Description of the Trend Level tool

The Trend Level tool provides a way of rapidly displaying the time variation in the concentration level of air pollutants in a condensed format. In a single plot, the variation in the concentration of a pollutant can be shown by time of day, month of year and year. The plot therefore provides information on trends, seasonal effects and diurnal variations. Each year is presented sequentially in a separate pane. In the default version the hour of the day is represented on the vertical axis whereas months throughout each year are represented in the horizontal axis to show the seasonality. The concentration level is represented by the colour scale on the right of the plot.

The pollutants measured at each site are shown in the variables box once a site has been selected from the drop-down list of site names at the top of the page. Trend level plots can only be produced for one site and one pollutant at a time.

Data can be plotted over user defined date ranges by clicking on the drop-down lists containing the day, month and year and using these to change the start and end dates accordingly. Data can be chosen on an annual basis by highlighting the 'Specific Years' option and using the year drop-down menu to alter the start and end year.

The Y Variable drop-down menu allows the user to select options other than the default of hour of the day for the variable used to control the vertical axis. Options available here include 'Weekday', 'Wind Direction' and 'Wind Speed' – wind direction and speed using the modelled wind direction and speed from the UK air quality forecast. The tool has been set up here to summarise the data using the mean concentration level.