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- ItemAerosol physical properties and processes in the lower marine boundary layer: A comparison of shipboard sub-micron data from ACE-1 and ACE-2(Milton Park : Taylor & Francis, 2016) Bates, Timothy S.; Quinn, Patricia K.; Covert, David S.; Coffman, Derek J.; Johnson, James E.; Wiedensohler, AlfredThe goals of the IGAC Aerosol Characterization Experiments (ACE) are to determine and understand the properties and controlling processes of the aerosol in a globally representative range of natural and anthropogenically perturbed environments. ACE-1 was conducted in the remote marine atmosphere south of Australia while ACE-2 was conducted in the anthropogenically modified atmosphere of the Eastern North Atlantic. In-situ shipboard measurements from the RV Discoverer(ACE-1) and the RV Professor Vodyanitskiy(ACE-2), combined with calculated back trajectories can be used to define the physical properties of the sub-micron aerosol in marine boundary layer (MBL) air masses from the remote Southern Ocean, Western Europe, the Iberian coast, the Mediterranean and the background Atlantic Ocean. The differences in these aerosol properties, combined with dimethylsulfide, sulfur dioxide and meteorological measurements provide a means to assess processes that affect the aerosol distribution. The background sub-micron aerosol measured over the Atlantic Ocean during ACE-2 was more abundant (number and volume) and appeared to be more aged than that measured over the Southern Ocean during ACE-1. Based on seawater DMS measurements and wind speed, the oceanic source of non-sea-salt sulfur and sea-salt to the background marine atmosphere during ACE-1 and ACE-2 was similar. However, the synoptic meteorological pattern was quite different during ACE-1 and ACE-2. The frequent frontal passages during ACE-1 resulted in the mixing of nucleation mode particles into the marine boundary layer from the free troposphere and relatively short aerosol residence times. In the more stable meteorological setting of ACE-2, a significant nucleation mode aerosol was observed in the MBL only for a half day period associated with a weak frontal system. As a result of the longer MBL aerosol residence times, the average background ACE-2 accumulation mode aerosol had a larger diameter and higher number concentration than during ACE-1. The sub-micron aerosol number size distributions in the air masses that passed over Western Europe, the Mediterranean, and coastal Portugal were distinctly different from each other and the background aerosol. The differences can be attributed to the age of the air mass and the degree of cloud processing.
- ItemAn overview of the Lagrangian experiments undertaken during the North Atlantic regional Aerosol Characterisation Experiment (ACE-2)(Milton Park : Taylor & Francis, 2016) Johnson, Doug W.; Osborne, Simon; Wood, Robert; Suhre, Karsten; Johnson, Randy; Businger, Steven; Quinn, Patricia K.; Wiedensohler, Alfred; Durkee, Philip A.; Russell, Lynn M.; Andreae, Meinrat O.; O’Dowd, Colin; Noone, Kevin J.; Bandy, Brian; Rudolph, J.; Rapsomanikis, SpyrosOne of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of doing this is in a Lagrangian framework where a parcel of air is sampled over several tens of hours and its physical and chemical properties are intensively measured. During the intensive observational phase of ACE-2, between 15 June 1997 and 24 July 1997, 3 cloudy Lagrangian experiments and 3 cloud-free, Lagrangian experiments were undertaken between the south west tip of the Iberian Peninsula and the Canary Islands. This paper gives an overview of the aims and logistics of all of the Lagrangian experiments and compares and contrasts them to provide a framework for the more focused Lagrangian papers in this issue and future process modelling studies and parametrisation development. The characteristics of the cloudy Lagrangian experiments were remarkably different, enabling a wide range of different physical and chemical processes to be studied. In the 1st Lagrangian, a clean maritime air mass was sampled in which salt particle production, due to increased wind speed, dominated the change in the accumulation mode concentrations. In the 2nd Lagrangian, extensive cloud cover resulted in cloud processing of the aerosol in a polluted air mass, and entrainment of air from the free troposphere influenced the overall decrease in aerosol concentrations in the marine boundary layer (MBL). Very little change in aerosol characteristics was measured in the 3rd Lagrangian, where the pollution in the MBL was continually being topped up by entraining air from a residual continental boundary layer (CBL) above. From the analysis of all the Lagrangian experiments, it has been possible to formulate, and present here, a generalised description of a European continental outbreak of pollution over the sub-tropical North Atlantic.