The elemental composition of PM on lters collected with the NanoMOUDI impactor was determined in solution with an Agilent 8800 inductively coupled plasma triple quadrupole mass spectrometer (ICP-MS) (Agilent Technologies, Santa Clara, CA, USA).
The lters were digested with a mixture of 65% HNO 3 (1.5 mL), 37% HCl (5 mL), 40% HF (0.6 mL) and 5 mL H 3 BO 3 (puriss p.a., Sigma Aldrich-Merck, Darmstadt, Germany) in temperature-controlled vessels using a Multiwave PRO microwave oven (Anton Paar, Graz, Austria). The digests were diluted to 50 mL with 18.2 MU cm deionised water (Merck Millipore, Billerica, MA, USA) aer adding 200 mL internal standard solution (25 mg L À1 Ga, Ge and In). The ICP-MS was tuned with a matrix matched solution prior to each run, and matrix matched standards were used for calibration. Blank samples and the standard reference material Portland Cement (blended with slag and y ash) 1881a (National Institute of Standards and Technologies, Gaithersburg, MD, USA) were prepared in the same manner as the samples. Recoveries are presented in the ESI (Table S2 †).
The lters were digested with a mixture of 65% HNO 3 (1.5 mL), 37% HCl (5 mL), 40% HF (0.6 mL) and 5 mL H 3 BO 3 (puriss p.a., Sigma Aldrich-Merck, Darmstadt, Germany) in temperature-controlled vessels using a Multiwave PRO microwave oven (Anton Paar, Graz, Austria). The digests were diluted to 50 mL with 18.2 MU cm deionised water (Merck Millipore, Billerica, MA, USA) aer adding 200 mL internal standard solution (25 mg L À1 Ga, Ge and In). The ICP-MS was tuned with a matrix matched solution prior to each run, and matrix matched standards were used for calibration. Blank samples and the standard reference material Portland Cement (blended with slag and y ash) 1881a (National Institute of Standards and Technologies, Gaithersburg, MD, USA) were prepared in the same manner as the samples. Recoveries are presented in the ESI (Table S2 †).