Major ions: Certified reference materials (CRM; ERM-CA616 groundwater standard and ERM-CA408 simulated rainwater standards; Sigma-Aldrich) were measured regularly for quality control and yielded an accuracy of 97 % for nitrate.

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 snow (n = 118) aerosol samples (n = 35), the average uncertainty values obtained for d15N were 0.5 ppth and 0.5 ppth respectively.

N/A denotes values below the detection limit.