1 Introduction

This interactive report is run on a weekly basis and provided to City of London Corporation on 25 February 2024 morning. It shows a weekly summary for the period Monday February 19 2024 to Sunday February 25 of up to date provisional monitoring data for the site and presents a simple summary of the current concentrations in the context of recent weather patterns and anticipated short term weather. It can be used by the Local Authority to monitor current levels, understand and explain any recent episodes and to subjectively forecast and act on the potential for evolving episodes. The reports provides:

• a time series plot
  
• an hourly measurement table
  
• a summary table
  
• air mass origins maps (forecast back trajectories)
  
• weather forecast for the coming week (‘Windytv’)

Concentrations are provisional, pending ratification. All gaseous pollutant mass units are at 20 degrees Celcius and 1013 mb. NO_X mass units are NO_X as NO₂ μgm^-3.

Many elements of the report are interactive and can be intuitively manipulated by the reader to provide a more useful and engaging information resource. For example, the floating table of contents to the left, follows the user throughout the report and is a simple, convenient, expanding/collapsing navigation tool. The tables display the first 10 rows initially but the user can expand this to their own preference to see the complete table in a single view using the dropdown option. The user can also search the table using the box to filter the appropriate entries, for example by a specific start or end date, by site, by metric or by site type etc. Other interactive components of the report are briefly summarised in each relevant section.

4 Air Mass Origins (weekly)

The back trajectory plot shows data from the HYSPLIT forecast model (NOAA HYSPLIT). This shows the air mass back trajectories for the previous day (2024-02-24), the trajectory for the current day (2024-02-25) and the forecast for the coming days.

Some examples of air mass origins and their impact on measured concentrations are described below:

• When trajectories demonstrate strong winds (long gaps between markers) this generally (but not always!) suggests effective dispersion of local emissions. When the markers are closer together this generally represents lower wind speed which might be responsible for poorer dispersion and the accumulation of pollution from local emissions sources.
• Strong winds can represent increased long range transport of pollutants. For example, trajectories that extend over the Sahara can indicate transport of African dust to the UK and air masses which originate from Russia can transport primary particles from Russian forest clearance and burning to the UK.
• When trajectories extend far out over the Atlantic Ocean where there are no sources (except perhaps sea salt particles), the concentrations of pollutants measured in the UK are expected to be low.
• In the UK during anti-cyclonic (high pressure) conditions, the trajectory lines are often tightly curled, close together and represent low wind speed recirculation of air. This kind of stagnation episode is characterised by increased concentrations.
• When trajectories extend over other land masses (e.g. Europe) there is potential for transboundary pollution like particles and ozone or ozone precursors to be imported to the UK. During spring time (typically March), sustained winds from the continent can produce secondary aerosol episodes seen as elevated PM_2.5 concentrations (and finer fractions) associated with emissions from agriculture. Generally, by late April, temperatures are high enough that the ammonium nitrate responsible breaks down in the atmosphere before concentrations accumulate.

Wind speeds and directions can be seen in even more detail in the dynamic map in the section below which shows strong winds as larger arrows and light winds as smaller arrows.

5 Weather forecast

The animation and daily forecast bar below show the weather forecast in London for coming days. The animation is interactive: press the play button at the bottom of the animation to view the animated forecast. Hover the cursor over the animation and a bar to the right hand side will appear. From this bar the user can select to view the forecast wind speed and direction, cloud cover, temperature, rain, waves or pressure. This weather forecast is sourced from Windytv and is the current forecast for the day at which this document is viewed (and therefore is directly relevant at the time the report is provided but which will not refer to the period that monitoring data covers if viewed at a later time).