Fig. 5: Relationships between the proportions of dimethylarsinic acid (DMAs^V) in precipitation (sub)events and contributing moisture sources, time spent in the Atlantic boundary layer, and the sea salt proxy.

a shows the variability of DMAs^V proportions (% sum of arsenic (As) species concentrations) in the clusters C1-C4 defined by different contributing moisture sources in Fig.3b. The boxplots show the interquartile range, representing the middle 50% of the data, which fall between the upper quartile (75% data below that score) and the lower quartile (less than 25% below that score). The whiskers refer to the 5th/95th percentiles. b and c show the variability of DMAs^V (circles) and methane sulfonic acid (MSA, diamonds) as a function of the time spent in the Atlantic boundary layer (b), and as function of the sodium to strontium concentration ratio (Na:Sr, sea salt proxy, c). All symbols in b and c are colour coded according to moisture source clusters. Different letters (x, y) in a denote significance levels based on Mann-Whitney-U test (p < 0.05). For all clusters with the same letter, the difference between species proportion is not statistically significant. Indicated correlation coefficient in b and c are significant (n = 26, p < 0.01) and the error bars represent the uncertainty of quantification by HPLC-ICP-MS/MS performed in duplicate (relative standard deviation of all DMAs^V measurements: 6 ± 4%), with invisible error bars indicating standard deviations within the symbol. The corresponding figure for trimethylarsine oxide (TMAs^VO) and inorganic As (iAs) is shown in Supplementary Fig. 14.