Opinion: From "Error" to "Signal": Rethinking Method Discrepancies in Black Carbon Measurements and Goal-Oriented Standardization

Abstract. Black carbon (BC) aerosol measurements face a fundamental dilemma: results from different methods applied to the same sample can differ by a factor of several, far exceeding instrumental uncertainty (±5–10 %). Petzold et al. (2013) established that equivalent black carbon (eBC), elemental carbon (EC), and refractory black carbon (rBC) are distinct concepts defined by different operational definitions. This paper argues that such discrepancies are inevitable, originating at the conceptual definition level rather than as a purely technical problem. Instrument calibration can address comparability among “same method, different instruments” but cannot eliminate conceptual differences among “different methods.” We propose two mutually reinforcing pathways. At the epistemological level, method discrepancies are reframed as interpretable “signals”—sources of information about aerosol state rather than errors to be eliminated. At the practical level, we advocate “goal-oriented standardization”—abandoning the pursuit of a singular “true BC” with all methods and instead matching measurement protocols to specific research objectives (e.g., climate modeling, health assessment, source apportionment). These pathways together respond to the dilemma of universal standardization, complementing metrological work on within-method comparability. This opinion article aims to stimulate debate on the future of black carbon measurement standardization.