Rainwater chemical composition at the end of the mid-summer drought in the Caribbean shore of the Yucatan Peninsula

Rainwater was sampled at Puerto Morelos in the Caribbean coast of the Yucatan Peninsula during two sampling periods: from 27 August to 6 September 2000 (at the end of the summer drought), and from 7 to 27 September 2000 (just after the summer drought). Rainwater chemical composition (except nitrate concentration) was similar to that observed in other regions in the Caribbean Sea. Sodium and magnesium presented the highest average concentrations, sea-salt aerosols being almost the unique source, of 91% and 95%, respectively. Excess potassium, calcium and sulfate originated from non-sea-salt sources. Excess sulfate concentration was 0.65 mg l−1, which agrees with the background concentration of 0.48 mg l−1, as it was expected. However, rainwater nitrate concentration was much higher than that observed in the rest of the Caribbean Sea region. Nitrate concentration was as high as nearly 20 times the background value of 0.155 mg l−1. According to wind data analysis, only nitrate concentration depended on wind direction. Nitrate concentration peaked when wind blew from the Yucatan Peninsula, and it was higher when forest fires were spotted, therefore forest fires must have been the main nitrate source during the dry period. Forest fires emit nitrogen oxides that are rapidly converted to HNO3. This was strongly supported by the fact that the lowest pH value (3.48) was measured in the rainwater sample with the highest nitrate concentration (4.51 mg l−1), and that 80% of the rain samples presented pH values below 5.6. Also, there was a significant difference in nitrate concentrations between the dry and humid periods, whereas the rest of the ionic species did not present significant differences. However, nitrate concentration was still higher than in other Caribbean regions during the humid period, suggesting that other sources are important too. These sources could not be identified.