Chemical composition of precipitation at coastal and marine sampling sites in Mexico

Chemical characteristics of wet precipitation were measured in Acapulco, Cancun,
Tapachula, Puerto Morelos and Tropical Pacific Ocean; from August 1999 to October
2001. The samples were analyzed for pH, Na+, K+, Ca2+, Mg2+, NH4
+, SO4
2-, NO3
- and Cl-.
Ionic abundance revealed a significant variation, however, the most abundant ions were
sodium, chloride and magnesium in all sites. pH values observed were within the normal
pH value considered for rainwater, and agree with that reported by other authors in
marine and coastal sites; sea-salt aerosol contributed with most of Na+, Cl- and Mg2+,
whereas, a significant proportion of K+ and Ca2+ originated in particles from soil; finally
sulfate excess concentration was in agreement with the background hemispheric values.
In the other hand, in Puerto Morelos, during sampling campaign performed at the end of
the mid-summer drought, NO3
- levels were the highest and also higher than SO4
2-
concentrations, exceeding background level; and pH values observed were low.
Excluding, Puerto Morelos, where other sources influenced the rain chemistry
composition; all remaining sampling sites were clearly influenced by marine aerosol,
showing ionic concentrations typical of sites with a minimal anthropogenic influence, and
therefore, their ionic concentrations can be considered as representative of background
values for this region.