NowCast (air quality index)

The PM (particulate matter) NowCast is a weighted average of hourly air monitoring data used by the United States Environmental Protection Agency (USEPA) for real-time reporting of the Air Quality Index (AQI) for PM (PM10 - particles less than 10 micrometers, or PM2.5 - particles less than 2.5 micrometers).

The PM NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing.

The PM NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored.

At each ozone monitoring site, partial least squares regression is used to develop a predictive relationship between the 1-hr and 8-hr averages using the previous 2 weeks of ozone data.

represent the hourly PM concentrations for the most recent 12-hour period, with

represent the minimum and maximum hourly concentration for the 12-hour period.

Define: and let With these definitions the PM NowCast[2] is given by: For the special case where there is no variability in the hourly values,

According to the equation above, the Nowcast is 144/2 μg/m3=72.0 μg/m3 the hour after the pulse, two hours later it is 144/4 μg/m3=36.0 μg/m3, three hours later it is 144/8 μg/m3= 18.0 μg/m3, and four hours later it is 144/16 μg/m3= 9.0 μg/m3.To calculate the corresponding AQI values, each NowCast concentration is substituted into the AQI equation in place of the 24-hour average PM2.5 concentration: where: and:[3][4] Thus, the four NowCast concentrations correspond to air quality indices[5] of 163, (AQI Color Code Red, Air Quality: Unhealthy), 102, (AQI Color Code Orange, Air Quality: Unhealthy for Sensitive Groups), 68 (AQI Color Code Yellow, Air Quality: Moderate ), and 50, (AQI Color Code Green, Air Quality: Good) respectively.

After the day is over and all of the hourly data is available, the AQI for the day is calculated from the 24-hr average; 144/24 μg/m3= 6.0 μg/m3, an AQI of 33 (Color Code Green, Air Quality: Good).