Description of the Smooth Trend tool

Smooth Trend calculates and fits a smoothed trend line to measurements of one atmospheric pollutant over a user selected time range at one or multiple sites. The smoothed line permits trends in pollutant concentrations to be shown.

Monitoring sites are chosen from the drop-down site name list. Multiple sites can be selected/de-selected by pressing and holding the CTRL key whilst highlighting the desired sites using the left mouse button. If there are no common measurements at the sites selected, 'Sorry, no pollutants found for the data options selected' will be displayed. As multiple sites are selected the number of pollutants displayed in the variables list may decrease to reflect the extent of corresponding pollutant measurements undertaken at different sites within the network. To confirm which sites a pollutant is measured at, first choose the pollutant from the variables menu below. This will reduce the list of sites to only those at which the pollutant is measured. Then select the sites to be used.

Use the variables drop-down menu to select the required atmospheric pollutant. Only one pollutant can be plotted 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.

This function produces a time series plot showing monthly average measurements as red lines and markers, with a red fitted trend line, and the grey region showing the 95% confidence intervals of the fit. Openair allows a data capture threshold to be set where averaging is applied using this tool. A data capture threshold of 75% has been set for the Smooth Trend function, meaning that 75% of data has to be present for the average to be calculated.

Type allows the user to split the data in several ways. The default option will produce a single plot using the entire data over the selected time range at one or more sites. Other types include 'Hour' (for hour of the day), 'Weekday' (for day of the week), 'Season' (for normal seasons), and 'Wind Direction' (split into eight compass point sectors using the modelled wind direction from the UK air quality forecast).

At the bottom of the page there is a tick box which provides the option to 'Deseasonalise' the data i.e. remove seasonal variation in the data to provide a clearer indication of the overall pollutant concentration trend over the selected time range at one or more sites. Removing the seasonal cycle is more effective for pollutants (or locations) where the seasonal cycle is stronger, e.g. for ozone and background sites.