AIRNow DMC Real-time Data Quality Control (QC - AirNow-Tech

AIRNow DMC Real-time Data
Quality Control (QC) Guidance
How you can take control of your data
Introduction
The AIRNow program provides real-time air quality maps and information via the AIRNow
website and a variety of other media outlets, including the Weather Channel and CNN. Data
quality can potentially be an issue. This is especially true considering the caveats accompanying
continuous air quality measurements. Given their high degree of public visibility, AIRNow
products should be created with the utmost care, using data that has undergone several measures
of quality control (QC). This document describes the QC measures taken by the AIRNow Data
Management Center (DMC) and details how agencies reporting data to the DMC can contribute
to QC efforts.
The QC process
Air quality data are sent to the AIRNow DMC each hour from more than 100 state, local, federal
agencies, and tribes for more than 1,500 monitoring sites. When an observation (OBS) file is
received from an agency, each hourly measurement is compared against a set of QC criteria.
These criteria help identify potentially erroneous measurements. Measurements violating one or
more QC criteria are viewed by an AIRNow operator for approval. Data that fail the QC checks
are marked as erroneous in the AIRNow database and are assigned a value of -980. These
measurements are not used to calculate the AQI, nor are they used to produce AIRNow maps or
related information. In effect, the public is prevented from viewing what would otherwise be an
incorrect indication of real-time air quality.
QC Criteria
QC criteria are defined by parameter and hour of the day at every site. Each hour, data for
ozone, PM2.5, PM10, and a number of other pollutants and meteorological parameters are
compared against the following QC criteria:
Buddy Average: The difference allowed between the incoming data value and the average
concentration at all buddy sites. If the observed value at the flagged site is greater than the
Buddy Average, the value is pushed to the AIRNow database but is flagged for further review. If
deemed erroneous, a value of -980 is input into the database and products are recreated.
Number of Buddy Sites: The total number of sites against which an observation is compared.
There is a limit of 5 sites.
Max Suspect: Observations greater than or equal to Max Suspect are input into the database and
may be used to produce products, but are flagged for review. If deemed erroneous, a value of 980 is input into the database and products will be recreated with the observations removed.
Max Severe: Observations greater than or equal to Max Severe are assigned a value of -980 and
withheld from all AIRNow products. However, each value flagged is reviewed by DMC staff.
If deemed correct, the value originally reported is used in products.
Rate of Change: Compares the current hour’s value to the previous hour’s value. If the
difference is greater than or equal to the Rate of Change, the observation is assigned a value of 980 and withheld from all AIRNow products. However, each value flagged is reviewed by
DMC staff. If deemed correct, the value originally reported is used in products.
Number of Sticking Hours: The number of hours a value can be observed consecutively after
which the value is flagged for review. If deemed erroneous, a value of -980 is input into the
database and products are recreated.
Sticking Value: Observations above this value are subject to the Number of Sticking Hours QC
check. Observations below this value are not checked against the Number of Sticking Hours QC
check. For example, a constant solar radiation measurement of 0 Watts/m2 at night would be
considered valid and shouldn’t be considered by the Sticking Value check.
Minimum Drift: Observations below this value are flagged as bad, changed to -980, and placed
in the queue for review.
Setting QC Criteria Using the AIRNow Tech Website
AIRNow Tech (www.airnowtech.org) is a feature rich website which allows state and local
agencies to view air quality data, as well as manage information regarding their agency and
monitoring networks.
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View and update QC criteria by visiting AIRNow Tech and choosing Sites from the
main menu. You are shown a list of all sites in your agencies network.
Select a site from the list by clicking on the site name. You are then taken to a second
page listing specific information about that particular site.
Select the “View QC Criteria” tab.
The screen expands to show the QC criteria for that site and parameter. View QC
criteria for additional parameters by choosing the desired parameter from the drop
down menu.
QC criteria use default values set by the AIRNow DMC. Alter defaults by selecting
the Enable Editing tab located either above or below the QC criteria table. The cells
which were grey turn white and are now available for editing.
Alter defaults by either editing a single cell, or by using the “Set All” entry box
located at the bottom of the QC table. Enter a value you want to input in all cells for
a particular hour, and then select “Set All Hourly Values”.
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Additional QC related criteria involve choosing a sites buddy site or sites. Buddy
sites are a group of sites against which hourly observations at a site are compared.
For example, if three sites are located within a few miles of each other you could set
the two sites as the others “buddy”. When the concentration at the site is greater than
the average concentration at its “buddy sites”, the observation is considered suspect
and flagged for review.
Choose your sites buddies and then specify the buddy average in the QC table.
Anytime a site observes a value greater then the value specified at the buddy average,
that observation is flagged for review.
Once the QC table is set to your liking, choose “Save QC Criteria”.
Finally, when you are finished editing QC criteria, select “Disable Editing” to secure
your recent edits from being overwritten.
Backfilling Data
Occasionally data delivery to AIRNow is disrupted, resulting in gaps of missing data. In these
cases agencies delivering data are encouraged to resend OBS files. OBS files can be resent
directly via ftp, or by emailing the files as an attachment to the AIRNow DMC
([email protected]). While data not received in real-time may not be used to
reproduce products, values are still accessible from the AIRNow Tech web page.

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