The Effect of Cloudiness on New Particle Formation: Investigation of Radiation Levels

The seasonal cloud effects on NPF events (E) and the radiation levels during the nucleation have not been systematically investigated. In this research, we study the cloudiness effects on E using a relative radiation intensity parameter I/Imax as an indicator of cloudiness level. The I/Imax parameter has been defined as the ratio of measured global-radiation I (w m-2) at a given time of day divided by the modeled clear-sky global radiation Imax (w m-2) at the same time. The ratio data are provided for all days including E, non-event (NE) and undefined (i.e. neither E nor NE) days during 2002-2012 for SMEAR II located in Hyytiälä , Southern Finland and during March 2002- April 2005 for San Pietro Capofiume (SPC) measurement station, Po Vally, Italy. We analysed the ratio I/Imax only for the duration between nucleation start and end times for Es, and from one hour after sunrise until noon for NE and undefined days. We averaged the ratio I/Imax values of each day over the determined time windows (average ratio) for E and NEs, and calculated the fraction (%) of days which are E (ΔE) or NE (ΔNE) in the given ranges of average ratio with the interval widths of 0.05. The results show that the higher (lower) the average I/Imax ranges, the higher (lower) ΔE (ΔNE) values. The ΔE values in the ranges lower than 0.95 in particular low ranges (i.e. the presence of cloud) is much less than Hyytiälä, indicating that presence of cloud during the period when E is going on is much more probable in Hyytiälä than SPC. We have investigated the seasonal distribution of Δ values in Hyytiälä. Interestingly, ΔE statistics in the ranges with interval widths 0.05 limited to 0.5-0.9, in autumn and spring is much higher than summer. In addition, the ΔE values have plateaued in the ranges with the intervals 0.05 limited to [0.65-0.8] in summer (201 Es), in contrast to spring (442 Es) and autumn (164 Es) where an increasing trend is observed. A few Es in which Clouds are present for a substantial period of (or whole) 3nm-time-window, in all seasons have been seen. Either very low condensation sink (CS) values or very high SO2 values favor such Es despite of cloudiness in both sites. In addition, there are some NE days in clear sky condition (i.e. high I/Imax values). Investigation of such NEs with the I/Imax values > 0.8 reveals that, very high CS values in both sites with average values 5.3 ×10-3 s-1 and 19.2 ×10-3 s-1 in Hyytiälä and SPC, respectively, has prevented the occurrence of nucleation event. In addition, high amounts of H2O in Hyytiälä and high NO2 values in SPC explain the non-occurrence of nucleation event under clear sky condition. Fraction (%) of days which are Event (ΔE) and Non-event (ΔNE) in different average ratio I/Imax ranges in (Left) Hyytiälä during period 2002-2012 and (Right) SPC during March 2002-April 2005. Q=Quantifiable (clear) event, NQ=non-Quantifiable (weak) event.