Search

Historical smoke concentrations at monthly resolution for the early twentieth century are found for Kew Observatory, London, using the atmospheric electricity proxy technique. Smoke particles modify the electrical properties of urban air: an increase in smoke concentration reduces air's electrical conductivity and increases the Potential Gradient (PG). Calibrated PG data are available from Kew since 1898, and air conductivity was measured routinely between 1909 and 1979 using the technique developed by C.T.R. Wilson. Automated smoke observations at the same site overlap with the atmospheric electrical measurements from 1921, providing an absolute calibration to smoke concentration. This shows that the late nineteenth century winter smoke concentrations at Kew were approximately 100 times greater than contemporary winter smoke concentrations. Following smoke emission regulations reducing the smoke concentration, the electrical parameters of the urban air did not change dramatically. This is suggested to be due to a composition change, with an increase in the abundance of small aerosol compensating for the decrease in smoke.

Molecular cluster ions are produced throughout the troposphere by radon isotopes and cosmic rays, causing the slight electrical conductivity ( σ ) of atmospheric air. In a field campaign at the Weybourne Atmospheric Observatory on the Norfolk coast of the UK (52°57′23′′N 1°7′40′′E), aerosol size spectra were obtained simultaneously with σ measurements and background radioactivity, from 11th to 17th May 2004. σ varied with wind direction, from the modulation of the total aerosol number concentration Z rather than changes in ion production from radioactivity. σ was at its greatest when the air originated over the sea. Cluster ion properties were deduced using the σ data and the ion balance equation. The ion mobility μ decreased as Z decreased, corresponding to an increase in average cluster ion size. Using the attachment timescale of ions to aerosol as the typical time during which the ions can grow, negative ion growth occurred at ∼3 nm hr −1 . Aerosol concentration changes modulate the time in which the ions can grow and the availability of condensable vapour.

Aerosol particles reduce the electrical conductivity of air, through removal of the small ions causing the conductivity. Visual range is also affected by aerosol. Early 20th century work linked increased smoke pollution concentrations with decreased air conductivity and visual range. By analysing newly-recovered monthly atmospheric electricity data from Kew Observatory, near London, for 1968–1979, averages of the visual range and air conductivity show a close empirical relationship: visual range is reduced when air conductivity is also reduced. The visibility–conductivity correlation is likely to be due to a common modulation of both atmospheric properties by the aerosol number concentration. For the urban air data from Kew, the visual range D and monodisperse aerosol number concentration Z were related by Z D x = A , with x and A found empirically: A is a function of the mean aerosol radius. Through this relationship, local meteorological observations of visual range can provide additional information on aerosol concentration or the electrical properties of air, when no direct measurements are available.

Scavenging by water droplets is a mechanism for aerosol removal near clouds. Numerical methods are developed to quantify the removal of charged radioactive aerosols, including the electrical image force's contribution, attractive at small separations. Charging of radioactive aerosols is found to have significant effects on their collision efficiency and scavenging coefficient. The effect depends on the aerosol charge, and therefore, on the radioactive aerosol's decay rate and number concentration, but it does not depend significantly on the charge carried by the water drops. Scavenging coefficients are calculated for radioactive aerosols. For small particles at low aerosol concentrations (Z∼10–100 cm−3), charging can increase the scavenging coefficients by up to an order of magnitude. Electrification will, therefore, encourage the removal of small radioactive aerosols from the atmosphere, more rapidly than equivalent non-radioactive aerosols. The increase in removal at low radioactive-aerosol concentration may account for underpredictions of surface concentrations and will contribute to spatial variations in aerosol removal.

A "eld experiment was conducted to study the mean and turbulent characteristics of air#ow within, and above, a street under neutral strati"cation. Pro"les of the mean wind and turbulent statistics were obtained and compared with #ow over smoother terrain. Our results modify the conventional picture of a persistent re-circulation within the street with small-scale turbulent #uctuations about this mean (Oke, 1987). Instead, the mean re-circulation within the street was found to be much weaker than the unsteady turbulent #uctuations. Hence, the mean #ow is merely a residual of an unsteady turbulent re-circulation. The re-circulation in the street is coupled to the wind aloft through a shear layer that develops at the roof-level. The shear layer is unstable, through Kelvin}Helmholtz instability, which leads to intermittency in the re-circulation in the street, and thence to ventilation of the air in the street. This "nding is likely to be important for dispersion in urban areas. ( 2000 Elsevier Science Ltd. All rights reserved.

Krypton-85 is a chemically inert radioactive gas present in the atmosphere, concentrations of which have been greatly increased by nuclear reprocessing and weapons testing since 1945. The long half-life (10.7 yr), allows the gas to mix thoroughly in the atmosphere. Ionization caused by krypton-85 increases the electrical conductivity of atmospheric air. Further increases in krypton-85 emissions seem inevitable. The increase in air conductivity due to release of krypton-85 will vary with height, and be larger over the oceans than over the land. Increases in conductivity will produce uncertain effects on atmospheric phenomena, so changes are compared in magnitude with other factors perturbing the conductivity, such as combustion aerosol burdens, volcanic eruptions and nuclear weapons testing. Conductivity changes are expected to have the greatest significance for meteorological phenomena close to the source.

Atmospheric electrical measurements provide proxy data from which historic smoke pollution levels can be determined. This approach is applied to infer autumnal Parisian smoke levels in the 1890s, based on atmospheric electric potential measurements made at the surface and the summit of the Eiffel Tower (48.7°N, 2.4°E). A theoretical model of the development of the autumn convective boundary layer is used to determine when local pollution effects dominated the Eiffel Tower potential measurements. The diurnal variation of the Eiffel Tower potential showed a single oscillation, but it differs from the standard oceanic air potential gradient (PG) variations during the period 09-17UT, when the model indicates that the Eiffel Tower summit should be within the boundary layer. Outside these hours, the potential changes closely follow the clean air PG variation: this finding is used to calibrate the Eiffel Tower measurements. The surface smoke pollution concentration found during the morning maximum was 60±30μgm-3, substantially lower than the values previously inferred for Kew in 1863. A vertical smoke profile was also derived using a combination of the atmospheric electrical data and boundary layer meteorology theory. Midday smoke concentration decreased with height from 60μgm-3 at the surface to 15μgm-3 at the top of the Eiffel Tower. The 19th century PG measurements in both polluted and clean Parisian air present a unique resource for European air pollution and atmospheric composition studies, and early evidence of the global atmospheric electrical circuit. © 2003 Elsevier Ltd. All rights reserved.