Difference between revisions of "Help:Using the Concentration Anomaly Tool"

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A concentration anomaly is a deviation from the normal. This tool permits the calculation of concentration anomalies.  
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Back to <[[Evidence_for_Flagging_Exceptional_Events#Step_4:_The_Event_is_in_Excess_of_the_.22Normal.22_Values|Evidence for Flagging Exceptional Events]]<br>
  
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A useful measure of the "normal" concentration is the median value over an extended period of time for a given station. In the tool settings below we have chosen a time setting of +/- 15 days (one month window), which is longer than a typical exceptional event. As seen below in the time series view of a particular site, an aggregation of three years of PM concentration data is used to compare whether a given day's concentration can be considered exceptional. While it is sufficiently short to preserve the seasonal variation of the concentration pattern, the magnitude of the event excess concentration is calculated as the concentration difference between the current daily concentration and the 84th percentile concentration. All of these parameters are customizable to fit a given event's time range.  A time window larger or smaller than the one month window used here can be chosen, the data aggregation time range can be extended beyond three years or shortened, and the excess concentration can be defined in several ways, including the difference between the concentration of a day of interest and varying percentiles (i.e. 84) or the median concentrations in the given time window. See [[Using the PM2.5 Concentration Data Browser]] to learn how to change these parameters.
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Link to '''[http://webapps.datafed.net/datafed.aspx?page=ARC/FRMPM25_Day&datetime=2007-05-24T18:00:00 Concentration Anomaly Tool]'''<br><br>
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A useful measure of the "normal" concentration is the 84th percentile (+1 sigma) for a given station. A concentration anomaly is a deviation from the normal. This tool permits the calculation of concentration anomalies. In the illustration below, a time windows of +/- 15 days (one month window) was chosen. This period is longer than a typical exceptional event, but it is sufficiently short to preserve seasonality. In order to establish the normal values the concentrations can be averaged over multiple years for the given time window measured in Julian days, i.e. days between 160 and 190. Hence, a particular sample is considered anomolously high (deviates from the normal) if its value are substantially higher than the 84th percentile of the multi-year measurements for that "month" of the year.  
  
 
[[Image:070524_FRMpm25_TimeSeries_280870001.png|800px]]<br>
 
[[Image:070524_FRMpm25_TimeSeries_280870001.png|800px]]<br>
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The time parameters of the Concentration Anomaly Tool can be adjusted, changing: (1) the event day, (2) time window and (3) the time range that the time window is averaged over. 
  
 
There is also need for flexibility in defining the 'normal' when calculating the deviation above normal. For example in figures below, the excess concentration is plotted based on 'normal' defined as the 50th, 84th (+ sigma) and 95th percentile of the distribution. Clearly, on a given day, the excess above the 95th percentile is much smaller than excess above the 50th percentile.   
 
There is also need for flexibility in defining the 'normal' when calculating the deviation above normal. For example in figures below, the excess concentration is plotted based on 'normal' defined as the 50th, 84th (+ sigma) and 95th percentile of the distribution. Clearly, on a given day, the excess above the 95th percentile is much smaller than excess above the 50th percentile.   
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[[Image:PM25 DiffAboveNorm95Perc.png|400px]]
 
[[Image:PM25 DiffAboveNorm95Perc.png|400px]]
  
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The anomaly calculation parameters of the Concentration Anomaly Tool can also be adjusted, changing: (1) the percentile that is defined as "normal" and (2) the calculation to find the  difference from normal.
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The tool has three layers that can be viewed and changed: Time Instance, Time Range and computed.
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=== Time Parameters ===
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====Change the event day====
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# Select Time Instance Layer and change the date in the Viewer, click go.
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[[Image:TimeInstance.png|400px]]
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====Change the Time Range ====
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# In the time view, click the magnify glass
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# Change time_min and time_max. Click Ok and then in the viewer click the red "Go" Button. The Time view will refresh with the new range.
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# Note: This changes the window that the anomaly calculation uses to average over.
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[[Image:TimeRange.png|400px]]
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* Note: Time Range can exclude the event year.
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====Change the Time Window ====
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# Click Map Service Program
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# Select GetCoverage data_range
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# In the Time box, change Julian Day Window - days before/days after.
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# Save and update browser.
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[[Image:TimeWindow.png|400px]]
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=== Anomaly Calculation Parameters ===
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====Change the "Normal" High====
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# Click Map Service Program
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# Click Time Aggregate for the Time Range layer.
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## To change the function click drop down box that says 'Percentile' and select desired function (e.g. Median, Avg, Sum)
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## To change percentile edit the text box below.
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[[Image:NormalHigh.png|400px]]
  
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In the figures below, the concentration patterns are illustrated for two days 2007-05-24 and 2007-05-27. For each day, the leftmost figure shows the measured day average PM2.5 concentration. The circles are color coded using the same coloring scheme as the contour for the concentration field. The middle figure shows the contour field for the 84th percentile PM2.5 concentrations. The color coded circles still represent the concentration for the selected day. The rightmost figure shows the excess concentration of the current day values over the 30-day period compared to the 84th percentile values. While the rightmost figures show that the excess concentrations are high, these by themselves cannot establish whether the origin is from controllable or exceptional sources.
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====Change the Anomaly Calculation====
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# Click Map Service Program
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# Click Evaluate Exp on the Computed Layer and modify the expression
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[[Image:DiffExpression.png|400px]]
  
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=== Normal and Exceptional Pattern Analysis Tool ===
 
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The pattern of air air pollution pattern varies in space, time and also depends on the pollutants. In case of PM, it also depends on the species in the PM chemical mix. The sulfate pattern, for example, is very different from nitrate, organics or dust. An event being exceptional, requires the it deviates from the "normal", but what is normal?
 
  
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[[Image:CATT Map ServiceProgram.png|400px]]
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[[Media:ConcentrationAnomalyTool_help.ppt|Images for Help]]

Latest revision as of 15:28, June 6, 2008

Back to <Evidence for Flagging Exceptional Events

Link to Concentration Anomaly Tool

A useful measure of the "normal" concentration is the 84th percentile (+1 sigma) for a given station. A concentration anomaly is a deviation from the normal. This tool permits the calculation of concentration anomalies. In the illustration below, a time windows of +/- 15 days (one month window) was chosen. This period is longer than a typical exceptional event, but it is sufficiently short to preserve seasonality. In order to establish the normal values the concentrations can be averaged over multiple years for the given time window measured in Julian days, i.e. days between 160 and 190. Hence, a particular sample is considered anomolously high (deviates from the normal) if its value are substantially higher than the 84th percentile of the multi-year measurements for that "month" of the year.

070524 FRMpm25 TimeSeries 280870001.png

The time parameters of the Concentration Anomaly Tool can be adjusted, changing: (1) the event day, (2) time window and (3) the time range that the time window is averaged over.

There is also need for flexibility in defining the 'normal' when calculating the deviation above normal. For example in figures below, the excess concentration is plotted based on 'normal' defined as the 50th, 84th (+ sigma) and 95th percentile of the distribution. Clearly, on a given day, the excess above the 95th percentile is much smaller than excess above the 50th percentile.

PM25 DiffAboveNorm50Perc.png PM25 DiffAboveNorm84Perc.png PM25 DiffAboveNorm95Perc.png

The anomaly calculation parameters of the Concentration Anomaly Tool can also be adjusted, changing: (1) the percentile that is defined as "normal" and (2) the calculation to find the difference from normal.

The tool has three layers that can be viewed and changed: Time Instance, Time Range and computed.

Time Parameters

Change the event day

  1. Select Time Instance Layer and change the date in the Viewer, click go.

TimeInstance.png

Change the Time Range

  1. In the time view, click the magnify glass
  2. Change time_min and time_max. Click Ok and then in the viewer click the red "Go" Button. The Time view will refresh with the new range.
  3. Note: This changes the window that the anomaly calculation uses to average over.

TimeRange.png

  • Note: Time Range can exclude the event year.

Change the Time Window

  1. Click Map Service Program
  2. Select GetCoverage data_range
  3. In the Time box, change Julian Day Window - days before/days after.
  4. Save and update browser.

TimeWindow.png

Anomaly Calculation Parameters

Change the "Normal" High

  1. Click Map Service Program
  2. Click Time Aggregate for the Time Range layer.
    1. To change the function click drop down box that says 'Percentile' and select desired function (e.g. Median, Avg, Sum)
    2. To change percentile edit the text box below.

NormalHigh.png

Change the Anomaly Calculation

  1. Click Map Service Program
  2. Click Evaluate Exp on the Computed Layer and modify the expression

DiffExpression.png


Images for Help

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