Authors Daniel M Brookes and Andrew J Kent
Compilation date 13 September 2018
Customer Gibraltar Environmental Agency
Approved by Andrew J Kent
Copyright Ricardo Energy & Environment
EULA http://ee.ricardo.com/cms/eula/

Contract reference … Report reference …

1 Introduction

1.1 Air quality monitoring in Gibraltar

The Gibraltar Air Quality Data Digest provides an annual summary of the air pollutant measurements from the Gibraltar Air Quality Monitoring Network for 2017.

The Gibraltar Air Quality Monitoring Network is operated on behalf of the Government of Gibraltar by the Environmental Agency, with assistance from Ricardo Energy & Environment. The principal objective of the Gibraltar Air Quality Monitoring Network is for compliance reporting against the European Air Quality Directive (AQD) on ambient air quality and cleaner air for Europe (2008/50/EC) and the Fourth Daughter Directive (DD4) (2004/107/EC).

The Gibraltar Air Quality Monitoring Network includes a mixture of automatic pollutant monitoring stations and non-automatic air pollutant monitoring sites. The automatic air quality monitoring stations measure:

  • Oxides of nitrogen (NOx) and nitrogen dioxide (NO2)

  • Sulphur dioxide (SO2)
  • Ozone (O3)

  • Carbon monoxide (CO)

  • Particulates (PM10, PM2.5)

  • Heavy metals: Arsenic (As), Cadmium (Cd), Nickel (Ni), Lead (Pb)

  • Benzene (Bz) and

  • Polycyclic aromatic hydrocarbons (PAH), including Benzo[a]Pyrene (B[a]P).

Non-automatic sites measure monthly average concentrations of NO2 and benzene using a passive, diffusion tube, based method. Diffusion tube measurements have a much higher uncertainty than automatic measurements and are purely indicative of air pollutant concentrations. As such they cannot be compared against the specific legislative values specified in the European Air Quality Directive. Further information on the air pollutants measured in Gibraltar can be found on the Air Quality in Gibraltar website.

The sites within the Gibraltar automatic air quality monitoring network provide a combination of high resolution hourly measurements and daily average air pollutant concentrations. High resolution information is communicated rapidly to the public. The main platform for information dissemination of air quality measurement in Gibraltar is the Air Quality in Gibraltar website. Non-automatic measurement sites provide an indicative measure of roadside and background air pollutant concentrations at thirty-two sites throughout Gibraltar.

The major objectives of the Gibraltar Air Quality Monitoring Network are:

  • Checking if statutory air quality standards and targets are met (e.g., EC Directives)

  • Informing the public about air quality

  • Identifying long-term trends in air pollution concentrations

  • Assessing the effectiveness of policies to control pollution in Gibraltar, as laid out in the Gibraltar Air Quality Action Plan.

The air pollutant measurements from Gibraltar Air Quality Monitoring Network are used for:

  • Reporting to the European Commission as required by the Air Quality Directives

  • Comparison with air quality objectives prescribed by the Air Quality Directives

  • Providing the public with information through air quality bulletins

  • Forecasting future air quality levels

  • Policy development for human health protection.

This document provides graphical analysis and explanatory summaries of the air pollutant measurements made in Gibraltar. Compliance plots (as discussed in Section 2.1) are used to demonstrate instances in 2017 where Gibraltar complied with the Limit Values (LVs) or Target Values (TVs) given in the European Air Quality Directives. The other plots presented here provide useful information from which the source of air pollutants can be inferred.

1.2 Limit Values and Target Values

A summary of the relevant legislative thresholds prescribed in the Air Quality Directive and Fourth Daughter Directive are presented in Table 1. This shows each pollutant and the relevant metric and threshold applicable. Also shown is the type of threshold: Limit Value (LV), Target Value (TV), Long Term Objective (ozone only) and the year in which each LV or TV has to be achieved. These are represented in the ‘compliance plots’ as red dashed lines and the corresponding metrics are also presented in the summary data tables in this document. The air pollutants covered under the two Directives are:

  • Air Quality Directive (AQD)
    • NOx and NO2

    • PM10

    • PM2.5
    • CO
    • SO2
    • Ozone
    • Benzene

    • Lead

  • Fourth Daughter Directive (DD4)

    • Benzo[a]Pyrene (B[a]P)

    • Arsenic
    • Cadmium

    • Nickel


Table 1: Summary of current Limit Values and Target Values (the number of permissible exceedances per year are given in brackets next to the Value)

Pollutant Type Year in force Metric Value
NO2 LV 2010 Annual mean 40 μg m-3
NO2 LV 2010 1-hr mean 200 μg m-3 (18 allowed)
PM10 LV 2005 Annual mean 40 μg m-3
PM10 LV 2005 24-hr mean 50 μg m-3 (35 allowed)
PM2.5 TV/LV (Stage 1) 2010/2014 Annual mean 25 μg m-3
PM2.5 LV (Stage 2) 2020 Annual mean 20 μg m-3
CO LV 2005 Maximum daily running 8-hr mean 10 mg m-3
SO2 LV 2005 1-hr mean 350 μg m-3 (24 allowed)
SO2 LV 2005 24-hr mean 125 μg m-3 (3 allowed)
O3 LTO Not defined Maximum daily running 8-hr mean 120 μg m-3
Benzene LV 2010 Annual mean 5 μg m-3
Lead LV 2005 Annual mean 0.5 μg m-3
B[a]P TV 2013 Annual mean 1 ng m-3
Arsenic TV 2013 Annual mean 6 ng m-3
Cadmium TV 2013 Annual mean 5 ng m-3
Nickel TV 2013 Annual mean 20 ng m-3


1.3 Air quality monitoring locations

Information on the air pollutant monitoring methods and the composition of the Gibraltar Air Quality Monitoring Network can be found on the Gibraltar Air Quality Monitoring Network website. Table 2 provides a summary of the air pollutant monitoring stations and air pollutants measured at the automatic monitoring stations and non-automatic monitoring sites in Gibraltar in 2017. Figures 1 and 2 show the locations of the automatic monitoring stations and the non-automatic sampling points in Gibraltar in 2017, respectively.


Table 2: Summary of automatic monitoring stations and non-automatic monitoring sites in Gibraltar in 2017.

Network Station name Pollutants measured
Automatic monitoring sites Bleak House NOx, NO2, PM10 and ozone
Rosia Road NOx, NO2, PM10, PM2.5, CO, SO2, benzene, PAH and metals
Withams Road NOx and NO2
Non-automatic monitoring sites Multiple sites NO2 and/or benzene


Figure 1: Location of automatic air quality monitoring stations in Gibraltar, 2017


Figure 2: Location of non-automatic air quality monitoring sites in Gibraltar, 2017


Table 3: Co-ordinates of non-automatic air quality monitoring sites in Gibraltar, 2017, as shown in Figure 2.


1.4 Data processing

The data summaries and plots presented this document were generated using openair, an open-source air quality analysis toolset written in the R statistical programming language.

1.5 Meteorological observations

A summary of 2017 meteorological observations are presented to provide a context for the air pollutant concentrations measured in Gibraltar. The influence of the Rock on the flow of the wind field is evident: wind speed and direction measurements vary markedly when correponding measurements are compared to one another. Wind speeds measured using the Gibraltar Airport meteorological mast (reference height, z = 10 m), tend to be higher than those measured at the three automatic air quality monitoring stations, where wind speeds are measured at a height of 6-8 m, and are therefore correspondingly lower.

2 Description of plot types

This section provides a summary of the main plot types used in the Gibraltar Air Quality Data Digest. The caption of each plot is cross-referenced to this section to allow the reader to easily find the summary description of the plot.

2.1 Compliance plot

The compliance plots show the mean concentrations, expressed as annual mean or an appropriate equivalent percentile for the year. This allows comparison of the mean value with a legislative threshold, e.g., an air quality Directive Limit Value (LV) or Target Value (TV) which is represented by the dashed red line. In instances where the European air quality Directives specify a permissible number of occasions above a threshold, the plotted data are presented as the percentile equivalent to that metric. For example, 99.8th percentile of hourly mean NO2 concentrations is equivalent to 18 permissible exceedances of the hourly NO2 LV (200 μg m-3). Compliance is indicated by values below 200 μg m-3. Likewise for PM10, the 90th percentile of daily mean PM10 is equivalent to 35 permissible exceedances of the daily PM10 LV (50 μg m-3). The plots are scaled to best represent the concentrations so where the red dashed line does not appear on the plot, this means that the measured concentrations are sufficiently below the threshold that it does not appear on the plot. PM2.5 has two thresholds specified by the Directive: A Stage 1 LV and TV at 25 μg m-3 and a Stage 2 LV at 20 μg m-3.

2.2 Wind rose

The wind rose plot shows the variation in wind speed and direction at a site. The classified colour scale represents different wind speeds (in m s-1). The magnitude of the wind speed is shown by the colours indicated in the colour bar. The orientation of the wind sectors around the plot origin represents the wind direction binned in 30° sectors. The distance outwards from the plot origin represents percentage of time the wind speed originated from a particular direction. Therefore by adding each percentage together should sum to 100.

2.3 Trend plot

The trend plot shows monthly mean air pollutant concentrations (indicated by a thin line joining closed circles) and the non-linear trend fitted to the data (bold line). The shaded grey bands represent the 95% confidence interval of the data. These plots demonstrate temporal trends over months and/or years and can be used to determine long-term trends. In some plots this data has been split according to wind direction sectors by using associated meteorological data at the site for which the trends are being plotted – in this way trends associated with air from different directions (and associated sources) can be shown. When splitting data according to wind direction (and applying a data capture threshold to the aggregated statistic to ensure it is representative for the period it is covering) there is a reduction in data volume to plot by wind sector. For this reason some of the plot panes are omitted where the data volume is too low to plot. In Gibraltar this can result from the topographical effects on the local meteorology (from the air quality monitoring station) at the point of measurement (e.g. fewer meteorological measurements of a particular wind direction as a result of the Rock acting as a barrier).

2.4 Contribution plot

Contribution plots show the influence of the wind direction on mean air pollutant concentrations. As with the wind rose, the orientation of the wind sectors around the plot origin represents the wind direction binned in 30° sectors. The radial scale indicates the percentage of time the wind from a particular wind sector contributed to the overall mean concentration. The plots show the contribution of each wind sector to the overall mean air pollutant concentration; this is particularly useful for showing the influence of wind direction on high air pollutant concentrations.

2.5 Calendar plot

Calendar plots are laid out in a similar style to a wall calendar and show the daily mean air pollutant concentrations disaggregated by day-of-week and month-of-year. The colour of each box corresponds to the daily mean pollutant concentration indicated by the colour bar. The prevailing daily wind speed is indicated by the length of arrow in each box, and the wind direction, by the arrow orientation. This allows the daily mean air pollutant concentrations to be seen relative to one another. It also allows seasonal patterns in air pollutant concentrations, trends, and the variation of the local wind field, as well as the influence of meteorology on air pollutant concentrations to be seen.

2.6 Polar plot

Polar plots show the relationship between air pollutant concentrations and wind speed and wind direction. The wind speed is represented by the distance from the plot origin and the wind direction is the orientation about the origin (as in other plots described above). The continuous colour scale represents the air pollutant concentration. The plot provides a continuous surface by smoothing between observed data points.

2.7 Time variation plot

The time variation plot shows changing concentrations over several different time scales:

  • Hour of the day and day of the week (upper pane)

  • Hour of the day (diurnal variation, in the lower left pane)

  • Month (seasonal variation, in the lower central pane)

  • Day of week (in the lower right pane).

The time variation plot is useful for representing changes due to different source activities. For example, fugitive dust emissions from building work, which may cause an increase in the PM10 mass concentration, might occur at specific hours of the day or days of the week; likewise NOx and NO2 associated with road traffic exhausts are typically higher during week days than weekend and at morning and evening rush hours.

This plot requires high-resolution (hourly) measurements of air pollutant concentrations in order to produce the temporal plot shown across the top panel and on the bottom right hand-side of the plot. PM10 in Gibraltar is measured using two techniques: The FDMS, which provides hourly mean PM10 mass concentrations, and by the Partisol, which provides daily mean PM10 mass concentrations. PM2.5 is only measured using the Partisol method. Therefore it is not possible to produce a time variation plot for PM2.5 mass concentrations in Gibraltar.

2.8 Spot concentration plot

The spot concentration plot shows the measured air pollutant concentration for a range of locations. This plot is ideal for summarising the results from the NO2 and benzene diffusion tube measurements from the Gibraltar non-automatic monitoring network. Air pollutant concentrations from the non-automatic monitoring network are measured at multiple sites

Two plots are presented for both pollutants. The first showing the annual mean air pollutant concentration at each site. The second shows the difference in the annual mean for the current year versus the long-term average, for each site, to show the long trends in air pollutant concentrations. This can be used to indicate whether air quality is improving or declining at these locations.

It should be noted that diffusion tube measurements are purely indicative of air pollutant concentrations. As such they cannot be compared against the specific legislative values specified in the European Air Quality Directive.

2.9 Daily trend plot

Daily trend plots show the variation in air pollutant concentrations across a single year with vertical bars representing the daily mean concentrations of one or more pollutants.

3 Summary Plots

3.1 Wind speed and direction


Figure 3: Frequency plot of wind speed and direction at the airport and Gibraltar automatic air quality monitoring sites, 2017 (see wind rose plot in subsection 2.2)


3.2 NOx and NO2


Figure 4: Annual mean NO2 measured at Gibraltar automatic air quality monitoring sites, 2005-2017 (see compliance plot in subsection 2.1)


Figure 5: 99.8th percentile 1-hr NO2 measured at Gibraltar automatic air quality monitoring sites, 2005-2017 (see compliance plot in subsection 2.1)


Figure 6: The year-on-year NOX concentration trend at automatic air quality monitoring sites, 2005-2017 (see trend plot in subsection 2.3)


Figure 7: The year-on-year NO2 concentration trend at automatic air quality monitoring sites, 2005-2017 (see trend plot in subsection 2.3)


Figure 8: The percentage contribution of wind direction to annual mean NOX concentration measured at automatic air quality monitoring sites, 2017 (see contribution plot in subsection 2.3)


Figure 9: Daily mean NO2 concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 10: Daily mean NO2 concentrations at Withams Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 11: Variation in NOX and NO2 concentrations measured at Rosia Road automatic monitoring site for different time periods, 2017 (see time variation plot in subsection 2.7)


Figure 12: Variation in NOX and NO2 concentrations measured at Withams Road automatic monitoring site for different time periods, 2017 (see time variation plot in subsection 2.7)


Figure 13: Polar plots of NOX concentrations for Gibraltar automatic air quality monitoring site, 2017 (see polar plot in subsection 2.6)


Figure 14: Annual mean NO2 diffusion tube concentrations, 2017 (see spot concentration plot in subsection 2.6)


Figure 15: Difference in annual mean NO2 diffusion tube concentrations 2017 minus the mean of 2005-2016 concentrations (see spot concentration plot in subsection 2.6)

3.3 Particulate matter


Figure 16: Annual mean gravimetric PM10 measured at Gibraltar automatic air quality monitoring sites, 2005-2017 (see compliance plot in subsection 2.1)


Figure 17: 90th percentile gravimetric PM10 concentrations measured at Gibraltar automatic air quality monitoring sites, 2005-2017 (see compliance plot in subsection 2.1)


Figure 18: Annual mean gravimetric PM2.5 measured at Rosia Road automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 19: The year-on-year gravimetric PM10 and PM2.5 concentration trend at Rosia Road automatic monitoring site, 2005-2017 (see trend plot in subsection 2.3)


Figure 20: The year-on-year gravimetric PM10 concentration trend at Rosia Road automatic monitoring site, 2005-2017, split by wind direction sector (see trend plot in sub-section 2.3)


Figure 21: The year-on-year gravimetric PMcoarse concentration trend at Rosia Road automatic monitoring site, 2005-2017, split by wind direction sector (see trend plot in subsection 2.3)


Figure 22: The percentage contribution of wind direction to daily mean gravimetric PM10, PM2.5 and coarse PM concentrations measured at Rosia Road automatic monitoring site, 2017 (see contribution plot in subsection 2.3)


Figure 23: Daily mean gravimetric PM10 concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 24: Daily mean gravimetric PM10 concentrations at Bleak House automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 25: Daily mean gravimetric PM2.5 concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 26: Daily mean gravimetric coarse PM concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)


Figure 27: Variation in PM10 concentrations (TEOM-FDMS data) measured at Rosia Road automatic monitoring site for different time periods, 2017 (see time variation plot in subsection 2.7)


Figure 28: Polar plots of daily mean gravimetric PM10, PM2.5 and coarse PM concentrations measured at Rosia Road automatic monitoring site, 2017 (see polar plot in subsection 2.6)


3.4 SO2


Figure 29: p99.18 of 24-hr mean SO2 concentrations measured at Rosia Road automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 30: p99.73 of 1-hr mean SO2 concentrations measured at Rosia Road automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 31: The year-on-year SO2 concentration trend at Rosia Road automatic monitoring site, 2005-2017 (see trend plot in subsection 2.3)


Figure 32: Daily mean SO2 concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)

3.5 CO


Figure 33: Maximum daily 8-hr CO concentrations measured at Rosia Road automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 34: The year-on-year CO concentration trend at Rosia Road automatic monitoring site, 2005-2017 (see trend plot in subsection 2.3)


Figure 35: Daily mean CO concentrations at Rosia Road automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)

3.6 Ozone


Figure 36: Maximum daily 8-hr ozone concentrations measured at Bleak House automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 37: The year-on-year ozone concentration trend at Bleak House automatic monitoring site, 2005-2017 (see trend plot in subsection 2.3)


Figure 38: Daily mean ozone concentrations at Bleak House automatic monitoring site over the year, 2017 (see calendar plot in subsection 2.5)

3.7 Benzene


Figure 39: Annual mean benzene concentrations measured at Rosia Road automatic monitoring site, 2005-2017 (see compliance plot in subsection 2.1)


Figure 40: Annual mean benzene diffusion tube concentrations, 2017 (see spot concentration plot in subsection 2.6)


Figure 41: Difference in annual mean benzene diffusion tube concentrations 2017 minus the mean of 2005-2016 concentrations (see spot concentration plot in subsection 2.6)


4 Summary Statistics


Table 4: Summary NO2 statistics (automatic data), 2005-2017.


Table 5: Summary PM10 statistics (gravimetric data), 2005-2017.


Table 6: Summary PM2.5 statistics (gravimetric data), 2005-2017.


Table 7: Summary SO2 statistics, 2005-2017.


Table 8: Summary CO statistics, 2005-2017.


Table 9: Summary O3 statistics, 2005-2017.

*AOT 40 is the sum of the differences between hourly concentrations greater than 80 μg m-3, i.e. 40 ppb, and 80 μg m-3, over a given period (the months of May-July) using only the 1-hr averages measured between 08:00 and 20:00 hrs.


Table 10: Summary Benzene statistics (automatic), 2005-2017.


Table 11: Summary Metals (arsenic, cadmium, nickel, lead) statistics, 2008-2017.


Table 12: Summary PAH (BaP) statistics, 2008-2017.



For further information, please contact:

Name Andrew J Kent
Address Ricardo Energy & Environment, Gemini Building, Harwell, Didcot, OX11 0QR, United Kingdom
Telephone 01235 753629
Email andrew.kent@ricardo.com