XRF is an acronym for x-ray fluorescence, a process whereby electrons are displaced from their atomic orbital positions, releasing a burst of energy that is characteristic of a specific element. This release of energy is then registered by the detector in then XRF instrument, which in turn categorizes the energies by element.
https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/handheld-xrf/how-xrf-works.html
The relatively poor air quality over the past week or two is mostly due to recent meteorology conditions. The persistent high pressure over California right now leads to cooler temperatures, a low inversion layer, light winds (no storms), and poor vertical mixing. This meteorology pattern traps our area’s emissions within the area and closer to the ground, leading to higher measured concentrations. Hopefully this will ease up a bit later this week.
I have passed along the information that the AirNow site is listing “Fairfield” on the Current AQI and AQI Loop of the “NowCast” values (as in the screenshot below). However, even though the header says Fairfield, the measured AQI listed on the AirNow page is not specific to any given monitor. The map and current AQI listed is the highest AQI measured for that hour at any site in the reporting zone for the zip code that is entered. There are five reporting zones in the Bay Area – and they are shown on the map here: http://www.baaqmd.gov/about-air-quality/current-air-quality and described here http://www.sparetheair.org/stay-informed/todays-air-quality/reporting-zones. Vallejo is in the Northern Zone, so for our AQI forecasts and AirNow current AQI, it is reflecting the maximum AQI in that whole area for a given day or hour.
I think a better site to track real-time measured concentrations at a specific Air District-operated monitor is the query page for the CARB AQMIS database located at https://www.arb.ca.gov/