New Particle Formation in Marine Air
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Marine aerosols contribute significantly to global climate directly by absorbing or scattering solar radiation, as well as indirectly by altering the reflectance and persistence of clouds. This work presents results of several investigations into the physicochemical properties of particulate matter over the North East Atlantic ocean. A suite of specifically designed aerosol instrumentation was used to perform an analysis of the characteristics of aerosol size distributions measured in air masses advecting over the Mace Head Atmospheric Research Station during the year 2008. During this time twelve aerosol size distribution clusters were identified as systematically occurring, which were further categorised into four groups with similar characteristics: coastal nucleation category (occurring 21.3 % of the time), open ocean nucleation category (occurring 32.6% of the time), background clean marine category (occurring 26.1% of the time) and anthropogenic category (occurring 20% of the time). Analysis and observations of open ocean new particle production are also reported, where new particle formation events were observed to form a distinct peak in the size distribution with a mode at ~15 nm and grow to a mode of ~50 nm over periods of 24-48 hours, during which time air masses were calculated to have advected over biologically-rich waters in the North Atlantic before detection. A study of size distribution measurements carried out at Mace Head over a seven year period, showed that these nucleation events also exhibit a seasonality, with a monthly average occurrence of 5.7 per percentage occurrence of clean air, peaking in May. In an investigation of new particle formation from Laminaria digitata macroalgae, aerosol nucleation in a range of I2 (0.3 - 76 ppbv) and O3 (<3 - 96 ppbv) mixing ratios was found to be significant, as well as correlated (R2 = 0.95) with I2 for low O3 mixing ratios (<3 ppbv). In experiments where particle production as a function of laboratory-generated I2 over a mixing ratio range of 1-8 ppbv was conducted under moderate O3 mixing ratios (~24 ppbv), a 100-fold or greater increase in the aerosol number concentrations and mass fluxes was observed compared to the low O3 experiments. A linear relationship (R2 = 0.81) between particle concentration and I2 was also found and this relationship suggests an I2 mixing ratio range of 6-93 pptv for particle production events frequently observed at Mace Head. Simulations examining the ability of the modal aerosol microphysical model M7 to predict new particle formation and growth from condensable iodine vapours also yielded large particle number concentrations. From a base case particle concentration of 222 cm-3 at radii >15nm, increases in concentrations to 366 cm-3 were predicted from a case where the nucleation was assumed to be from OIO-OIO, 722 cm-3 for a case where OIOH2SO4 case was assumed to be involved in the particle formation process, and 1584 cm-3 for a OIO-H2SO4 case where there was also additional condensing organic vapours. This work shows that despite being characterised by low water solubility, particles containing a large amount of organic matter in addition possess a high CCN activation efficiency. Also, investigation into marine particle size distributions shows that when volcanic ash is present in the marine atmosphere, a bimodal aerosol size distribution can be observed. In an examination of the Eyjafjallajökull ash plumes present in air masses arriving at Mace Head, a clear peak in the mass distribution at a diameter of 600 nm was observed, with the measured particles having been extremely hydrophilic. This work has elucidated some of the statistical characteristics of marine size distributions as well as providing new information on the underlying mechanisms of observed marine particle nucleation events, which in turn will allow for a more complete understanding of marine aerosols and their impact on climate change.
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