(673b) Volatility of Organic Aerosol – Bridging Ambient and Laboratory Studies

The volatility of atmospheric organic aerosol (OA) is one of the more important physical properties determining it’s partitioning between the gas and particulate phases. The evolution of the volatility of OA during its atmospheric lifecycle was investigated synthesizing laboratory studies of model systems and ambient measurements in a variety of environments. The model systems included secondary organic aerosol formed during the oxidation of biogenic and anthropogenic volatile organic compounds. The field measurements took place in a major urban area (Paris, France) and a remote area (Finokalia, Greece) in both the summer and winter seasons. All measurements were performed combining CMU’s variable residence-time thermodenuder (TD), the Aerodyne Aerosol Mass Spectrometer (AMS) and a Scanning Mobility Mass Spectrometer (SMPS).

The OA measurements in Paris were part of the MEGAPOLI campaigns and were collected for one month in the summer of 2009 and winter of 2010, providing contrasting meteorological conditions. During the summer the OA concentrations were relatively low, with mostly aged OA that was transported to the site from other areas. Half of the OA evaporated at 90°C and 90 percent at 190°C at a centerline residence time of 25 seconds.  During the winter the OA levels were higher with local sources, such as wood burning, becoming more important. In contrast to the summer, half of the winter OA evaporated at 80°C.

The most recent FAME campaign took place in October 2011 at a remote site on the island of Crete, Greece. This dataset was added to the collection of studies at Finokalia performed during the summer of 2008 and winter of 2009. This particular site is far from local emission sources, allowing the ambient aerosol to reach a highly aged, oxygenated state.  During periods with intense sunlight the aged OA had relatively low volatility (requiring a temperature of 120°C to evaporate half of its mass at a centerline residence time of 15 seconds).  During periods with lower photochemical activity the OA volatility increased with half the OA evaporating at 100°C.

Laboratory studies were also performed in order to target atmospherically relevant systems and manipulate their aging conditions. The volatility of secondary organic aerosol (SOA) generated from the ozonolysis of α-pinene, β-pinene, and limonene was studied under high and low NO_xconditions in low to moderate relative humidity. At a centerline residence time of about 16 seconds, nearly all three systems (under all conditions) fully evaporated at 90°C. The volatility of aerosol formed from the reaction of toluene with the hydroxyl radical was also investigated with a special focus on the change of the volatility as the oxidation reactions continued (chemical aging). 

In order to synthesize these measurements involving particles of different sizes and at different concentrations, a dynamic mass transfer model is used to estimate the OA volatility distribution of each case. The model accounts for non-equilibrium behavior in the TD and for the measurement specific parameters including varying initial particle size and OA concentration. In this way, the volatility is quantified for the different areas and seasons, evaluating effects of photochemical conditions and OA sources.