The particulate matter (PM) 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. The Ozone NowCast is an algorithm developed by the USEPA to predict the 8-hour ozone average centered on the current hour from the current 1-hr average ozone data. 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. USEPA uses the Ozone Nowcast for real time public reporting of the ozone AQI.[1]
Calculation of the PM NowCast Let c 1 , c 2 , . . . c 12 {\displaystyle c_{1},c_{2},...c_{12}} represent the hourly PM concentrations for the most recent 12-hour period, with c 1 {\displaystyle c_{1}} the most recent hourly value, and let c m i n {\displaystyle c_{min}} and c m a x {\displaystyle c_{max}} represent the minimum and maximum hourly concentration for the 12-hour period. Define:
w ∗ = 1 − c m a x − c m i n c m a x = c m i n c m a x {\displaystyle w^{*}=1-{\frac {c_{max}-c_{min}}{c_{max}}}={\frac {c_{min}}{c_{max}}}}
and let
w = { w ∗ if w ∗ > 1 2 1 2 if w ∗ ≤ 1 2 {\displaystyle w={\begin{cases}w^{*}&{\mbox{if }}w^{*}>{\frac {1}{2}}\\{\frac {1}{2}}&{\mbox{if }}w^{*}\leq {\frac {1}{2}}\\\end{cases}}}
With these definitions the PM NowCast is given by:
N o w C a s t = ∑ i = 1 12 w i − 1 c i ∑ i = 1 12 w i − 1 . {\displaystyle NowCast={\frac {\sum _{i=1}^{12}w^{i-1}c_{i}}{\sum _{i=1}^{12}w^{i-1}}}.}
For the special case where there is no variability in the hourly values, c m i n = c m a x , w = 1 {\displaystyle c_{min}=c_{max},w=1} , and the NowCast reduces to the twelve-hour average:
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