NowCast (air quality index)

Calculation of the PM NowCast

Let c₁,c₂,...c₁₂ represent the hourly PM concentrations for the most recent 12-hour period, with c₁ the most recent hourly value, and let c_min and c_max represent the minimum and maximum hourly concentration for the 12-hour period.

Define:

w ∗ = c m i n c m a x

and let

w = { w ∗ if  w ∗ > 1 2 , 1 2 if  w ∗ ≤ 1 2 .

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 .

For the special case where there is no variability in the hourly values, c_min = c_max, w = 1, and the NowCast reduces to the twelve-hour average:

N o w C a s t = ∑ i = 1 12 c i 12 .

For the special case where w=1/2:

N o w C a s t = c 1 + ( 1 2 ) c 2 + . . . + ( 1 2 ) 11 c 12 1 + 1 2 + ( 1 2 ) 2 + . . . + ( 1 2 ) 11

But 1/(1-x)=1 + x + x²+ ... , x < 1, so to a good approximation, when w = 1/2:

N o w C a s t = 1 2 c 1 + ( 1 2 ) 2 c 2 + . . . + ( 1 2 ) 12 c 12

Because the most recent hours of data are weighted so heavily in the NowCast when PM levels are changing, EPA does not report the NowCast when data is missing for c₁ or c₂.

Calculation of the Ozone NowCast

Let c₁,c₂,...c₈ represent the hourly ozone concentrations for the most recent 8-hour period, with c₁ the most recent hourly value, and let c_min and c_max represent the minimum and maximum hourly concentration for the 8-hour period.

Define:

w = c m i n c m a x

With these definitions the Ozone NowCast is given by

N o w C a s t = ∑ i = 1 8 w i − 1 c i ∑ i = 1 8 w i − 1 .

Note: USEPA generally truncates ozone values to whole PPB, so a value of 13.78 PPB would use a value of 13.0 in the NowCast calculation, and a value of 0.01378 PPM would use a value 0.013 PPM. Using un-truncated numbers may yield discrepancies with forecast data presented at the AIRNow site.

Example

Consider a day when the hourly average PM_2.5 concentration is zero for all hours of the day, except for a single hour from noon to 1 pm, where a monitor records a concentration pulse of 71 micrograms per cubic meter (µg/m³). According to the equation above, the Nowcast is 71/2 µg/m³=35.5 µg/m³ the hour after the pulse, two hours later it is 71/4 µg/m³=17.8 µg/m³ and three hours later it is 71/8 µg/m³= 8.9 µg/m³. To calculate the corresponding AQI values, each NowCast concentration is substituted into the AQI equation in place of the 24-hour average PM_2.5 concentration:

I = I h i g h − I l o w C h i g h − C l o w ( C − C l o w ) + I l o w

where:

I = the AQI, C = the 24-hour average PM_2.5 pollutant concentration, C l o w = the concentration breakpoint that is ≤ C , C h i g h = the concentration breakpoint that is ≥ C , I l o w = the index breakpoint corresponding to C l o w , I h i g h = the index breakpoint corresponding to C h i g h .

and:

Thus, the three NowCast concentrations correspond to air quality indices of 101, (AQI Color Code Orange, Air Quality: Unhealthy for Sensitive Groups), 63 (AQI Color Code Yellow, Air Quality: Moderate ), and 37 (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; 71/24 µg/m³= 3.0 µg/m³, an AQI of 12 (Color Code Green, Air Quality: Good). EPA has developed a calculator to compute the PM NowCast, AQI, and AQI category and color from the most recent 12 hours of monitoring data.