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maeshle measures the NO₂ pollution in each hexagon of approximately 36 km² on Earth. Each month since May 2018, maeshle gives each hexagon a score for its decrease in pollution. Most NO₂ comes from combustion. Thus NO₂ is a good indication of local air pollution.
The data source is monthly global NO₂ from the Sentinel-5P/TROPOMI satellite (Terrascope L3, 0.02° grid, quality-filtered). maeshle calculates the average of this data in each H3 resolution-6 hexagon, with weights for coverage. The values are tropospheric column densities in µmol/m². A value includes all the NO₂ in the air above a hexagon. The values are not surface concentrations.
maeshle calculates all its scores from m. m is the average NO₂ for the last 12 months. A 12-month window removes the seasonal cycle and decreases the measurement noise. m uses only the months before the current month. Thus a new month does not change the values for the closed months.
Some months have no satellite data, for example in winter at high latitudes. A 12-month average without the usual peak months of a cell is too low, because it is an average of the low months only. When the unobserved months usually hold a large part of the cell's annual NO₂, maeshle shows no value for m.
The Change layer shows the speed at which m changes. There are three horizons:
The three horizons frequently do not agree. The overall rate includes 2019–2021, when the values were almost constant. Thus a cell with only recent decreases has a lower overall rate than its 5-year rate.
The 5-year and last-year horizons need m values from that time. In the far north, the polar night causes months without data. Then a 12-month average is not possible, and those hexagons stay empty. The overall horizon has no empty hexagons, because it starts where the record of each cell starts.
The Credit layer shows the credit given in the last month, in the last 12 months, or in the full record; see below. The NO₂ layer shows the current value of m.
The chart in the hover panel agrees with the layer that you select. For NO₂, the chart shows m. For Change, the chart shows the change of m against the year before. For Credit, the chart shows the monthly credits as bars. The three views have the same month axis.
maeshle gives credit to a cell when m goes below the record minimum of that cell. The baseline B is the lowest m of the cell in the last five years. This period includes the last month. m must go below B by more than a noise margin of 1.2%. The credit is the relative decrease (B − m)/B. Thus a clean cell and a dirty cell receive the same credit for the same percentage decrease. maeshle removes records after five years. Thus each cell can receive credit again after some time.
The baseline includes the last month. Thus a cell must go below its own best value again to receive more credit. Each step down receives credit one time. For the full record of a cell, the total credit is approximately log(first m ÷ latest m). The speed of the decrease does not change this total.
A step smaller than the margin is not lost. maeshle measures the decrease from the level at which the cell last received credit. Thus small decreases collect until their sum is more than the margin. A month that the neighbourhood check (below) sets to zero collects the same way.
There are three more rules. Only cells with m ≥ 30 µmol/m² receive scores, because percent changes have no meaning at the background noise level. NO₂ moves tens of kilometres. A record minimum can occur because the plume of an adjacent cell went to a different area for one year. Thus maeshle gives the full credit only when the region of approximately 1,770 km² around the cell is also below its own baseline. maeshle sets these isolated, improbable records to zero. And a record must come from a window that saw the cell's usual peak months: a low average over the low months only is not a low level. A record from such a window is banked, and maeshle pays it when the coverage returns.
On the Credit layer, the hover chart shows a bar for each month in which the hexagon received credit. A taller bar shows more credit. The tooltip shows the exact figure. The line below the chart gives the count of months with credit and the best month. This line stays visible on the other layers. The bars are usually not frequent, because a hexagon receives credit for a decrease, not for a low constant level.
The site is a set of static files, with no backend. It uses MapLibre GL and H3. Click the ⬡ code of a hexagon to copy a link that goes directly to that hexagon.
Contains modified Copernicus Sentinel-5P data, processed by VITO/Terrascope. Basemap © OpenStreetMap contributors © CARTO.