Marine fresh carbon pool dominates summer carbonaceous aerosols over Arctic Ocean
2.1 Sampling information Aerosol samples (n=109) were collected by the Chinese Research Vessel (R/V) Xuelong on three Arctic cruises during the summer (July-September) of 2014, 2016, and 2018. Detailed information about three Arctic cruises are provided in Table S1. Total suspended particulate (TSP) samples and field blanks were collected using large-flow air samplers at a flow rate of 1.05 m3/min. The samples were collected for 24h, while the field blanks were collected for 1min. Large-flow air samplers were placed on the top deck of R/V Xuelong (upstream of the exhaust) to avoid contamination. All samples and blanks were collected with a quartz filter and stored in a refrigerator at - 4°C until laboratory analysis. We divided the sea areas passed by the route into three regions (Northwest Pacific, Bering Sea, and Arctic Ocean) according to latitude. 2.2 Chemical analysis Carbonaceous matter in aerosols was analyzed using a multi-band thermal-optical carbon analyzer (DRI Model 2015, Desert Research Institute, USA). A small disc with an area of 0.5 cm3 was obtained with the filter samples and fed into the instrument to measure organic carbon (OC) and elemental carbon (EC). The IMPROVE_A warming procedure was used to analyze the OC and EC components of the filter samples, and total carbon (TC) was obtained by calculating the sum of OC and EC. During carbonaceous matter measurements, the effects of carbonate were ignored according to previous studies [Cavalli et al., 2010; Sciare et al., 2003]. Considering the low levels of carbonate in marine aerosol [Kundu and Kawamura, 2014] and referring to the experimental method of related studies [Ceburnis et al., 2011; Kimitaka Kawamura et al., 2012; Xiao et al., 2018], the effect of carbonate is not considered in this study regarding aerosol carbonaceous components. The δ13C values of TC were determined using an elemental analyzer (EA; Flash 2000; Thermo) in conjunction with an isotope ratio mass spectrometer (IRMS; MAT253; Thermo). Filter samples (3.14 cm2 area) were wrapped in a tin cup and subsequently burned at 980°C. During combustion, all carbonaceous materials were converted to CO2, and these gaseous components were separated with a packed column in a gas chromatograph, mounted in the EA before measurement using IRMS. Calibrations of δ13CTC were made using the international standard IAEA-600 (-27.771‰) and the working standard urea (-41.30 ± 0.04‰).
National Natural Science Foundation of China