Skip First Level Navigation | Skip All Navigation

Department of Environmental Protection - Bureau of Air Quality
Air Quality - Online Data Service

SERVICE INFORMATION

Frequently Asked Questions

  1. Why is data reported as missing?
  2. What determines the location of an air monitoring site?
  3. Why are some pollutants measured in both parts per million and parts per billion?
  4. Why are there multiple listings for the same pollutant at a site?
  5. Does the Bureau collect any other air quality data?
  6. What is Preliminary Data?
  7. How do I get copies of the final validated data?
  8. How do I get copies of the final validated data?

1. Why is data reported as missing?

There are a variety of reasons as to why data may be reported as missing on the web page.  Missing data can be described as either data that has not been retrieved from the monitoring site or data that is not considered valid.  The monitor may be a seasonal monitor, such as most of the ozone sites, and only operate for part of the year.  For the time that it is not in operation the data will be reported as missing.  There are some basic data quality tests that are conducted as the data is collected.  If the data does not pass those tests it will be reported as missing.  Most of the data reported is collected through the use of dial up modems.  In some cases the computer is not able to connect to the site and the current hour of data is then reported as missing.  As soon as the computer is able to make a connection it will collect any missing data that is available and fill in those hours with actual data.  Finally, data that has been collected and has appeared on the web page may be invalidated later as additional quality control checks are conducted on the data.


2. What determines the location of an air monitoring site?

Monitoring sites are located around the state for several different purposes.  Sites can be located to:

  • Monitor the impact of a large source of air pollution.
  • Measure the pollution levels in a densely populated area.
  • Determine compliance with an air quality standard.
  • Determine background/baseline levels of air quality for licensing of air emission sources.
  • Determine long-term trends in air pollution levels.


3. Why are some pollutants measured in both parts per million and parts per billion?

The measurement scale is usually determined by the expected concentrations, the standard for a particular pollutant and the type of monitor that is being used.  In most cases where data is being collected to determine compliance with a standard or to monitor the impact of a large source, a standard monitor will be used and data will be collected in the parts per million scale.  If data is being collected in a rural location for background concentrations or for research work a trace level monitor may be used and the data will be collected in parts per billion.  In those cases the monitors are usually more sensitive and designed to work at very low concentrations.  Data reported as parts per million can be converted to parts per billion by moving the decimal point three places to the right.


4. Why are there multiple listings for the same pollutant at a site?

The basic unit of data is an hourly average.  However, the standards may be written for an hourly average, three hour average, eight hour average, twenty-four hour average or an annual average.  If there are multiple standards for the same pollutant based on different time or averaging periods then the data is calculated for each of those averaging periods.    For example, sulfur dioxide has a three hour and a twenty-four hour standard. The data is collected and reported for each hour.  At the same time the current hour and the previous two hours are averaged and the average is reported as a separate parameter for comparison with the three hour standard.  Similarly, the current hour and the previous 23 hours are averaged and the average is reported as a separate parameter for comparison with the 24 hour standard.


5. Does the Bureau collect any other air quality data?

Yes, the Bureau operates a number of monitors that collect data over a longer time period, typically 24 hours.  Those samples have to be processed in a laboratory and the results may not be available for several weeks.  The Bureau may also operate some temporary monitoring sites in remote locations where it is not feasible to have a phone line installed or does not have good cell phone reception.  In those cases the data is collected on a routine basis (usually weekly or biweekly) and will be entered into the EPA database after quality control review.


6. What is Preliminary Data?

Preliminary data is data that comes straight from the monitoring site.  This is the data that gets automatically displayed and stored in the database that is available through the web site.  Some quality control has been provided through the data acquisition system.  Additional data review may determine that some of this data does not meet all of our quality assurance guidelines and if it does not it will be removed from the database.  The final validated data is available through the EPA database.  We will attempt to correct any known errors in the data that is available on our website but the EPA database will continue to be the recommended site to retrieve final validated data.


7. How do I get copies of the final validated data?

Once the air quality data has been reviewed and validated it is stored in the national EPA database for air quality data.  Copies of data can be downloaded for the Maine sites as well as the rest of the country.  The EPA database has summary data for all of the air quality data that has been collected and stored in the system.  They also have available the raw hourly data for the most recent ten years.  Final data can be reviewed and retrieved at the following web address:  http://www.epa.gov/air/data/index.html


8. How do I get copies of the final validated data?

Once the air quality data has been reviewed and validated it is stored in the national EPA database for air quality data.  Copies of data can be downloaded for the Maine sites as well as the rest of the country.  The EPA database has summary data for all of the air quality data that has been collected and stored in the system.  They also have available the raw hourly data for the most recent ten years.  Final data can be reviewed and retrieved at the following web address:  http://www.epa.gov/air/data/index.html