C signatures of aerosol organic and elemental carbon from major combustion sources in China compared to worldwide estimates.

Carbon isotope signatures are used to gain insight into sources and atmospheric processing of carbonaceous aerosols. Since elemental carbon (EC) is chemically stable, it is possible to apportion the main sources of EC (C3/C4 plant burning, coal combustion, and traffic emissions) using a dual C-C isotope approach. The dual-isotope source apportionment crucially relies on accurate knowledge of C source signatures, which are seldom measured for EC. In this work, we present C signatures of organic carbon (OC) and EC for relevant sources in China. EC was isolated for C analysis based on the OC/EC split point of a thermal-optical method (EUSAAR_2 protocol). A series of sensitivity studies were conducted to investigate the EC separation and the relationship of the thermal-optical method to other EC isolation methods. Our results show that, first, the C signatures of raw materials and EC related to traffic emissions can be separated into three groups according to geographical location. Second, the C signature of OC emitted by the flaming combustion of C4 plants is strongly depleted in C compared to the source materials, and therefore EC is a better tracer for this source than total carbon (TC). A comprehensive literature review of C source signatures (of raw materials, of TC, and of EC isolated using a variety of thermal methods) was conducted. Accordingly, we recommend composite C source signatures of EC with uncertainties and detailed application conditions. Using these source signatures of EC in an example dual-isotope source apportionment study shows an improvement in precision. In addition, C signatures of OC were measured at three different desorption temperatures roughly corresponding to semi-volatile, low-volatile, and non-volatile OC fractions. Each source category shows a characteristic trend of C signatures with desorption temperature, which is likely related to different OC formation processes during combustion.