Rainfall composition in Minnesota: integrating the chemistry, synoptic meteorology and numerical modelling

Clear quantitative differences (particularly with NH4+ and NO3−) were observed in the chemical composition of rain collected by an in situ, refrigerated event sampler versus using a non-refrigerated, composite, weekly sampling protocol, at a site in the upper Midwestern USA. There was a non-linear, parabolic relationship between pH and SO42− or NO3− concentrations. Such relationships could be stratified by the occurrences of three different classes of rainfall chemistry, governed by three different types of branching air trajectories preceding them. These overall processes were influenced by source-receptor issues. A Fourier modulated, three-way parabolic spline regression model was used to explain the non-linear relationships between H+ and SO42− or NO3−. This model performed much better (R2: 0.48–0.61) than the linear (R2: 0.008–0.31) and non-modulated three-way parabolic spline (R2: 0.22–0.36) regression analyses. Overall, the study allowed an integration of rainfall chemistry with synoptic meteorology and numerical modelling. ©