An outdoor site adjoining Durham Observatory was instrumented to measure the atmospheric potential gradient and precipitation current at the ground during periods of steady, quiet precipitation. A system was constructed to automatically record these quantities and to present the data in a form suitable for input to a computer. Examination of most periods of quiet precipitation between January and June 1972 shows that rain is usually positively charged, with the potential gradient being negative, while during snow these signs are reversed. There is nearly always significant correlation between the two electrical quantities, with during rain variations in potential gradient most often leading those in precipitation current by several minutes; during snow the precipitation current leads by a similar amount. These two effects correspond to the so-called "inverse relation" and "mirror-image- effect" often quoted in previous work. These results are shown to be consistent with the precipitation charge being due to two charging processes, one acting on solid precipitation within the cloud, and the second occurring during the melting of snow to rain. Examination of aerological data shows that the time lags between corresponding variations of the electrical quantities can be explained during rain by the effects of the wind shear between the cloud and the ground on the falling precipitation. The different electrical behaviour of a few periods of precipitation can be explained by ,the effects of a slow-moving cloud where electrical development is taking place, rather than by a passing cloud with constant electrical activity. Comparison of periods of quiet precipitation with "disturbed" periods, when the electrical activity is much greater, suggests that the transition to disturbed precipitation occurs when the rate of electrical sign reversals exceeds 2 per hour and the precipitation rate exceeds 1-0 mm hr ,^ The connection between the degrees of electrical and meteorological activity agrees with the suggestion that the electrical activity reflects the degree of atmospheric stability within the cloud.

Evidence for the existance of volcanic lightning is produced from a literature survey and the charge sources which have been suggested are listed and those, of the 18th and 19th centuries, which involved boiling water are reviewed in detail. Studies of saline contact charging, undertaken by Blanchard et al, when Surtsey emerged in 1963 are reviewed and lead to the continuation, in this investigation, of the Leidenfrost form of boiling and the discovery of an associated charging process. This discovery resolves a two centuries long dispute as to whether the Leidenfrost phenomenon of boiling produces charge as well as showing its important relevance to volcanic lightning where solution contact charging is in evidence. The mechanisms by which charged particles are emitted and formed into hollow solute spheres have been investigated and their charge and size distributions determined. Their structure has been compared with the structure of particles produced by flash boiling and spraying of saline solution. The energy conversion involved in the particle emission mechanism has also been studied. Charging mechanisms and some charging processes have been reviewed and by comparing results from other investigations speculations as to which mechanisms produce charge in the Leidenfrost process have been attempted. The comparison of results leads to the suggestion that Blanchard's observations are more likely to be Leidenfrost boiling than, as he assumed, flash boiling. The high average charge per particle of 10^-14C on particles from Leidenfrost boiling indicate that the number density is smaller, by a factor of 10⁴, than that previously suggested to produce the fields measured in volcanic clouds. Exploratory experiments have established that in addition to saline solution other aqueous solutions will also produce charge.

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A new experimental methodology was developed to measure the non-volatile particle number concentration, using a thermodenuder (TD). The TD was coupled with a high resolution time-of-flight aerosol mass spectrometer, measuring the chemical composition and mass size distribution of the submicrometer aerosol and a scanning mobility particle sizer (SMPS) that provided the number size distribution of the aerosol in the range from 10 to 500 nm. The method was evaluated with a set of smog chamber experiments and achieved almost complete evaporation (98 %) of secondary organic aerosol (SOA) as well as freshly nucleated particles, using the TD temperature of 400 C. This experimental approach was applied in a winter field campaign in Athens and provided a direct measurement of non-volatile particles from major pollution sources. During periods in which the contribution of biomass burning sources was dominant, more than 80% of the particles survived the intense heating, suggesting that nearly all biomass burning particles had a non-volatile core. These particles consisted mostly from black carbon (BC), while organic aerosol (OA) were responsible for another 40 %. Organics that survived through the TD were mostly biomass burning OA (BBOA) and oxygenated OA (OOA) that had not evaporated, contributing 90% of the organic mass concentration, while the other 10% was hydrocarbon-like OA (HOA) and cooking OA (COA). For periods that traffic contribution was dominant, mostly during the rush hour, 50-60% of the particles had a non-volatile core, while the rest evaporated at 400 0C. The remaining particles consisted mostly from BC, with an 80% contribution, while organics were responsible for another 15-20 %. Organics were mostly HOA and OOA, with a contribution of >95% to the organic mass concentration, while <5% was from BBOA and COA. / Μια νέα πειραματική μεθοδολογία αναπτύχθηκε για τη μέτρηση της συγκέντρωσης αριθμού μη πτητικών σωματιδίων, χρησιμοποιώντας εναν θερμοαπογυμνωτή. Ο θερμοαπογυμνωτής ήταν συζευγμένος με ένα υψηλής-ανάλυσης χρόνου-πτήσης φασματόμετρο μάζας (HR-ToF-AMS), το οποίο μετρούσε τη χημική σύσταση και τη κατανομή μεγέθους μάζας των υπο-μικρομέτρου αεροζόλ και ένα σαρωτή κινητικότητας μεγέθους σωματιδίων (SMPS) που παρείχε τη κατανομή μεγέθους αριθμού των αεροζόλ στο εύρος απο 10 έως 500 nm. Η μέθοδος αξιολογήθηκε με μια σειρά πειραμάτων σε θάλαμο προσομοίωσης ατμόσφαιρας και πέτυχε σχεδόν πλήρη εξάτμιση (98 %) των δευτερογενών οργανικών αεροζόλ (SOA), καθώς και φρέσκων σωματιδίων προερχόμενων απο πυρηνογένεση, χρησιμοποιώντας το θερμοαπογυμνωτή στη θερμοκρασία των 400 °C. Αυτή η πειραματική προσέγγιση εφαρμόστηκε σε μια χειμερινή καμπάνια πεδίου στην Αθήνα και παρείχε μια άμεση μέτρηση των μη πτητικών σωματιδίων απο τις κύριες πηγές ρύπανσης. Κατά τη διάρκεια περιόδων όπου η συνεισφορά των πηγών καύσης βιομάζας ήταν κυρίαρχη, περισσότερο απο 80 % των σωματιδίων επέζησαν απο την έντονη θέρμανση, γεγονός που υποδηλώνει ότι σχεδόν όλα τα σωματίδια καύσης βιομάζας είχαν μη πτητικό πυρήνα. Αυτά τα σωματίδια αποτελούνταν ως επί το πλείστον από μαύρο άνθρακα (BC), ενώ τα οργανικά αεροζόλ (ΟΑ) ήταν υπεύθυνα για το υπόλοιπο 40 %. Τα οργανικά που επιβίωσαν έπειτα απο τη διέλευση τους απο το θερμοαπογυμνωτή ήταν κυρίως απο καύση βιομάζας (ΒΒΟΑ) και οξυγωνομένα ΟΑ (ΟΟΑ), συμβάλλοντας στο 90 % της συγκέντρωσης οργανικής μάζας, ενώ το υπόλοιπο 10 % ήταν οργανικοί υδρογονάνθρακες (ΗΟΑ) και οργανικά αεροζόλ προερχόμενα από μαγείρεμα (COA). Για τις περιόδους όπου η συμβολή της κυκλοφορίας ήταν κυρίαρχη, κυρίως κατά τη διάρκεια ωρών αιχμής, 50 έως 60 % των σωματιδίων είχαν μη πτητικό πυρήνα, ενώ τα υπόλοιπα εξατμίστηκαν στους 400 °C. Τα υπολοίποντα σωματίδια αποτελούνταν κυρίως από BC, με 80 % συνεισφορά, ενώ τα οργανικά ήταν υπεύθυνα για το 15 με 20 % της συγκέντρωσης μάζας. Τα οργανικά αποτελούνταν από ΗΟΑ και OOA, με μια συμβολή > 95 % στη συγκέντρωση οργανικής μάζας, ενώ < 5 % ήταν από BBOA και COA.