WAAQS relies on actual observations to determine neighborhood air quality. Because of this, only years with significant amount of data can be used. Ozone estimates can reasonably be calculated back to 1990, while PM2.5 estimates can only be estimated back to 2002. In addition, because AQI is a combination of PM2.5 and Ozone, it is limited to the PM2.5 period of record and can only be estimated back to 2002.
Why are PM2.5 and AQI not available for my location?
PM2.5 and AQI have not been estimated for neighborhoods above about 1,000 ft. Estimating PM2.5 concentrations is significantly more challenging than ozone. Localized impacts, limited historical monitoring and uncertainty in grid based models reduced the District’s confidence in PM2.5 estimates at higher elevation sites. Because AQI is dependent on both ozone and PM25, AQI was not estimated for neighborhoods without PM2.5 data. District staff will continue to refine estimation methods and provide estimates after these challenges have been addressed.
What is Population Exposure?
The number of days the population in a selected area was exposed to air pollution concentrations exceeding the standard.
What are Location averages?
When under population exposure, the number of days a person at the selected location was exposed to air pollution concentrations exceeding the standard.
When under AQI, the number of days a person at a selected location experienced good/unhealthy air quality.
What are County averages?
When under population exposure, the average population exposure for residents in the county of a selected location.
When under AQI, the average number of days with good/unhealthy air quality experienced by the population in the county of a selected location.
What are SJ Valley averages?
When under population exposure, the average population exposure for residents in the San Joaquin Valley air basin.
When under AQI, the average number of days with good/unhealthy air quality experienced by the population in the San Joaquin Valley air basin.
How was historical air quality for my neighborhood determined?
Air quality has been estimated for over 3,800 neighborhoods in the Valley using models developed from statistical relationships between monitoring site observations and grid based air quality models used for attainment projections. From these statistical relationships, daily 8-hr average ozone, daily 24-hr average PM2.5 and daily AQI were estimated. Monthly statistics for all neighborhoods were then calculated using these daily estimates.
How accurate are air quality estimates?
An evaluation of accuracy for this type of model is challenging, because most estimates are made for locations without air quality monitors. In general, the estimated air quality readings for neighborhoods close to existing or historical monitoring sites are close to or exactly match observations, while sites further from monitors are less accurate. Accuracy for ozone was evaluated using more than 22,795 observations from eight monitoring sites not included in the predictive model for various reasons. Estimates for these sites had a mean error of 1.1 ppb and a mean absolute error of 5.1 ppb. A similar analysis with 4,776 PM2.5 observations from four sites resulted in a mean error of -1.4 µg/m3 and a mean absolute error 4.2 µg/m3.