Correcting aethalometer black carbon data for measurement artifacts by using inter-comparison methodology based on two different light attenuation increasing rates

摘要

In black carbon (BC) measurements obtained using the filter-based optical technique, artifacts are a major problem. Recently, it has become possible to correct these artifacts to a certain extent by using numerical methods. Nevertheless, all correction schemes have their advantages and disadvantages under field conditions. In this study, a new correction model that can be used for determining artifact effects on BC measurements was proposed; the model is based on two different light attenuation (ATN) increasing rates. Two aethalometers were used to measure ATN values in parallel at aerosol sampling flow rates of 6 and 2 L min. In the absence of sampling artifacts, the ratio of ATN values measured by the two aethalometers should be equal to the ratio of the sampling flow rates (or aerosol deposition rates) of these two aethalometers. In practice, the ratio of ATN values measured by the two aethalometers was not the same as the ratio of the sampling flow rates of the aethalometers because the aerosol loading effects varied with the aerosol deposition rate. If the true ATN value can be found, then BC measurements can be corrected for artifacts by using the true ATN change rate. Therefore, determining the true ATN value was the primary objective of this study. The proposed correction algorithm can be used to obtain the true ATN value from ATN values acquired at different sampling flow rates, and the actual BC mass concentrations can be determined from the true ATN change rate. Before BC correction, the BC concentration measured at the sampling flow rate of 6 L min was smaller than that measured at 2 L min by approximately 13 and 9% in summer and winter seasons, respectively. After BC correction by using the true ATN value, the corrected BC for 6 L min can be exactly equal to the corrected BC for 2 L min. Field test results demonstrated that loading effects on BC measurements could be corrected accurately by using the proposed model. Additionally, the problem of enhanced light ATN caused by light scattering at the unloaded filter can be overcome without using any light scattering coefficient. Therefore, the correction algorithm can be applied to a newly designed instrument to determine actual real-time BC concentrations by using two sampling spots for different aerosol deposition rates. Moreover, a simple empirical correction scheme for post-processing for correcting the existed aethalometer BC data is also presented. While this simple correction scheme is dependent on the aerosol type, it can be used to correct BC data when the primary source of BC and the weather conditions are similar to those in this study. Furthermore, two existed aethalometers with appropriate flow control can be used to create correction schemes suitable for different environments.