Chemistry of fog waters in California's Central Valley—Part 3: concentrations and speciation of organic and inorganic nitrogen
Introduction
Intense radiation fog events are common in the Central Valley (CV) of California during winter. These fogs usually form late at night and dissipate 3 or 4 h after sunrise, although they can persist for several days without fully breaking up (Sagebiel and Seiber, 1993). In foggy regions such as the CV, fog events may strongly influence atmospheric chemistry and air quality. For example, fog formation can significantly increase the removal rates of atmospheric particles, accelerate gas to particle partitioning, and lead to reactions in the aqueous phase and at the gas–liquid interface (Blando and Turpin, 2000; Collett et al., 1999a; Fuzzi et al., 1997; Gustafsson and Gschwend, 1999; Hoag et al., 1999; Laj et al., 1997; Lillis et al., 1999; Seinfeld and Pandis, 1998). In addition, since the deposition of fog water can be an important source of water, nutrients, and pollutants, fogs can also play crucial roles in the maintenance and decline of ecosystems, especially in coastal and mountain regions (Finlayson-Pitts and Pitts, 2000; Weathers, 1999).
Understanding the effects of fogs on atmospheric chemistry and ecological health requires detailed information on their chemical composition. While inorganic compounds have been studied extensively in fog waters, relatively little is known about the organic constituents. As shown by previous studies, the concentrations of organic carbon in fog waters can be quite high (Collett et al., 1999b; Facchini et al., 1999; McGregor, 2000). For example, reported levels in samples from the CV range from 420 to 9250 μM C (Collett et al., 1999b; McGregor, 2000). The presence of high concentrations of organic compounds may significantly affect the chemistry and toxicity of fog waters (Blando and Turpin, 2000; Collett et al (1999a), Collett et al (1999b); Facchini et al., 1999; Glotfelty et al (1987), Glotfelty et al (1990); Lo and Lee, 1997; Sagebiel and Seiber, 1993; Schomburg et al., 1991; Suzuki et al., 1998). For example, organic compounds likely contribute considerably to the buffering capacity recently identified in CV fog waters (Collett et al., 1999a). Also, organic species are undoubtedly involved in the chemical reactions that occur in foggy or cloudy atmospheres (Anastasio et al., 1997; Anastasio and McGregor (2000), Anastasio and McGregor (2001); Blando and Turpin, 2000; McGregor and Anastasio, 2001). In addition, the occurrence of surface-active organic materials can cause substantial enrichment of hydrophobic toxic substances, such as pesticides, in fog waters (Glotfelty et al (1987), Glotfelty et al (1990); Sagebiel and Seiber, 1993; Schomburg et al., 1991).
Past studies have found that measured individual organic compounds can only account for a small fraction of the total organic carbon (TOC) in fog waters. For example, in samples from the CV identified low molecular weight carboxylic acids and carbonyl compounds typically accounted for less than 20% of TOC (Collett et al (1999a), Collett et al (1999b)). A complementary approach to studies of individual organic compounds is classification of organic carbon based on functional groups, polarity, or molecular weight (e.g. Anastasio et al., 1997; Decesari et al., 2000; Facchini et al., 1999; Grosjean and Wright, 1983). While this latter approach is more general than compound-specific studies, it typically accounts for a larger fraction of organic carbon and can have important implications for the bulk properties of organic compounds in atmospheric waters. For instance, Facchini et al. (1999) found that ∼40%, on average, of water-soluble organic carbon in Po Valley fog waters was in macromolecular compounds (>500 Da), but analysis for 120 individual organic compounds could only account for less than 5%, on average, of the total dissolved organic carbon.
One broad class of organic compounds that has not been examined yet in fog waters is nitrogen-containing organic compounds. Based on measurements of dry and wet deposition, organic nitrogen (ON) is widespread in atmospheric condensed phases and usually represents a significant portion of the total nitrogen (Anastasio and McGregor, 2000; Cornell et al (1995), Cornell et al (1998), Cornell et al (2001); Jassby et al., 1994; Russell et al., 1998; Scudlark et al., 1998; Timperley et al., 1985). In addition, a number of other studies have shown that amino compounds are also common in atmospheric deposition (McGregor and Anastasio, 2001, and references therein), although the contribution of amino compounds to ON in atmospheric particles or drops has not been previously determined. Based on the chemical and physical properties of amino compounds and other ON functional groups, these compounds could significantly affect the formation and chemistry of fog drops if present in sufficient concentrations.
The overall goal of this study was to quantify and speciate the organic and inorganic nitrogen in fog waters collected from Davis, California. We report here the major results from this study, including: (1) the concentrations and relative amounts of dissolved inorganic and organic nitrogen in fog waters and (2) the contribution of free and combined amino compounds to the pool of ON in the fog waters. Finally, we compare nitrogen data in fog waters with those in co-located PM2.5 samples to explore the potential importance of ON compounds in fog and aerosol processing.
Section snippets
Sample collection and processing
Fog water samples (see Table 1) were collected in Davis, CA, during winters from 1997 to 2001 using a Caltech Active Strand Cloudwater Collector (CASCC2; Demoz et al., 1996). Fog waters were collected directly into high density polyethylene (HDPE) bottles and were filtered (0.45 μm Teflon; MSI Tefsep) right after collection. Filtered samples were stored frozen (−20°C) until analysis. Two cloud waters from Tenerife (Canary Islands) were also studied. To minimize contamination, the collection
Controls
As a check for possible contamination introduced during sample collection and processing, collector rinse waters were collected and analyzed (see Section 2.1). For cations and anions, concentrations in the rinse waters were all less than 10% of the median values in the bulk fog waters (Table 1). Concentrations of DOC in rinse waters ranged from 100 to 400 μM C, except for one rinse water where the aliquot for analysis was apparently contaminated (Anastasio and McGregor, 2001). Excluding this one
Conclusions
We measured considerable amounts of TDN in wintertime fog waters collected in Davis, California. While inorganic nitrogen species, primarily NH4+ and NO3−, accounted for the bulk (72–96%) of this fog water nitrogen, DON, measured here for the first time in fog waters, was also a significant component. Concentrations of DON ranged from 120 to 1630 μM N and accounted for 3.9–28% (median=16%) of TDN. Ratios of C : N in the fog water dissolved organic compounds ranged from 1.6 to 12, with a median
Acknowledgements
The authors thank Keith McGregor (UC Davis) for TOC analysis and assistance with fog sampling, Mike Jimenez-Cruz (UC Davis) for assistance with fog sampling, Zengshou Yu (UC Davis) for assistance with sample hydrolysis, Eli Sherman and Jeff Collett Jr. (Colorado State University) for the Tenerife cloud samples, and Tony Andreoni and Dean Bloudoff (California Air Resources Board) for loan of the CASCC2. This work was supported by the U.S. EPA (R819658 and R825433) Center for Ecological Health
References (75)
- et al.
Chemistry of fog water in California's Central Valley1. In situ photoformation of hydroxyl radical and singlet molecular oxygen
Atmospheric Environment
(2001) - et al.
Secondary organic aerosol formation in cloud and fog dropletsa literature evaluation of plausibility
Atmospheric Environment
(2000) - et al.
Formation of pollution particulate nitrogen compounds by NO-soot and NH3-soot gas-particle surface reactions
Atmospheric Environment
(1975) - et al.
Internal acid buffering in San Joaquin Valley fog drops and its influence on aerosol processing
Atmospheric Environment
(1999) - et al.
Water-soluble organic nitrogen in atmospheric aerosola comparison of UV and persulfate oxidation methods
Atmospheric Environment
(1999) - et al.
Urea in rainwater and atmospheric aerosol
Atmospheric Environment
(1998) - et al.
On the Caltech active strand cloudwater collectors
Atmospheric Research
(1996) - et al.
Fog droplets—an atmospheric source of secondary biological aerosol particles
Atmospheric Environment
(1997) - et al.
Water-soluble primary amine compounds in rural continental precipitation
Atmospheric Environment
(1992) - et al.
Carbonyls in urban fog, ice fog, cloudwater and rainwater
Atmospheric Environment
(1983)