This study identified the relationships between aerosol and precipitation chemistry during the monsoon season (June 15- September 15) in Arizona by using four co-located IMPROVE and NADP NTN sites: Chiricahua National Monument (Chir NM), Grand Canyon, Organ Pipe National Monument (Organ Pipe NM), and Petrified National Park (Petrified NP). Relationships between 1999 and 2014 were determined by using the using a two-tailed student’s t-test (95% confidence).
In Chir NM and Grand Canyon, decreasing significant annual trends of sulfate aerosol fractions coincided with increasing significant annual trends of rain pH. This result suggests that in Chir NM and Grand Canyon, the decrease in sulfate aerosol concentrations result in more alkaline pH. Aerosol and rain interrelationships in all sites showed that calcium (Ca), magnesium (Mg), and potassium (K) were highly related to each other suggesting their origin from common dust sources. Similarly based on these interrelationships, sodium (Na) and chloride (Cl-) were related to sea salt in all sites. At all sites, the highest correlation values between aerosol and precipitation concentration correlations were found for Ca, K, Mg, and Na (in decreasing order), suggesting the role of dust and sea salt as cloud condensation nuclei (CCN) or ice nuclei (IN).
The highest grand average of rain accumulation was found in Chir NM and this coincided with the most acidic pH and lowest rain concentrations. In Chir NM, the highest mass fractions and amount of moles in rain of Sulfate and Nitrate were found which justify the acidic pH. The opposite trends are found in Petrified NP, which has the lowest grand annual average of rain accumulation, most alkaline pH, highest rain concentrations, and lowest mass fractions and moles in rain of nitrate and sulfate. The use of rain concentrations are misleading due to the dilution of rain which does not provide information on the absolute amount and abundance of certain ions. Hence it is necessary to analyze the amount of moles and mass fractions in rain of acidic anions to determine the effect on rain pH.
At all sites, the significant correlations between rain accumulation and aerosol and rain concentrations were negative, suggesting that the monsoon rain acts as a sink. However, the significant correlations between rain accumulation and amount of moles in rain were positive for all species in all sites. The positive relationships are justified by the uptake of aerosol concentrations by rain by cloud seeding and scavenging. Of these relationships, sulfate and nitrate exhibited the highest correlated values (r-values) where sulfate and nitrate gaseous precursors are capable of directly entering rain drops.
Inverse relationships between air and rain mass fractions of nitrate were found with respect to pH where nitrate was more alkaline in the air and more acidic in the rain. The result suggests that in the air, precursor forms of nitrate which can react with dust emissions are found, and the product of this reaction (highly water soluble) serves as cloud condensation nuclei where the presence of nitrate in rain is acidic. In addition, the slope of the correlation value for air or rain mass fraction can be used as a proxy to determine whether it is acidic or alkaline. Acidic mass fractions would exhibit positive values with respect to sulfate air mass fractions and rain accumulation and negative values for all other aerosol and precipitation data.
To determine the effects of moisture source on aerosol and precipitation chemistry, it was necessary to choose one site that had sources of moisture from the Gulf of California and compare this to another site that had moisture sources from the Gulf of Mexico. The Kruskal Wallis test was applied to aerosol concentrations to determine if sites shared similar air mass sources. The results of this test indicate that Chir NM, Grand Canyon, and Petrified NP share similar air mass sources while Organ Pipe NM did not share any similarities. Next, based on these results and previous studies, Chir NM and Organ Pipe NM were chosen to determine the effects of air mass source on the aerosol and precipitation chemistry. Air mass back trajectories revealed that Chir NM and Organ Pipe NM shared similar moisture sources from the Gulf of California and few dates were found where they differed. Future work will seek to compare sites in western Arizona (Organ Pipe NM) to other co-located NADP and IMPROVE sites in New Mexico.
The results of this work suggest that the reactions between dust and precursors of nitrate and sulfate are commonly found in Arizona. During the monsoon period, higher rain accumulation is observed which uptakes more dust containing acidic ions and acidic precursor gases via scavenging, resulting in increasingly acidic rain pH. Therefore, the aquatic and terrestrial ecosystems found in the area are exposed to acidic rain during the monsoon season. However, in 1999 to 2014, the reduction of sulfate aerosol concentrations due to air regulations has possibly led to an increase in alkalinity of rain pH
