Elsevier

Atmospheric Environment

Volume 43, Issue 27, September 2009, Pages 4171-4177
Atmospheric Environment

Organic carbon, total nitrogen, and water-soluble ions in clouds from a tropical montane cloud forest in Puerto Rico

https://doi.org/10.1016/j.atmosenv.2009.05.049Get rights and content

Abstract

Chemical characterization to determine the organic and nitrogen fractions was performed on cloud water samples collected in a mountaintop site in Puerto Rico. Cloud water samples showed average concentrations of 1.09 mg L−1 of total organic carbon (TOC), of 0.85 mg L−1 for dissolved organic carbon (DOC) and of and 1.25 mg L−1 for total nitrogen (TN). Concentrations of organic nitrogen (ON) changed with the origin of the air mass. Changes in their concentrations were observed during periods under the influence of African dust (AD). The ON/TN ratios were 0.26 for the clean and 0.35 for the AD periods. Average concentrations of all these species were similar to those found in remote environments with no anthropogenic contribution. In the AD period, for cloud water the concentrations of TOC were 4 times higher and TN were 3 times higher than during periods of clean air masses associated with the trade winds. These results suggest that a significant fraction of TOC and TN in cloud and rainwater is associated to airborne particulate matter present in dust. Functional groups were identified using proton nuclear magnetic resonance (1H NMR) spectroscopy. This characterization led to the conclusion that water-soluble organic compounds in these samples are mainly aliphatic oxygenated compounds, with a small amount of aromatics. The ion chromatography results showed that the ionic species were predominantly of marine origin, for air masses with and without African dust influence, with cloud water concentrations of NO3 and NH4+ much lower than from polluted areas in the US. An increase of such species as SO42−, Cl, Mg2+, K+ and Ca2+ was seen when air masses originated from northwest Africa. The changes in the chemical composition and physical properties of clouds associated with these different types of aerosol particles could affect on cloud formation and processes.

Introduction

Aerosols interact with radiation and play an important role in climatic processes. They affect the Earth's radiative balance directly by scattering and absorbing of solar radiation, and indirectly by acting as cloud condensation nuclei (CCN), affecting cloud droplets and cloud albedo (NRC, 1996). Clouds provide important removal mechanisms for atmospheric aerosols. Clouds contain a variety of organic and inorganic compounds that can be suspended and dissolved. Large concentrations of various species can accumulate, even in a small volume of cloud water. Little is known about the organic composition of clouds and how clouds process organic compounds. Therefore, it is important to understand interactions between clouds and carbonaceous aerosol particles.

It is known that organic matter comprises a large fraction of fine particulate matter in some environments, and it is possible that aqueous reactions in cloud drops are important sources of secondary organic aerosol (Herckes et al., 2002, Kerminen et al., 2005, Prenni et al., 2007, Ervens et al., 2008, Wex et al., 2009). Activation of carbonaceous particles to form cloud drops may also significantly alter optical properties of clouds, indicating that these particles can be effective CCN and contributing to the number and size of cloud droplets formed (Rissman et al., 2007). Some organic compounds present in aerosols are surface-active and their presence in CCN can affect the surface tension of cloud droplets (Gill et al., 1983, Capel et al., 1990, Shulman et al., 1996, Facchini et al., 2000, Facchini et al., 1999a, Facchini et al., 1999b, Svenningsson et al., 2006, Engelhart et al., 2008). The nature of these surface-active compounds in clouds and rainwater is not well understood and there is very little information about their content in remote tropical environments.

Organic aerosols can constitute a significant fraction of CCN and in certain environments they may be present in concentrations similar to or even higher than sulfate aerosol (Novakov and Penner, 1993, Rivera-Carpio et al., 1996). Other studies have provided strong evidence of the CCN ability of organic aerosols (Novakov and Corrigan, 1996, Noone et al., 1996, Cruz and Pandis, 1997, Hansson et al., 1998). Therefore, the organic fraction should be taken into account when determining the hygroscopic behavior and CCN ability of atmospheric aerosols.

Nitrogen is another important component of clouds solutes. Total nitrogen (TN) in cloud water can include both be organic and inorganic forms. Organic nitrogen (ON) has been largely ignored for decades (Veneklass, 1990, Eklund et al., 1997, Keene et al., 2002, Cornell et al., 2003). Inorganic nitrogen (IN) has been more extensively studied due to its impact on acid rain. Recently, organic nitrogen (ON) has been shown to influence climate and environment by means of its light absorption properties, photochemical reactivity, and its contribution to the acid buffering capacity of cloud water (Zhang and Anastasio, 2001, Zhang and Anastasio, 2003, Zhang et al., 2002, Mace et al., 2003, Nakamura et al., 2006).

Northeastern Puerto Rico is a relatively unpolluted marine location because of its proximity to the Atlantic Ocean and distance from upwind land masses with anthropogenic activity. However, the Caribbean region, including Puerto Rico, is influenced by African dust incursions, especially during the summer months (Prospero and Lamb, 2003). Mineral dust has a great impact on visibility, climate forcing and public health. It may even reduce or suppress precipitation (Prospero and Lamb, 2003) and will alter the chemical composition of cloud water.

Chemical characterization of clouds is fundamental to determine the impact of aerosols on cloud chemistry. This study focuses on the chemical characterization of cloud water samples collected in a tropical montane cloud forest in northeastern Puerto Rico. There clouds frequently envelop the mountain peaks facilitating study of the interaction of aerosols with clouds over extended periods of time. We sought to relate the cloud chemical composition to air mass origin, and to determine the organic and nitrogen fraction of aerosols.

Section snippets

Cloud sampling

Cloud water samples were collected at East Peak (EP), part of the El Yunque National Forest (EYNF), Puerto Rico (Fig. 1). East Peak is located ∼35 km from San Juan and ∼20 km from the South Atlantic Ocean at 1051 m.a.s.l. in a tropical rainforest. The area is a humid mountaintop with frequent abundant cloud coverage fed by high altitude trade winds. During the sampling period, the average annual precipitation was 4200 mm and the annual mean temperature was 18 °C. The results presented here are

TOC, TN, and DOC

Table 1 presents the concentrations (mg L−1) of TOC, DOC, TN, and the liquid water content (LWC) for cloud samples collected at EP. LWC was calculated using the collection rate (mL min−1) that is then converted to LWC (g m−3) according to the approach used by Demoz et al. (1996). They calculated that 2.5 mL min−1 corresponds to an LWC of approximately 0.5 g m−3 using a CASCC2.

Average concentrations of TOC, DOC, and TN were 1.09, 0.85, and 1.25 mg L−1, respectively. Concentrations of TOC as high

Conclusions

The chemical composition of cloud water samples at EP was investigated to determine the influence of the origin of sampled air masses on the concentrations of organic, inorganic, and nitrogen species. Cloud chemical composition was influenced by African dust, anthropogenic pollution, and sea salt. Concentrations of TOC, DOC, and TN were similar to those measured at other remote sites. Concentrations of TOC and TN increased during AD events, suggesting that concentrations of organic compounds

Acknowledgements

Thanks to the El Yunque National Forest for allowing the use of their facilities to collect the cloud samples, to the Atmospheric Chemistry Group at the University of Puerto Rico, Río Piedras Campus for their unconditional help and support, and to the Minority Access to Research Careers (MARC) program. From the University of Puerto Rico, Río Piedras Campus, we also acknowledge Dr. Jorge Ortiz-Zayas for allowing the use of the ion chromatograph and Dr. José R. Martínez for his contribution to

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