Fraction Of Organic Aerosol Mass Research Articles

Phase Behavior of Ammonium Sulfate with Organic Acid Solutions in Aqueous Aerosol Mimics Using Microfluidic Traps.

Water-soluble organic acids such as dicarboxylic acids are known to form a significant fraction of organic aerosol mass, yet the chemical composition and interactions between components in an organic acid-inorganic salt mixed particle remain unclear. In this study, phase behavior of different mixing ratios of the salt and organic acids, here 3-methyl glutaric acid and 3-methyl adipic acid, are investigated with respect to their water activity. A microfluidic pervaporation approach is used to study different phase transitions of internally mixed aqueous droplets. Single droplets of varied compositions are trapped and stored in microfluidic wells until dehydration, where both the water content and the solution volume of the droplet decrease slowly with time. The volume is calculated by imaging techniques and correlated with the initial known concentration of the solution to determine concentrations at each time interval. The phase transitions of the droplets with changing concentrations are also observed under an inverted microscope. This study will help determine the concentration at which a mixture droplet, mimicking organic and inorganic atmospheric aerosols, changes phase.

Submicron organic aerosol in Tijuana, Mexico, from local and Southern California sources during the CalMex campaign

The CalMex campaign was conducted from May 15 to June 30 of 2010 to study the properties and sources of air pollution in Tijuana, Mexico. In this study, submicron organic aerosol mass (OM) composition measured by Fourier Transform Infrared Spectroscopy (FTIR), Aerosol Chemical Speciation Monitor (ACSM), and X-ray spectromicroscopy are combined with statistical analysis and measurements of other atmospheric constituents. The average (±one standard deviation) OM concentration was 3.3 ± 1.7 μg m−3. A large source of submicron aerosol mass at this location was determined to be vehicular sources, which contributed approximately 40% to the submicron OM; largely during weekday mornings. The O/C ratio estimated from ACSM measurements was 0.64 ± 0.19; diurnal variations in this value and the more oxygenated fraction of OM as determined from Positive Matrix Factorization and classification analyses suggest the high degree of oxygenation originates from aged OM, rather than locally-produced secondary organic aerosol. A large contribution of this oxygenated aerosol to Tijuana from various source classes was observed; some fraction of this aerosol mass may be associated with non-refractory components, such as dust or BC. Backtrajectory simulations using the HYSPLIT model suggest that the mean wind vector consistently originated from the northwest region, over the Pacific Ocean and near the Southern California coast, which suggests that the origin of much of the oxygenated organic aerosol observed in Tijuana (as much as 60% of OM) may have been the Southern California Air Basin. The marine aerosol contribution to OM during the period was on average 23 ± 24%, though its contribution varied over synoptic rather than diurnal timescales. BB aerosol contributed 20 ± 20% of the OM during the campaign period, with notable BB events occurring during several weekend evenings.

Surface tension depression by low-solubility organic material in aqueous aerosol mimics

Surface-active material, including long-chain fatty acids (LCFAs), comprises a significant fraction of organic aerosol mass. Surface-active species are thought to form a film at the gas–aerosol interface, with implications for aerosol heterogeneous chemistry and cloud formation. However, LCFA phase behavior and surface-bulk partitioning has not been characterized under most conditions typical of tropospheric aerosol water (i.e. acidic, high ionic content), making it challenging to predict surface film formation in aerosols. In this study, we present measurements of the surface tension of aqueous solutions containing the slightly soluble LCFAs oleic and stearic acid. The effect of varying pH, organic concentration, and inorganic salt content was tested for each system. We observe surface tension depression compared to water of up to ∼30 and 45% for aqueous solutions containing stearic or oleic acid at pH 0–8 and high inorganic salt concentrations (NaCl and (NH4)2SO4). This suggests that surface film formation is favorable for these species in atmospheric aerosols.

A Method for Forecasting Cloud Condensation Nuclei Using Predictions of Aerosol Physical and Chemical Properties from WRF/Chem

AbstractModel investigations of aerosol–cloud interactions across spatial scales are necessary to advance basic understanding of aerosol impacts on climate and the hydrological cycle. Yet these interactions are complex, involving numerous physical and chemical processes. Models capable of combining aerosol dynamics and chemistry with detailed cloud microphysics are recent developments. In this study, predictions of aerosol characteristics from the Weather Research and Forecasting Model with Chemistry (WRF/Chem) are integrated into the Regional Atmospheric Modeling System microphysics package to form the basis of a coupled model that is capable of predicting the evolution of atmospheric aerosols from gas-phase emissions to droplet activation. The new integrated system is evaluated against measurements of cloud condensation nuclei (CCN) from a land-based field campaign and an aircraft-based field campaign in Colorado. The model results show the ability to capture vertical variations in CCN number concentration within an anthropogenic pollution plume. In a remote continental location the model-forecast CCN number concentration exhibits a positive bias that is attributable in part to an overprediction of the aerosol hygroscopicity that results from an underprediction in the organic aerosol mass fraction. In general, the new system for predicting CCN from forecast aerosol fields improves on the existing scheme in which aerosol quantities were user prescribed.