Major ions: At the beginning, end and during the course of the day, a certified reference material (CRM; ERM-CA408 simulated rainwater standards; Sigma-Aldrich) was measured for quality control and yielded an accuracy of 107 %, and precision of 10 % for nitrate.

Major elements: At the beginning, end and during the course of the day, a certified reference material (CRM; ERM-CA408 simulated rainwater standards; Sigma-Aldrich) was measured for quality control and yielded an accuracy of 94 and 113 %, and precision of 10 and 13 % for sodium and magnesium.

Nitrate stable isotope ratios: Certified reference materials (IAEA USGS-32, USGS-34 and USGS-35; Böhlke et al., 1993; Böhlke et al., 2003) were prepared (matrix match 1 M NaCl solution; ACS grade) and subject to the same analytical procedures as snow and aerosol samples. For each batch of 60 samples, the overall accuracy of the method is estimated as the reduced standard deviation of the residuals from the linear regression between the measured reference materials (n = 16) and their expected values. For the ice core (n = 800), the average uncertainty values obtained for d18O, d17O, D17O and d15N were 2.8 ppth, 2.1 ppth 0.9 ppth and 2.9 ppth respectively.