Description of the Time Plot tool

The Time Plot function is designed to quickly plot time series of data, for one or several pollutants, for one or more years.

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 pollutants 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 drop-down 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.

The pollutants measured at each site is 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. The number of pollutants measured at each site varies across the network. Multiple pollutants can be selected/de-selected by pressing and holding the CTRL key whilst highlighting the desired pollutants using the left mouse button. Modelled wind speed and wind direction from the UK air quality forecast are available in the variables list along with the pollutants and can be selected in the same way.

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 resultant time series plots are stacked to allow easy comparison of pollutant concentrations if multiple pollutants are selected.

Averaging time sets the averaging period to be applied to the data for plotting. The options are: 'Default', i.e., no averaging applied, 'Hour', 'Day', 'Week', 'Month', 'Year' and 'Season'. 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 Time Plot function, meaning that 75% of data has to be present for the average to be calculated.

The Colours parameter allows the user more control over the colour scheme used to plot which can be useful if there are lots of sites/variable plotted at one time.

Type allows the user to split the data in several ways. Types include 'Hour' (for hour of the day), 'Weekday' (for day of the week), 'Month' (for months of the year), 'Wind Direction' (split into eight compass point sectors), and 'Wind Speed' (split into four quartiles) – wind direction and speed using the modelled wind direction and speed from the UK air quality forecast.

If the 'Stack by Year' tick box is selected then the time series will be plotted as separate series stacked by year. This is useful to see detail where a long time series is plotted particularly if the averaging period is short.

The 'Normalise' option can be set to normalise the plotted time series by their mean values. This is useful to observe the relative change in the plotted values, for example to compare the change in pollutants with concentration levels of different magnitudes.