Using bright sunshine at low-elevation angles to compile an historical record of the effect of aerosol on incoming solar radiation

In seeking an indicator of multi-decadal aerosol effects on incoming solar radiation, the records of bright sunshine from a standard meteorological instrument, the Campbell–Stokes sunshine recorder, have been examined using two new metrics: (i) the frequency of ‘burn’ for solar elevation angles below 2°, (ii) an efficiency of ‘burn’ measure at low-elevation angles. Proof-of-concept results from a site in north-west England, U.K., show signals in the relevant bright sunshine record consistent with known events of volcanic injection into the stratosphere, and with a brightening in the last decade compared with the 1970s. This brightening is consistent with regional surface air-pollution records and is most evident in winter – suggesting that it is due to a reduction of particulate emissions from space heating. Between the late 1970s and early 2000s, there was a nearly four-fold increase in the number of evenings and mornings around the solstices that recorded bright sunshine at elevation angles below 2°. Current meteorological practice assumes bright sunshine will not be recorded below 3°, and could therefore need amending. The method avoids the need for high-frequency cloud-cover data by normalising low-elevation sunshine against daily sunshine totals. This makes the method applicable at a much wider range of meteorological observation sites. Because of the extensive past use of Campbell–Stokes recorders, the method opens the possibility of generating a new archive of airborne particulate changes based on the attenuation of direct insolation at low-elevation angles.