The “direct effect” of aerosols is well known: particles in the atmosphere scatter solar radiation back out to space, reducing the amount of solar radiation reaching the surface of the Earth. Atmospheric aerosols originate from both natural sources (such as dust and wind-blown sea salt) and from anthropogenic sources – such as sulphate, nitrate and organic particles from industry and traffic. What then, is the impact of urban areas, which by definition have greater industry and traffic, thereby producing more pollution and atmospheric aerosol, on the amount of solar radiation reaching the surface, and can we measure this effect?
A new network of weather stations has been set up in schools across London, by the Open Air Laboratories (OPAL, www.opalexplorenature.org) and the London Grid for Learning, measuring temperature, pressure, wind speed and direction, rainfall, UV index, and solar radiation. Data is measured every minute, and can be viewed at http://weather.lgfl.org.uk. The OPAL London weather stations are probably the densest urban weather station network in the world, providing us with detailed information on how weather changes across the city.
We have used measurements of solar radiation from the OPAL network over the course of a year to examine how the amount of sunlight reaching the surface changes across London, from the suburbs, where we expect smaller amounts of pollution, to the city centre where pollution levels are higher. The image below shows the solar radiation anomaly across London relative to the annual midday average of 338Wm-2, based on measurements from 29 weather stations. The double black line shows the location of the M25, the thin black line the Thames River, and solid single black lines show +/- 15 Wm-2, the standard deviation from the measurements.
Clearly central London received less solar radiation compared to outer London, with the exception of the far northeast, and a concentric pattern is discernible. This is what we might expect from typical aerosol pollution patterns – more aerosol in central London depleting the amount of sunlight reaching the surface – but do aerosol measurements support this hypothesis?
London also has a network of air quality measurements: the London Air Quality Network (LAQN, www.londonair.org.uk). LAQN sites measure concentrations of ozone, carbon monoxide, nitrogen dioxide and sulphur dioxide, but also of particulate matter (PM) of two types – PM10 (particles smaller than 10 microns in size), and PM2.5 (particles smaller than 2.5 microns in size). The annual average midday concentrations (micrograms per cubic metre) of PM2.5 and PM10 are shown below.
Clearly the above images show that there are higher concentrations of aerosol towards central London compared to outer London, but can these be related to the observed changes in solar radiation across London?
Analysis of the data reveals that while solar radiation, PM2.5 and PM10 show a statistically significant relationship to distance from central London (which is officially defined as the statue of Charles I at the junction of the Strand, Whitehall and Cockspur Street, in case you were wondering), and that an increase of 1 microgram per cubic metre was found to correspond to a decrease in solar irradiance of 2.27 Wm-2.
Since these figures are only correlations and cannot be classed as causal, we undertook some modelling of solar radiation to try and calculate whether the measured aerosol or potential changes in cloud amount could be causing the observed differences in solar radiation across London. Our calculations showed that the aerosol optical depth (the amount of light extinguished in the atmosphere due to aerosol) would have to change by 0.18 across London, or cloud optical depth would have to change by 1.9 across London. A new AERONET site at UCL in London has been measuring aerosol optical depth, and typical values in London are around 0.18, compared to 0.15 measured at Chilbolton AERONET site, well outside of London. This difference is only 0.03, and therefore we conclude that spatial changes in aerosol across London are unable to explain the observed changes in solar radiation. However, our calculations do suggest that the aerosol might cause around 33-44% of the differences in measured solar radiation.
So if the aerosol changes across London are unable to explain the solar radiation measurements, what else could be responsible?
It is possible that the aerosol changes are having an effect on cloud optical thickness via the indirect aerosol effect. I.e. higher concentrations of aerosol particles allow more, smaller cloud droplets to form, scattering more sunlight away from the surface and decreasing the solar radiation at the surface. Based on other studies which have examined the relationship between aerosol and clouds and using the aerosol measurements from the PM sites, we estimate that the aerosol indirect effect might cause somewhere between 27 to 50% of the difference in surface solar radiation measured across London. The large range in this percentage reflects the large uncertainty in the magnitude of the indirect effect.
Therefore it appears that of the total changes in surface solar radiation across London, 33-40% could be caused by the direct radiative effect of aerosols, and 27-50% could be caused by the indirect effect. These figures involve large uncertainties, and do not cover the remaining 10 to 50% of the observed measurements, which might be caused by effects such as enhanced convection (and therefore cloud optical depth) over the urban heat island, changes in water vapour, column ozone, aerosol optical properties and surface albedo across London. Additionally, it’s probable that aerosols would also exert a longwave radiative effect which may counteract the shortwave cooling to some extent.
Nevertheless, the solar radiation measurements show a clear difference in measured solar radiation between central and outer London. The implications of these differences could be wide ranging – from the effects of lowered solar heating in city centres which could affect urban energy balance models, to the possibility that solar panels in city centres may not be as effective as those in rural areas.