air pollution – Don't hold your breath

Getting used to extremes

On Sunday morning I woke to the laboured groan of the air purifier in our bedroom. There was a smell of dust in the air. When I drew the curtains I saw a wan light outside. I opened a website which gives the air quality index anywhere in the country and saw something shocking. At 6 AM the dust count index had suddenly maxed out at 500. I have an older purifier in the study. Within the hour it switched off; its filters had clogged due to the dust. Popular web sites were still calling the air quality reasonable, because they average the count over 24 hours. It would be a day before they noticed that there was something wrong. By then the daytime temperature had fallen to a new record for January in Mumbai, in a belated vindication of Carl Sagan’s 40 years old paper on nuclear winters. When I looked at satellite photos I saw two things: clouds and dust. There were masses of white clouds over the Himalayas and the Tibetan plateau, and over the Indian Ocean, but not much over the landmass of India. And there were long gray fingers of dust reaching across the sea from Iran, Balochistan, and the Arabian peninsula towards Mumbai, Goa, and Trivandrum. Everyone I knew was talking about it, but news channels wouldn’t pick up on it till late Monday.

Air Quality in Mumbai’s dockyards on Monday afternoon

The nature of the problem is shown in this screen shot of the air quality in the dockyards of Mumbai. You can see that gaseous pollution is not a concern, but the dust content of the air is as bad as it can be. Dust comes in two varieties: PM2.5 and PM10. The letters PM mean particles of matter suspended in air, and the numbers give the size of the particles measured in microns. As you can see from the graphic here, these are much thinner than a strand of human hair and can easily pass into the lungs. In the short term inhaled dust causes inflammation and allergies. In the long term it is more damaging (go here for more). We had a stock of N95 masks at home, so we put one on immediately. I surprised myself by thinking well of the pandemic for once.

News of a massive dust storm in Balochistan in the middle of the previous week had largely been ignored in the media. But you can see the roiling dust in the atmosphere in the satellite photo from Sunday. Long thin fingers of dust stretch down south from it and then curl eastwards towards India. Another branch stretches west across Pakistan and towards northern India. By the next day, the dust over the Arabian sea has been entrained into two lanes: one passing over Goa, the other over Trivandrum. The southern lane stretches across the Bay of Bengal to Odisha, Bengal and further east. Mumbai is at the edge of the lane. Meanwhile the main mass of dust has diffused over most of northern India. In Tuesday’s photo the whole of the Indian landmass has a thin dust cover, and the dust over the sea seems to be settling. In most parts of India the air quality was not as bad, but a general drop in temperature signaled the effect of dust high in the atmosphere.

When we talk of weather it is usually just the temperature and the various forms of water: fog, rain, snow, and hail. But I suppose we now have to broaden our mind to include anything carried by air. Dust storms of this kind, called the loo were not unusual in the northern plains between April and June one, although they have decreased in frequency over the years. But this is probably triggered by a different weather system: the extreme La Nina event this year in the east Pacific. It has already brought record snows to the Hindu Kush and the western Himalayas, will probably bring tsunamis to the west Pacific later, extreme hot weather to the US in summer, and enhance the monsoon rainfall over India. And the disturbance has churned up this storm. The event is said to be one extreme of the normal, but when the extreme is normal, what can you say about change?

Altitude

We took an hour’s walk inside Binsar National Park, a short climb to its highest point. This Zero Point, as it is called is at an altitude of just over 2400 meters. The cool air at this height smelt clean, with a flavour of green trees. The view at the top showed smoky valleys, and the high Himalayas were almost invisible because of the haze. But just around this part of the park the winds and the cool heights had together managed to confine the smoke below. I’m sure that the air here is usually much cleaner, but at that time it still felt better than city air. The walk through an oak forest was wonderful, and a great change from sitting inside a car all day. At this height you get Himalayan white oak (Quercus leucotrichophora), easily identified by the fact that the oval leaves with serrated edges are white on the reverse. The dry tree fern that you see in the featured photo is just one of many things which grow on oaks.

Coming back to our hotel, we felt the change in the air. Warmer, of course, now that we were half a kilometer lower, and also more haze. Fortunately there was no smell of smoke in the immediate vicinity. We were told that a short shower the previous day had put out fires locally, and cleared the haze a little. I admired a red sunset as I walked up the steep forested path from the road to the hotel. Pollution gives you interesting sunsets.

Bad days

To the right of the building that you see in the featured photo, above the trees, is my view of the sea. I can often see ships on the horizon, waiting to dock in the Mumbai harbour. Not now. For the last two weeks, I have not seen the horizon because of the pollution. It is specially galling, because we’ve had wonderfully clear air for almost a year, since late March 2020. I took winter pollution in my stride before, hiding behind masks and switching on air purifiers, but this year I reminded myself of the reasons behind this.

The reason for the annual winter pollution is the formation of an inversion layer in the atmosphere. When the sea is colder than the land, the hot air over the city rises, and cold air from the sea blows in. This happens daily, through the year. In winter, the sea air is colder, and the sun is not high enough in the sky to warm this layer fast enough that the breeze sustains itself through the day. As a result, a cold layer stays put over the city, as human activity pumps more pollutants into it. Since cold air is denser, the layering is stable, and the static dense layer just gets more and more dirty. On a relatively warmer day this layer can get heated enough to rise, and one can suddenly see the air clear up. But then as the air cools again, it gets murky as the temperature inversion sets in. By comparing the maximum and minimum sea water temperature with atmospheric temperature, it seems to me that, as usual, we are in for bouts of bad air right until March.

What bothers me is the source of the pollution. Since businesses are still running in shifts, and people are largely home, the traffic is not as bad as it used to be. Sure, the rush hour has its share of snarls, but travel times are still only half of what they were last January. On the other hand, all the construction and repairs that were postponed by nine or ten months has suddenly started again. I can see roads dug up everywhere, many earth movers at work, and concrete being poured. I guess dust from construction, rather than traffic and industry, is the main component of the bad air now. This is actually worse than normal, and very bad news for respiratory health.

In other years I find that visits to Delhi or Kolkata in this season are likely to give me a bad throat. The reason is that the air pollution in both these cities comes from burning organic matter, which may cause fungal spores and bacteria to become airborne. These are directly responsible for throat infections. Winter pollution in Mumbai usually causes respiratory problems in more indirect ways. However, if dust is now a major component of air pollution in Mumbai, then the bacteria carried in soil have just added to the list of throat infections we can now get. Add this to our worries about COVID-19 and the possible cross over of the bird flu now killing poultry and crows through the country. Consider also that an inversion layer prevents the rapid dilution of pathogens that infected people breathe into the atmosphere. All told, the next couple of months could be bad.

Viper bowstring hemp

When I paused to take photos of this common garden ornamental, I was struck by how appropriate its common name snake plant is. Unfortunately, too many different plants are called by that name, so I could call it by its binomial, Dracaena trifasciata, or call it the viper bowstring hemp. This name comes from the fact that the fibers of this plant were used to make bowstrings by the Yoruba people who live in the native range of the plant: from the Congo westwards to Nigeria. There are so many varieties of this plant (another one in the photo below) that it is sometimes hard to believe that they are all in the same species. At least one study has tried to make sure that several plants that we lump into this species are indeed one.

Although the center of diversity of the 120 species of the genus Dracaena lies in west Africa, there is increasing evidence that the genus evolved in sub-tropical Asia. The main clue to this strange event is that the closest cousins of these species are found in tropical and sub-tropical regions of eastern Asia. They have gone extinct in that part of the world, but the oldest species of Dracaena seem to lie in Hawaii and parts of South America. This apparently also happened to the family of plants called the Begoniaceae (the Begonias). So there is the beginning of a mystery here: how did that first dispersal happen, and then a second dispersal to Africa. I’m on tenterhooks now, waiting for the solution.

The plant is easy to grow indoors, and we once had one which grew very well even away from direct sunlight. I find that different varieties as well as closely related species are being sold as “natural air purifiers”. This is not entirely wrong, since many papers have been written about its ability to slowly soak up volatile organic molecules like benzene and formaldehyde. Good ventilation is perhaps a more effective way of getting rid of those indoor air contaminants. There is no evidence that the plant gets rid of suspended particulate matter, which is a major component of air pollution in India.