Motivation

Once you start learning to identify lichen in your environment (see this activity), the next step is to use the species that you're seeing to draw some conclusions about the air quality in your area.

Method

Steps 1 and 2 can be skipped if your area already has a lot of lichen observations on iNaturalist.

Step 1: Go on a lichen walk

In your neighborhood and/or local park, look for lichen on the sidewalks, on rocks, and trees, and take some pictures. Make sure to record the image, date, and time!

Step 2: Photograph lichen(s) in your neighborhood

Upload your photos to iNaturalist to learn the scientific name(s) of the most common lichen in your area.

Step 3: Identify common lichen in the area

Search for the most lichen in your area by going to iNaturalist and searching for the "Common Lichen" species near your location and/or ranking the lichens you saw on the walk by frequency.

Step 4: Assess Pollution Sensitivity of species

Once you have a list of scientific names for common species in your area, search for them in the appropriate link for your region:

• Sensitivity Lists for Pacific Northwest
• Sensitivity Lists for Eastern US
• Sensitivity Lists for Western US
• Paper with Baseline sensitivity data in Table 16 for Washington, Oregon, and California

These links will give you information on the species' sensitivities to nitrogen (most likely in the form of NH₃ and the reaction product NH₄+) and/or sulfur pollutants. Oligotrophs (oligo- (“few”) +‎ -trophy (“nutrition”)) are the most sensitive to nitrogen pollution, followed by mesotrophs. Eutrophs are the most tolerant to nitrogen pollution.

You can also reference the tables below for common species that indicate good or poor air quality.

Indicators of Good Air Quality
If you're in an area with a lot of diversity of lichen, and a fair amount of hairy/bushy lichen, you're likely in a spot with good air quality and low concentrations of SO₂ and NO₂. You're most likely to find these during a hike or in a park with a lot of vegetation.

Common Name	Scientific Name	Image
Old Man's Beard For more examples of beard lichens, check out this site	Usnea Filipendula
Lace Lichen, Witch's Hair	Ramalina, Alectoria
Lobaria pulmonaria	Lungwort	Bernd Haynold - CC BY-SA 3.0 - Wikipedia

Indicators of Nitrogen Pollution
The following lichen are either tolerant to or thrive in high nitrogen environments (nitrophiles or eutrophs), and these are commonly found on trees in urban environments near power plants, automobile exhaust, industrial activities, or fertilizer application.

Some of the most obvious nitrophiles are bright yellow or orange.

Common Name	Scientific Name	Image
Common Sunburst Lichen	Xanthoria parietina
Candleflame Lichen	Calenderia Concolor
Common Greenshield Lichen	Flavoparmelia caperata
Gold Dust Lichen	Chrysothrix candelaris
Hooded Rosette Lichen	Physcia Adscendens

Note that this activity will not give you quantitative measurements of your local air quality, but it can indicate differentials in air quality across various spots.