By the late 20th Century (1980s/90s) sulphur dioxide concentration in the atmosphere was already in decline across most of England due to the decline in heavy industry and a gradual change from coal fired power stations to gas. Before then, pollution from coal burning severely affected lichens across much of Eastern and Midland England and this wasn’t restricted to cities, the pollution was widespread rather than strictly localised.
By the start of the current century, atmospheric sulphur dioxide had fallen to levels which were below the limiting factor for most lichens across Eastern and Midland England. The influence of nitrogen compounds became the dominant form of atmospheric pollution influencing lichen communities. Although the concentration of nitrogen compounds has increased, it was already high during the industrial days but at that time the effects of the nitrogen compounds were suppressed by the acidic sulphurous pollution. It has been a rather interesting time to be a lichenologist as the communities have changed dramatically in the past three decades and new species seem to appear in the region every year.
The following twitter thread shows some of the changes that have occurred on a single tree trunk over a period of thirteen years.
https://twitter.com/obfuscans3/status/1383391666954903562
This reply is a very brief summary but hopefully helps to explain the main change since the height of the sulphur dioxide pollution in the mid 20th Century.
