bhaktapur and Ratna Park areas. These measurements were also compared to air quality in cities across Nepal.
Air quality deteriorated to dangerous levels from the afternoon of 4 January till the morning of 6 January, whereas such counts were comparable to those recorded before and after the hazardous air level episode in Nepalganj, Dang, Pokhara, Hetauda.
A comparison of AQI PM2.5 daily average time series shows that their values on 4 and 5 January is comparable with the values on preceding days at the same stations outside the valley, whereas it dropped by 30% or more on 6-7 January for Nepalganj, Dang, and Pokhara stations.
However, the stations at Ratna Park and Bhaisepati recorded a 50% rise in pollution levels on 4 and 5 January, whereas the Bhaktapur station recorded only a 15% rise.
In contrast, the Nagarkot station recorded better air quality by about 40% or more from 4-7 January. Similarly, the concentration of ozone, nitrous oxides, carbon monoxide and black carbon-8 (soot particles) also came down by about 10% or more in Nagarkot during the period compared to preceding days, which is consistent with the reduction of AQI PM2.5 there.
It is worth noting that Nagarkot and surrounding regions on the eastern rim of the Valley are considered as the primary outflow region for wind-blown pollutants from inside the valley trough since upper-level wind is predominantly westerly across Nepal in winter. In fact, this was why the DHM selected Nagarkot as the site for establishing an air quality monitoring station.
The differences in the daily average for the AQI PM2.5 count for the stations in the Valley and that at Nagarkot are of particular interest. The measurements for 4-6 January could mean that the stations could be valuable in enhancing our understanding of the variation of ambient air quality in the valley over time, and the transport of pollutants.
Data from the radiosonde station at the premises of Tribhuvan University Kirtipur is being released regularly at 5:45AM since March 2019. Attached to a hydrogen balloon, the radiosonde measures temperature, relative humidity, wind speed and direction at different elevations and transmits the data to a ground station.
Globally, there are hundreds of such stations that make measurements once or twice a day, synchronously providing valuable insight into the global weather pattern at different altitudes.
Comparing the Kirtipur data on 1-5 January, there is a clear indication that ground-level inversion was present on both days. This happens when a layer of cold air is trapped by warmer air above it. Ground-level inversion occurs almost every day during winter in the Valley, and has the effect of concentrating overnight pollution.
The stronger the inversion, and deeper and stronger the stable layer, which makes it harder for a parcel of air to rise as it will be relatively colder, hence denser, than its surrounding. Consequently, it is squeezed downwards.
Hence, pollutants suspended in the bottom layer of the atmosphere are pretty much confined within the inversion layer. The shallower the depth of inversion, the higher is the concentration of air pollutants, even with the same amount of emission load, since they are confined in a smaller volume near the ground.
On most days during the winter, ground-level inversion is gradually established in the Kathmandu Valley from the evening and intensifies in the night, but it weakens as it is burned away by the sun during the morning, disintegrating by noon. Once the inversion decays, the bottom layer of air mixes with the higher levels of the atmosphere, leading to lowering of air pollution level at the surface.
In addition, as the wind picks up towards the afternoon, much of the air pollution is blown away to the east, leading to a reduction of the concentration of air pollutants in the Valley.
However, from the afternoon of 3 January till the morning of 6 January, Nepal was under the influence of a westerly disturbance, and the most parts of the country were covered by high clouds (Figure 3) with light scattered precipitation in the western parts of the country.