sensortoolkit.calculate._nowcast_pm25

This module calculates U.S.EPA’s NowCast for fine particulate matter (\(PM_{2.5}\)) for data recorded or averaged to 1-hour measurement intervals.

For reporting changes in air quality at high time-resolution, U.S. EPA’s NowCast calculates hourly \(PM_{2.5}\) concentration values via a 12-hour window of hourly \(PM_{2.5}\) measurements. Values are weighted based on the range of concentration levels observed within the 12-hour window, whereby large changes result in greater weighting of recent hours while steady air conditions result in more evenly distributed weighting of hourly concentrations.

Module Dependencies

  • Python (version >= 3.7):

    Programming language. Module tested with Python versions 3.7 and version 3.8.

  • NumPy (version >= 1.16.5):

    Mathematical operations library. Module tested with versions 1.16.5 through 1.20.1.

  • Pandas (version >= 0.25.1):

    Data analysis library. Module tested with versions 0.25.1 through 1.2.4.

Calculation

The NowCast for a selected (current) hour is computed via the following steps:

  1. Select a 12-hour window of PM measurements whereby the oldest hour in the selection is 11 hours preceding the current hour. The most recent (current) hour is the hour for which the NowCast will be computed. At least two of the last three hours (including the current hour) must have concentration values to compute the NowCast.

  2. From this 12-hour window, select the maximum and minimum PM concentrations present. Compute the range by subtracting the minimum from the maximum.

  3. Normalize the range by dividing by the maximum concentration value. This gives a measure of the rate of change of PM values within the 12-hour window.

  4. Compute the weight factor by subtracting the normalized rate of change from 1.

  5. If the weight factor is less than 0.5, round the value up to 0.5. The weight factor must fall within the range 0.5 to 1.0.

  6. Multiply each hour in the 12-hour window by the weight factor raised to the power of the number of hours ago that the value was recorded. The most recent (current) hour in the series is raised to the zeroth power and the oldest hour is raised to the 11th power.

  7. Sum the weighted PM values computed in the previous step for each hour in the 12-hour window.

  8. In a similar method to steps 6 and 7, compute the sum of the weight factor raised to the 0th through 11th power. This sum includes 12 terms, whereby the power of each term corresponds to the number of hours ago that a concentration value was recoded.

  9. Divide the weighted concentration sum calculated in step 7 by the sum determined in step 8. The result is the NowCast for the zeroth (current) hour in the 12-hour window.

Resources


@Author:
Samuel Frederick, NSSC Contractor (ORAU)
U.S. EPA / ORD / CEMM / AMCD / SFSB
Created:

Fri Jun 19 08:25:11 2020

Last Updated:

Thu Sep 7 16:41:00 2021

Functions

nowcast_pm25

Compute NowCast values for fine particulate matter (\(PM_{2.5}\)) 1-hour averages.