sky-truth air-chemistry comparison plan, written 2026-10-24, before any chemistry was read

Chemistry: EPA Chemical Speciation Network, Dallas Hinton (AQS 48-113-0069), PM2.5 elements by X-ray
fluorescence, 24-hour samples (midnight to midnight local), daily values as published by EPA.
For each element, the sampler position (POC) with the most samples over the whole record is used.
Values at or below zero are set to half the smallest positive value of that element at the site.

Day labels (computed from weather only, never from chemistry): NOAA GFS analyses every 3 hours at
200, 250, 300, 350 hPa over the city. A time has lasting-trail air when the Schmidt-Appleman test
(engine efficiency 0.35) says a trail forms and humidity is at least 100% over ice at any of those levels.
Over the sample day and the day before (16 times):
  lasting-trail day: 4 or more times;  no-trail day: none;  anything between is left out.
A day with any of the 16 times missing from the archive gets no label.
From 2021-04-01. Rain on the sample day (>= 1.0 mm, ERA5 via Open-Meteo) is recorded.

Primary measures: aluminum (88104), barium (88107), strontium (88168), each divided by silicon (88165) to take out wind-blown dust.
Test, one for each: difference in mean log(element / silicon), lasting-trail days minus no-trail days,
compared only within the same month (of the same year) and the same rain/no-rain group, weighted by
n1*n2/(n1+n2) per group. One-sided p-value from 20000 label shuffles within groups (seed 20261024).
Holm correction across the three. Significant only if adjusted p < 0.05.
No result is called until there are at least 30 lasting-trail and 30 no-trail sample days.

Also reported, clearly marked secondary: the same test on raw concentrations, and on every
XRF element (Holm across all of them). Nothing here is changed after the chemistry is seen.