Aethalometer

Abstract: . Aerosol black carbon is a unique primary tracer for combustion emissions. It affects the optical properties of the atmosphere and is recognized as the second most important anthropogenic forcing agent for climate change. It is the primary tracer for adverse health effects caused by air pollution. For the accurate determination of mass equivalent black carbon concentrations in the air and for source apportionment of the concentrations, optical measurements by filter-based absorption photometers must take into account the "filter loading effect". We present a new real-time loading effect compensation algorithm based on a two parallel spot measurement of optical absorption. This algorithm has been incorporated into the new Aethalometer model AE33. Intercomparison studies show excellent reproducibility of the AE33 measurements and very good agreement with post-processed data obtained using earlier Aethalometer models and other filter-based absorption photometers. The real-time loading effect compensation algorithm provides the high-quality data necessary for real-time source apportionment and for determination of the temporal variation of the compensation parameter k.

Abstract: A source apportionment study was performed for particulate matter in the small village of Roveredo, Switzerland, where more than 70% of the households use wood burning for heating purposes. A two-lane trans-Alpine highway passes through the village and contributes to the total aerosol burden in the area. The village is located in a steep Alpine valley characterized by strong and persistent temperature inversions during winter, especially from December to February. During two winter and one early spring campaigns, a seven-wavelength aethalometer, high volume (HIVOL) samplers, an Aerodyne quadrupole aerosol mass spectrometer (AMS), an optical particle counter (OPC), and a Sunset Laboratory OCEC analyzer were deployed to study the contribution of wood burning and traffic aerosols to particulate matter. A linear regression model of the carbonaceous particulate mass in the submicrometer size range CM(PM1) as a function of aerosol light absorption properties measured by the aethalometer is introduced to estimate the particulate mass from wood burning and traffic (PM(wb), PM(traffic)). This model was calibrated with analyses from the 14C method using HIVOL filter measurements. These results indicate that light absorption exponents of 1.1 for traffic and 1.8-1.9 for wood burning calculated from the light absorption at 470 and 950 nanometers should be used to obtain agreement of the two methods regarding the relative wood burning and traffic emission contributions to CM(PM1) and also to black carbon. The resulting PM(wb) and PM(traffic) values explain 86% of the variance of the CM(PM1) and contribute, on average, 88 and 12% to CM(PM1), respectively. The black carbon is estimated to be 51% due to wood burning and 49% due to traffic emissions. The average organic carbon/total carbon (OC/TC) values were estimated to be 0.52 for traffic and 0.88 for wood burning particulate emissions.

Abstract: Two extreme cases of aerosol optics from the Reno Aerosol Optics Experiment are used to develop a model-based calibration scheme for the 7-wavelength aethalometer. The cases include those of very white and very dark aerosol samples. The former allows fo ra nassessment of the scattering offset associated with this filterbased method, with the wavelength-dependent scattering measured from a 3-wavelength nephelometer, and interpolated and extrapolated to the 7 wavelengths of the aethalometer. A photoacoustic instrument operating at 532 nm is used to evaluate the filter loading effect caused by aerosol light absorption. Multiple scattering theory is used to analytically obtain a filter-loading correction function. This theory shows that the exponential behavior of light absorption in the strong multiple scattering limit scales as the square root of the total absorption optical depth rather than linearly with optical depth as is commonly assumed with Beer’s law. The multiple scattering model also provides a theoretical justification for subtracting a small fraction of aerosol light scattering away from measured apparent light absorption by the filter method. The model is tested against ambient measurements and is found to require coefficients that are situation specific. Several hypotheses are given for this specificity, and suggested methods for reducing it are discussed. Specific findings are as follows. Simultaneous aerosol lightscattering measurements are required for accurate interpretation of aethalometer data for high aerosol single-scattering albedo. Instantaneous errors of up to ±50% are possible for uncorrected data, depending on filter loading. The aethalometer overpredicts black carbon (BC) concentration on a fresh filter and underpredicts BC on a loaded filter. BC and photoacoustic light absorption can be