Evaluation and Correction of Micro-aethalometer MA200

Abstract:

Black carbon (BC) is a strong light absorber in the atmosphere, and have a great influence on the radiative balance of earth. Therefore, extensive and precise measurement of BC is necessary for accurate radiative calculation. However, the vertical distribution of BC is poorly characterized due to technical limitation. The micro-aethalometer MA200 is a recent instrument developed by Aethlabs to measure ambient BC. Its small size and light weight make it ideal for aerial measurement. In this paper, the performance of MA200 is evaluated by comparing MA200 with AE33 at different time resolutions. The results show that for 1-minute resolution, the correlation coefficient (R²) between MA200 and AE33 is ~0.97. For 1-second resolution, R² is less than 0.1 due to excessively large noise of MA200. To reduce noise, 1-second data of MA200 is weighted running averaged until time window is larger than 31 s and R² is improved to ~0.5. The calculation method of absorption coefficient from attenuation coefficient (s_ATN) measured from MA200 is developed based on comparison between MA200 and AE33, namely s_abs = (s_{ATN, MA} - s₀)/C_MA, where s₀ is compensation of s_{ATN, MA} and C_MA is scattering correction factor. s₀ spans from -15.3 Mm^-1 to -6.4 Mm^-1, and C_MA ranges from 2.65 at 880 nm to 3.21 at 375 nm with obvious wavelength dependence. Evaluation and correction of MA200 in this study can give later MA200 users suggestions about the accuracy and data processing of MA200 to make better use of MA200.