Lara S. Hughes

Closure between aerosol particles and cloud condensation nuclei at Kaashidhoo Climate Observatory

Predicting the cloud condensation nuclei (CCN) supersaturation spectrum from aerosol properties is a fairly straightforward matter, as long as those properties are simple. During the Indian Ocean Experiment we measured CCN spectra, size-resolved aerosol chemical composition, and aerosol number distributions and attempted to reconcile them using a modified form of Kohler theory. We obtained general agreement between our measured and modeled CCN spectra. However, the agreement was not as good during a time period when organic carbon comprised a quarter of the total mass of the aerosol in the submicron size range. The modeled concentrations overpredict those actually measured during that time period. This suggests that some component, presumably organic material, can inhibit the uptake of water by the electrolytic fraction of the mass.

Ambient single particle analysis in Riverside, California by aerosol time-of-flight mass spectrometry during the SCOS97-NARSTO

Single-particle measurements were made using aerosol time-of-flight mass spectrometry (ATOFMS) instruments in conjunction with the 1997 Southern California Ozone Study-North American Research Strategy for Tropospheric Ozone (SCOS97-NARSTO). The size and chemical composition of individual ambient particles in Riverside, CA during the summer of 1997 are described. Data collected using co-located micro-orifice uniform deposit impactors (MOUDI) impactors are used to scale the ATOFMS number counts, providing a unique picture of the particle population which complements information obtained using traditional sizing and composition analysis techniques in this and previous studies. Changes in single particle composition are observed over time, and compared and contrasted with observed changes in visibility, ozone, and PM_(10) concentrations. Details are provided on changes in the particle size and composition observed during three morning periods with low ozone and elevated PM_(10) versus three afternoon periods with both elevated ozone and PM_(10) levels between 21–23 August 1997. The ATOFMS size profiles show afternoon periods dominated by sub-μm particles composed of organic carbon coupled with ammonium nitrate, and morning periods with relatively high levels of super-μm dust particles. The observed changes in particle size and composition are consistent with differences in air mass trajectories arriving at Riverside during the morning and afternoon time periods. Temporal variations in single particle types detected over a 40 day sampling period are presented, demonstrating the type of unique information that can be obtained regarding atmospheric particle processes through long term sampling studies. The broader availability of single particle mass spectrometers coupled with recent advances in the field are now providing unique information on the associations of multiple chemical species (i.e. mixing state) within individual particles with high size and temporal resolution.

Aerosol optical properties during INDOEX based on measured aerosol particle size and composition

The light scattering and light absorption as a function of wavelength and relative humidity due to aerosols measured at the Kaashidhoo Climate Observatory in the Republic of the Maldives during the INDOEX field campaign has been calculated. Using size-segregated measurements of aerosol chemical composition, calculated light scattering and absorption has been evaluated against measurements of light scattering and absorption. Light scattering coefficients are predicted to within a few percent over relative humidities of 20–90%. Single scattering albedos calculated from the measured elemental carbon size distributions and concentrations in conjunction with other aerosol species have a relative error of 4.0% when compared to measured values. The single scattering albedo for the aerosols measured during INDOEX is both predicted and observed to be about 0.86 at an ambient relative humidity of 80%. These results demonstrate that the light scattering, light absorption, and hence climate forcing due to aerosols over the Indian Ocean are consistent with the chemical and physical properties of the aerosol at that location.

Determination of the particle counting efficiency and chemical sensitivities of an aerosol time of flight mass spectrometer under ambient sampling conditions

Abstract Recently developed aerosol time-of-flight mass spectrometer (ATOFMS) instruments are capable of determining the size and chemical composition of single particles (Noble and Prather, 1996, Gard et al., 1997). These instruments provide convenient determination of atmospheric aerosol composition on a single particle basis in real time. To date, data from these instruments have been used qualitatively to study both the sources of ambient aerosols (Liu et al., 1997) and the atmospheric transformation of particles (Gard et al., 1998). However, quantitative reconstruction of a continuous time series of the actual size distribution and chemical composition of atmospheric aerosols requires that the absolute counting efficiency and chemical sensitivities of these instruments be known.