Before the last AQB meeting, during a working lunch, a presentation was given by Kent Bott about how DAQ forecasts air quality, including the decisions to call green, yellow & red days.
It is pretty simple and complex at the same time. The color codes reflect predicted monitor values.
The ozone “color-changing limits” are as follows:
GREEN (GOOD) is 0 to 59 ppb;
GREEN (MODERATE) is 60 to 67 ppb;
YELLOW is 68 to 75 ppb;
UNHEALTHY FOR SENSITIVE GROUPS 76 to 95 ppb;
UNHEALTHY is 96 to 115 ppb.
These limits are all running 8-hours average concentration values.
For PM 2.5, 15 ug3 is GREEN
YELLOW is called when 24 is predicted,
RED is when 35, the standard, is predicted.
These limits are based on a 24-hour average.
Kent listed 11 different sources he checks for meteorology, and he uses this information along with current monitor values & trends to predict air quality. He consults with Cheryl – it seems they reach a joint
decision about questionable calls. He says he does all the forecasting, working every day during the bad air seasons.
He says forecasting ozone is more difficult, and at a national meeting, they were amazed that Utah attempted 3 day ozone forecasts. St George is particularly challenging to predict, the movement of air masses there is
not well understood yet.
PM2.5 is easier, a rule of thumb is that it doubles every 24 hrs until it plateaus. Thru this past winter, he correctly predicted 22 of 23 actual days when the monitors measured 35+ (one day ahead forecast)
-Kathy Van Dame