Measuring aerosol black carbon age with aethalometers

Aerosol mixing state is a parameter influencing optical properties of absorbing particles that is hard to measure. It was proposed that the lensing effect where transparent shell covers the absorbing particle can significantly enhance optical absorption (Bond et al., 2006). Aerosol mixing state can be assessed using advanced mass spectrometers ATOFMS (Healy et al., 2012), and with SP2 for particles with diameters above about 100 nm (Subramanian et al., 2010). Filter-based measurements of aerosol optical absorption are widely used to determine Black Carbon (BC) concentrations in real time. Measurements at multiple wavelengths permit the separation of contributions of BC from different combustion sources (Sandradewi 2008). However, filter-based methods are influenced by the non-linear response due to the »loading« effect, caused by the increasing sample deposit on the filter (Gundel 1984, Weingartner 2003, Arnott 2005, Virkkula 2007).The filterloading effect is the reduction of the filter based photometer sensitivity due to filter loading. It was shown that filter-loading effect differs between locations and seasons with indication that the filter loading effect in Aethalometer relates to the particle coating (Virkkula et al., 2007; Drinovec et al., 2015). Using Aethalometer model AE33 (Magee Scientific) it is possible to measure filter loading parameter k with high time resolution (Drinovec et al., 2015).Influence of coating on the parameter k was investigated both during the ambient and laboratory campaigns

Interpretation of the loading effect in absorption filter photometers - dual spot aethalometer measurements

Filter-based measurements of aerosol optical absorption are widely used to determine Black Carbon (BC) concentrations in real time. Measurements at multiple wavelengths permit the separation of contributions of BC from different combustion sources. Filterbased methods can suffer from non-linearity due increasing sample deposit on the filter. The dual-spot Aethalometer model AE33 provides a real-time determination of this loading effect, by determining the compensation parameter k in real time for each of the wavelengths. We present an interpretation of k in terms of aerosol composition. Optical properties of combustion aerosols were investigated under laboratory conditions. Optical properties of emitted aerosols varied greatly with the combustion regime. The type of combustion also influenced the parameter k, which is attributed to different particle coating thickness. Optical and chemical properties of aerosols were measured with high time resolution during summer and winter at different sites. The ratio of the sum of inorganic secondary and organic aerosol mass to BC (expected to be high for air parcels containing aged aerosols) correlates well with the parameter k measured by the AE33 at 880 nm. To investigate the influence of coating on parameter k, a drier and a thermodenuder were used to remove the coating. These results indicate that the compensation parameter k can be used for discrimination between fresh and aged aerosols

Evolution of soot particle morphology and mixing state in the atmosphere

Soot particles (aka black carbon) impact the environment and climate by affecting Earth\u27s radiation balance, cloud microphysics, and atmospheric chemistry. The complex morphology and mixing state of soot particles influence their optical properties and therefore their radiative forcing, the particles\u27 transport, lifecycle, and heterogeneous chemistry. How soot morphology and mixing state alter during transport from the source to remote areas is still not well understood. While aging, soot particles can change shape, oxidize and mix, and become coated by organic and inorganic materials. In this study, we investigate the morphological and mixing state evolution of single soot particles in different stages of their \u27life\u27 in the atmosphere. This analysis will include an overview of several samples collected in various locations and atmospheric conditions: 1) particles freshly emitted near freeway on-ramps in Southern Michigan (USA); 2) particles emitted in two biomass burning events in New Mexico (USA), one close to the sampling location and another hundreds of miles away; 3) particles in the urban atmosphere of Mexico City and in the uplifted boundary layer captured on the top of the Pico de Tres Padres Mountain (on the north edge of Mexico City); 4) particles collected in the Sacramento urban area and the Sierra Nevada foothills (CA, USA); 5) particles collected in Detling (UK), and mostly transported from London, and 6) long-range transported particles in the free troposphere and collected at the Pico Mountain Observatory, located near the top of the Pico Volcano in the Azores (Portugal). We analyzed a large number of individual particles using electron microscopy and X-ray spectroscopy followed by image analysis. The projected structural properties of soot particles were characterized using size (maximum length, maximum width, and area equivalent diameter) and shape descriptors (e.g., aspect ratio, roundness, and convexity). The particle mass-fractal dimensions were determined using the ensemble method. The mixing state was analyzed by classifying soot particles based on visual inspection of coating and morphology. Soot particles freshly emitted by anthropogenic sources show less coating and more open chain-like structures; on the other hand biomass burning and long-range transported soot particles appear to be mostly coated and exhibit very compacted shapes. However, soot processing in urban atmospheres results in a complex mixture of coated and uncoated particles with a variety of morphologies and mixing states

A Comparative Investigation of Ultrafine Particle Number and Mass Emissions from a Fleet of On-road Diesel and CNG Buses

Particle number, particle mass and CO2 concentrations were measured on the kerb of a busy urban busway used entirely by a mix of diesel and CNG operated buses. With the passage of each bus, the ratio of particle number concentration and particle mass concentration to CO2 concentration in the diluted exhaust plume were used as measures of the particle number and mass emission factors, respectively. With all buses accelerating past the monitoring point, the results showed that the median particle mass emission from CNG buses was less than 9% of that from diesel buses. However, the median particle number emission from CNG buses was six times higher than the diesel buses and the particles from the CNG buses were mainly in the nanoparticle size range. Using a thermodenuder to remove the volatile material from the sampled emissions showed that the majority of particles from the CNG buses, but not from the diesel buses, were volatile. Approximately, 82% of the particles from the CNG buses and 38% from the diesel buses were removed by heating the emissions to 300°C