The aerosol OC, EC, and total carbon (TC) concentrations were measured with a thermal-optical transmission analyzer (Sunset Laboratory, Tigard, OR, USA) using the National Institute for Occupational Safety and Health 5040 method. Potassium hydrogen phthalate was used as an external standard to verify the accuracy of the measurement of OC and TC during the analysis. These analyses gave an overall analytical uncertainty of <2.8% (1 SD, for n = 5).

The WSOC concentration was measured following earlier protocols [(33), with slight modification in (23)]: WSOC was extracted in water using ultrasonication, followed by centrifugation of the filter punches, and finally quantified in the filtered solutions using a high-temperature catalytic oxidation instrument (Shimadzu-TOC-VCPH analyzer; Shimadzu, Kyoto, Japan), following the nonpurgeable OC protocol. The presently used water-to-sample ratio was in the range of 0.10 to 0.24 cm3 m−3 (= volume extraction solvent/volume air sampled), which is well within the optimal water-to-sample range (>0.1 cm3 m−3), as suggested by theoretical considerations.

Concentration values of all measured parameters were blank-corrected by subtracting an average of the field blanks. The average relative SDs of triplicate analysis for TC and WSOC were respectively 3.1 and 7.0% at Delhi, 2.4 and 9.4% at BCOB, and 2.8 and 3.0% at MCOH. Detection limit for WSOC was 0.20 μg m−3 at Delhi, 0.16 μg m−3 at BCOB, and 0.14 μg m−3 at MCOH. EC was below detection limit in the blanks. The overall precision in the TC and WSOC concentrations and isotopic signatures was estimated, considering the analytical precision of concentration measurements (estimates from triplicate analysis), mass contributions from field blanks (estimates from several blanks), estimates from analysis of standard reference material NIST (National Institute of Standards and Technology) SRM (Standard Reference Material) 8785, and the precision of the isotope characterization (instrument precision) and the isotope signature of the field blanks.

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