Organic carbon, total nitrogen, and water-soluble ions in clouds from a tropical montane cloud forest in Puerto Rico
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
References (42)
- et al.
Solute deposition from cloud water to the canopy of a Puerto Rican montane forest
Atmospheric Environment
(1994) - et al.
The chemical composition of fogs and intercepted clouds in the United States
Atmospheric Research
(2002) - et al.
Organic nitrogen deposition on land and coastal environments: a review of methods and data
Atmospheric Environment
(2003) - et al.
A study of the ability of pure secondary organic aerosol to act as cloud condensation nuclei
Atmospheric Environment
(1997) - et al.
On the Caltech active strand cloudwater collectors
Atmospheric Research
(1996) - et al.
Seasonal variation of tropical precipitation chemistry: La Selva, Costa Rica
Atmospheric Environment
(1997) - et al.
Surface tension of atmospheric wet aerosol and cloud/fog droplets in relation to their organic carbon content and chemical composition
Atmospheric Environment
(2000) - et al.
Organic nitrogen in precipitation over Eastern North America
Atmospheric Environment
(2002) - et al.
Contribution of water soluble organic nitrogen to total titrogen in marine aerosols over the East China Sea and Western North Pacific
Atmospheric Environment
(2006) - et al.
Chemistry of fog waters in California's Central Valley, 3: concentrations and speciation of organic nitrogen
Atmospheric Environment
(2001)