Environmental Monitoring and Assessment (2005) 110: 99–120
DOI: 10.1007/s10661-005-6281-2
c Springer 2005
MONITORING OF FOGWATER CHEMISTRY IN THE GULF COAST
URBAN INDUSTRIAL CORRIDOR: BATON ROUGE (LOUISIANA)
S. RAJA1 , R. RAVIKRISHNA1 , R. R. KOMMALAPATI2 and K. T. VALSARAJ1,∗
1
Gordon A and Mary Cain Department of Chemical Engineering, Louisiana State University,
Baton Rouge, Louisiana, U.S.A.; 2 Department of Civil Engineering, Prairie View A&M University,
Prairie View, Texas, U.S.A.
(∗ author for correspondence, e-mail: [email protected])
(Received 7 September 2004; accepted 12 November 2004)
Abstract. Seventeen fog events were sampled in Baton Rouge, Louisiana during 2002–2004 as
part of characterizing wet deposition by fogwater in the heavily industrialized corridor along the
Louisiana Gulf Coast in the United States. These samples were analyzed for chemical characteristics such as pH, conductivity, total organic and inorganic carbon, total metals and the principal ion
−
2−
−
concentrations. The dominant ionic species in all samples were NH+
4 , NO3 , Cl and SO4 . The
pH of the fogwater sampled had a mean value of 6.7 with two cases of acidic pH of 4.7. Rainwater and fogwater pH were similar in this region. The acidity of fogwater was a result of NO−
3
but partly offset by high NH+
4 . The measured gaseous SO2 accounted for a small percentage of the
observed sulfate concentration, indicating additional gas-to-particle conversion of SO2 to sulfate in
fogwater. The gaseous NOx accounted for most of the dissolved nitrate and nitrite concentration in
fogwater. The high chloride concentration was attributable to the degradation of chlorinated organics in the atmosphere. The metal composition was traced directly to soil-derived aerosol precursors
in the air. The major metals observed in fogwater were Na, K, Ca, Fe, Al, Mg and Zn. Of these
Na, K, Ca and Mg were predominant with mean concentrations >100 µM. Al, Fe and Zn were
present in the samples, at mean concentrations <100 µM. Small concentrations of Mn (7.8 µM),
Cu (2 µM), Pb (0.07 µM) and As (0.32 µM) were also observed in the fogwaters, and these were
shown to result from particulates (PM2.5 ) in the atmosphere. The contribution to both ions and metals
from the marine sources in the Louisiana Gulf Coast was minimal. The concentrations of all principal ionic species and metals in fogwater were 1–2 orders of magnitude larger than in rainwater.
Several linear alkane organic compounds were observed in the fogwater, representing the contributions from petroleum products at concentrations far exceeding their aqueous solubility. A pesticide
(atrazine) was also observed in fogwater, representing the contribution from the agricultural activities
nearby.
Keywords: fogwater, ion composition, Louisiana, metals, organics
1. Introduction
The Gulf Coast region of Louisiana in the United States is a heavily industrialized corridor where the world’s most recognizable chemical and petrochemical
companies are situated (Petersen, 1999; Burby, 2000). In particular, the area between metro Baton Rouge and New Orleans along the banks of the Mississippi
River is of interest as it is alleged to be a highly polluted region (Allen, 2003).
100
S. RAJA ET AL.
Louisiana is also typical of a predominantly agricultural area in the northern
parishes, where various pesticides are in use. Thus, in the metro Baton Rouge
atmosphere, we have observed both petrochemical markers such as aliphatic and
aromatic hydrocarbons and agricultural markers such as pesticides (Subramanyam
et al., 1993). This region of the country also sees its share of dense fog events during
the fall season. Fog frequency averages 40 days during the year in the Louisiana
Gulf Coast region (Ahrens, 2000). Extremely low visibilities created by dense
fog have contributed to several multi-vehicle accidents resulting in injuries and
fatalities.
Fog is formed by condensation of water on sub-micron size solid particulates
in the near-surface atmosphere. Typically, fog-droplet dimensions range from 1 to
50 µm. There are various types of fog formed in the atmosphere; these are classified
as radiation fog, precipitation fog, advection fog, slope fog and valley fog. Fogwater
characteristics have been explored in various locations particularly in the nonurban, agricultural environments and a few in the more urban environments. The
characteristics of the fog formed in urban environments can be different, considering
the fact that they occur in the more polluted urban atmosphere.
The deposition of fogwater promotes the atmospheric processing of sub-micron
particles, organic molecules and inorganic ions. Typical characteristics of fog such
as pH, conductivity and ion composition have been studied for a number of locations (Waldman et al., 1982; Millet et al., 1996; Pruppacher and Klett, 2003).
Though much is known regarding the fog processing of particles and ions, much
less information exists on organic molecules in fog (Munger et al., 1983; Herckes
et al., 2002). A handful of compounds such as low-molecular-weight carboxylic
acids, linear alkanes, pesticides and polycyclic aromatic hydrocarbons have been
identified in some fog samples (Decesari et al., 2000a; Limbeck and Puxbaum,
2000; Collett et al., 1999; Capel et al., 1991; Glotfelty et al., 1987). More recent
efforts have concentrated on the surface-active components in fog (Capiello et al.,
2003). Much of the extensive fogwater characterization data in the United States
are in the California Central Valley, Los Angeles Basin and the northeast Atlantic
coast regions, where fog is a frequent occurrence (Collett et al., 2002). Although
the Gulf Coast region of Louisiana experiences fogs several times during the year,
the literature data on the fogwater characteristics is non-existent. Hence, we undertook a series of fog collection campaigns to obtain the baseline data on fog.
In particular, we were interested in (i) understanding whether fog in the heavily
industrialized corridor along the Gulf Coast differs from other urban areas of the
United States and the world and, (ii) elucidating the possible sources of various
species in the fog in this area. This study was in preparation for a more concentrated effort at size-fractionated, time-series analysis of fog composition to test the
theory we have advanced that small fog droplets are enriched in many organic compounds as a result of surface adsorption (Valsaraj et al., 1993, Raja and Valsaraj,
2004a,b).
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
101
2. Experimental
Baton Rouge, the capital city of Louisiana, is situated approximately 200 miles
inland from the Gulf of Mexico. Mississippi river makes its bend around the city of
Baton Rouge before flowing out into the Gulf of Mexico past New Orleans some
75 miles away. Radiation fog generally occurs in the Baton Rouge area and the Gulf
Coast during two distinct periods: Fall (October–December) and Spring (March–
April). In most cases, fog forms during the midnight hours and lasts typically
through the early morning hours till the sun comes up. During the period between
2002 through 2004 we obtained 17 fogwater samples in the metro Baton Rouge
area. Most of the samples were collected during the springtime in the month of
March and during the fall season in the months of October and November (Table I).
TABLE I
Description of conditions during fog collection
Date
Conditions during fog
3/15/02
11/11/02
11/20/02
3/5/03
Early morning fog
Heavy cloud cover
Heavy fog following heavy rain
Heavy fog. Fog covered the entire
Gulf Coast. Followed by rain
Preceded by rain 24 h earlier
Dense fog
Dense fog
Warm day prior to fog, cool night,
clear sky, light fog
Warm day prior to fog,
cool night, clear sky, heavy fog
Patchy fog, clear sky
Heavy fog, warm day,
cool night preceding
Cloudy sky, heavy fog
Thick fog, clear sky,
cool night preceding
Thin patchy fog
Warm day, cool night
preceding fog, clear sky
Hot to warm day, cool
night preceding, clear sky
Patchy fog
3/14/03
3/18/03
3/27/03
10/25/03
10/31/03
11/1/03
11/2/03
11/6/03
11/12/03
11/17/03
3/19/04
3/20/04
3/30/04
Duration of Duration of
<1/4th mile <1/8th mile
Range of wind
visibility (h) visibility (h) T (◦ F) velocity (mph)
3.5
1
4
8
1
1
2.5
2
68
Calm air
53–58 Calm air
58–61 3.5–5.7
2.5
5.5
2
2
1.5
1
1
0
57–59
55–65
60–62
66–69
5
4
63–67 0–8.0
4
3.5
0
0
63–65 3.5–5.7
60–64 0–5.7
1
5.5
0.25
4.5
62–68 0–5.7
60–69 0–3.5
1.5
3.5
0
1
67–68 Calm air
57–63 Calm air
3.5
2.5
57–62 Calm air
1
0
45–49 Calm air
4.6–8.0
3.5–9.2
0–5.7
0–5.7
Notes: The weather data is from the National Climatic Data Center. http://www.ncdc.noaa.gov/
servlets/ULCD.
102
S. RAJA ET AL.
Typically, these samples were obtained during the early morning hours in an open
area in the agricultural farm maintained by the LSU Agricultural Center. Figure 1
shows the location of the site.
2.1. D ESCRIPTION
OF FOG COLLECTION EQUIPMENT
The high-volume fog collector, depicted by the schematic in Figure 2, primarily
consists of a fan that draws air through a set of stainless steel screens enclosed in an
aluminum cylindrical chamber. This impaction collector was designed on Glotfelty
et al. (1986). The screens were axially attached to a motor that can rotate them in
either direction. The size of the screens fitted into the collector was 1200 and 74 µm.
The speeds of the fan and the screen rotations were changed using a motor controller.
When the fan pulled the surrounding air at a high flow rate (34.2 m3 /min), fogwater
impinged on the screens and collected in concentric grooves around the screens.
R
R
A vacuum pump pulled the collected fogwater through Teflon tubes into Pyrex
sampling bottles. An air sampling tube was fixed in the fog collection chamber
behind the screens. The air that was in contact with the fog droplets was sampled
at a fixed flow rate (∼0.8 L/min) by passing it through an XAD bed that trapped
the organics in the air stream. The XAD polymer traps were obtained from Supelco
Inc. Sample collection was started a few hours before the anticipated fog event and
continued till the fog dissipated. After a fog event, the sample bottles and the XAD
traps were sealed and transported back to the laboratory for storage or analysis.
The amount of fogwater collected varied between 20 and 125 mL, depending on
the duration of the event. The fog collection equipment was placed in a location
facing an open agricultural field, with no tall structures for at least 400 m in all
directions in front of the equipment. Typically, an intensity of fog equivalent to a
visibility if 1/4 mile or less was found to be necessary to collect any fogwater. The
approximate duration of actual fog collection was estimated from period of dense
fog (visibility < 1/8th mile) reported by the National Weather Service (NWS) for
the Baton Rouge Metro Airport nearby.
2.2. A NALYSIS
OF FOGWATER SAMPLES
The fogwater samples were filtered through a 0.45 µm cellulose acetate membrane
filter as soon as they were brought to the laboratory. The membrane was stored for
potential analysis. Liquid sub-samples were isolated for the measurement of pH,
conductivity, dissolved organic carbon (DOC), organic solutes and metals content.
R
The pH and conductivity were measured using Orion Model 210 pH meter and
Oakton Model 10, respectively. The analyses of ions (chloride, sulfate, nitrite,
nitrate, bromide, fluoride, phosphate and ammonium) were done using a Dionex
DX320 Ion chromatograph instrument. The DOC content was analyzed using a
Shimadzu 5050A TOC analyzer. For metal analysis, a 1-mL sub-sample was diluted
10 times with a 2% nitric acid solution. Analysis of selected metals was performed
Figure 1. Location of the sampling site.
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
103
104
S. RAJA ET AL.
Figure 2. Schematic of the fog collector.
according to EPA-SW 846 method 6020 using ICP-MS (Perkin-Elmer, Model Elan
9000). The organic analytes were extracted with dichloromethane (DCM) based on
EPA method 3510 using a liquid–liquid extraction. The sample and the extraction
solvent were shaken in a sealed bottle for about 25 min. The DCM was withdrawn
after separation in a separatory funnel. The DCM extract was then concentrated to
up to 100 µl for analysis using a mild stream of nitrogen gas. The organic analysis
was performed based on EPA method 8270 for semi-volatile organic compounds
using gas chromatography (Hewlett-Packard Model 5890, Series 2) and a mass
selective detector (GC-MSD) (Hewlett-Packard, Model 5971).
2.3. D ATA
ANALYSIS
The chromatograms from the organic analysis were analyzed by selecting compounds present with a spectral match of greater than 90%. Method blanks were
analyzed and the organic compounds that appeared in the method blanks were
removed from the fogwater organic analytes list. A list of commonly occurring
analytes was identified and standards were obtained for these. External calibration
standards and laboratory control samples (LCS) were used to quantify the GC-MSD
analysis results. The concentrations of the final organic extracts were obtained
from the chromatographic areas using the external calibration and the recovery
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
105
percentages from the LCS analysis. The organic extract concentrations were converted to concentrations in the fogwater samples using the ratio of the volume of
solvent to the fogwater sample. All other measurements were direct reports of the
concentrations in the fogwater and required no calculations.
3. Results and Discussion
3.1. A TMOSPHERIC
GAS CONCENTRATIONS AND METEOROLOGY
Air pollution resulting from smog episodes in Baton Rouge has resulted in several
health advisories being issued during the past several years. Ongoing efforts by the
Louisiana State Department of Environmental Quality (DEQ) make it possible to
obtain real-time concentrations of major pollutants (PM, O3 , NOx , SO2 and CO).
These are monitored in Baton Rouge at the Capitol monitoring site, which is located
on campus at LSU. Measurements in 2002–2004 have shown concentration levels
of these pollutants as summarized in Table II.
The most dominant pollutants are NO2 and O3 . Concentrations of ozone average
about 80–100 µg/m3 with occasional maximum values exceeding the National
Ambient Air Quality Standard (NAAQS) value of 240 µg/m3 (1-h average). SO2 ,
although not high, also plays a role in aerosol generation in the area. The high
concentration of ozone in the air indicates a high degree of photochemical activity
in the atmosphere. The reported number of days when the air quality index (AQI)
exceeded 100 in Baton Rouge was 8 (2002), 16 (2003), and 5 (2004). U.S. EPA has
the following emission estimates for principal pollutants in the parish of East Baton
Rouge for the year 1999: 170,736 tons of CO, 47,792 tons of NOx , 44,924 tons
of SO2 , and 11,877 tons of PM2.5 . In addition, the total volatile organic chemical
(VOC) emissions that lead to ozone problems were estimated to be 29,040 tons in
the parish in 1999. Similarly, the ammonia emissions in the parish were 1137 tons
TABLE II
Primary pollutant concentrations determined in Baton Rouge at the Capitol site on
LSU campus
Average annual mean or maximum concentration (µg/m3 )
Pollutant
2002
2003
2004
CO
NO2
O3
4228 (8-h maximum)
33 (mean)
262 (1-h maximum)
106 (mean)
8 (mean)
13 (mean)
3542 (8-h maximum)
30 (mean)
278 (1-h maximum)
80 (mean)
16 (mean)
13.7 (mean)
3085 (8-h maximum)
32 (mean)
212 (1-h maximum)
SO2
PM2.5
35 (mean)
15.7 (mean)
106
S. RAJA ET AL.
in 1999 with the surrounding Ascension parish contributing an additional 22,629
tons in 1999.
During the five fog sampling campaigns (3/15/02, 11/11/02–11/20/02, 3/5/03–
3/27/03, 10/25/03–11/17/03, and 3/19/04–3/30/04) 19 fog events were captured.
During this work, the atmosphere was in the neutral stability class with wind velocity
ranging from 0 to 9 mph and temperature ranging from 45 to 69◦ F. In most cases,
fog lasted for 6–8 h, starting at midnight and dissipating by mid-morning.
3.2. FOGWATER
COMPOSITION
Table III gives the characteristic properties such as pH, total organic carbon, total
inorganic carbon, and conductivity of fogwater. The mean, maximum and minimum
values observed are shown. The pH of the fogwater averaged 6.1 and varied between
4.7 and 6.9. This is similar to the observed variation in pH of rainwater in this region,
viz., 4.8–6.3 (Braun, 1984). The rainwater as well as fogwater in the Gulf Coast is
therefore not as acidic as in some other parts of the United States (West and Feagley,
1995; NADP, 2004). The low pH of 4.7 for two fogwater samples coincided with
high ion conductivities and organic carbon in the same samples. The mean organic
carbon concentration in the fogwater was 97 mg/L and varied between 22 and
218 mg/L, which is typical of most fogwater samples. The ion conductivity is an
indication of the ion concentrations in the samples and varied between 3 and 646 µS.
3.3. I ON
CONCENTRATIONS
The ion compositions of the fogwater samples are compared to other studies in
Table IV. The principal ions observed were ammonium, nitrate, sulfate and chloride.
2−
As in most urban industrial areas elsewhere, the same three ions (NO−
3 , SO4 ,
−
Cl ) contribute the most to the overall anion concentration in this study. Sulfate
concentration is higher than for other urban areas and shows a wide variation among
the 19 fog events in Baton Rouge samples. Nitrate and chloride concentrations are
more in line with the concentrations in other areas. One important difference noted
was the high chloride in the Baton Rouge samples.
The concentrations of ammonium observed in the Baton Rouge samples were on
the high side with a mean of 5764 µM. The high concentration of this anion should
be anticipated as the fog collection site was near the LSU Agricultural farm, which
uses ammonia–nitrogen fertilizers at various times during the year. Moreover, as
indicated earlier, the East Baton Rouge parish and the adjacent Ascension and St.
John parishes together contributed to 29,407 tons of NH3 emissions as per the
1999 US EPA estimate. Similar concentrations of ammonium ions have also been
observed in other agricultural areas such as Riverside, California, Po Valley, Italy
and Strasbourg, France. To a large extent, the near-neutral pH of the fogwater is a
result of the large ammonium concentration compensating for the high nitrate and
sulfate concentration in the fogwater.
Urban
Urban
Urban
Non-urban
Urban
Urban
Urban
Urban
Baton Rouge, Louisiana (1)
Riverside, California (2)
Pasadena, California (3)
Corvallis, Oregon (4)
Dubendorf, Switzerland (5)
Po Valley, Italy (6)
Strasbourg, France (7)
New Delhi, India (8)
18
43
10
15
16
8
N
6.1 (4.8–6.9)
4.3 (2.3–5.7)
2.92–5.25
5.6 (4.5–6.8)
4.02 (2.08–5.99)
(2.5–6.8)
(3.5–4.0)
(6.6–6.7)
pH
178 (10–507)
(78–281)
(15–108)
97 (22–548)
Organic
carbon (mg/L)
6 (1–23)
Inorganic
carbon (mg/L)
255 (3–646)
Conductivity
(µS)
0.06 (0.01–0.09)
(0.004–0.089)
(0.1–0.3)
(0.004–0.05)
0.020 (0.008–0.04)
Liquid water
content (g/m3 )
Notes: Values within brackets are minimum and maximum.
References: (1) This work; (2) Munger et al., 1983; (3) Waldman et al. (1982); (4) Muir (1991); (5) Johnson et al. (1987), Capel et al. (1991);
(6) Gelencser et al. (2000), Decesari et al. (2000a); (7) Millet et al. (1996); (8) Ali et al. (2004).
Urban or
non-urban
Geographical
location
TABLE III
Comparison of the general characteristics of fogwater collected in Baton Rouge and other parts of the United States and the world
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
107
SO2−
4
NO−
3
386 (76–1404)
–
–
–
–
(4–510)
–
–
NO−
2
Cl−
24 (14–34) 2740 (360–7183)
–
(100–750)
–
(480–730)
–
84 (16–164)
–
(113–11549)
–
(0–695)
–
(1880–3110)
–
(50–140)
Br−
50 (29–71)
–
(180–410)
–
–
–
–
(50–100)
F−
References as in Table III. Values within brackets represent reported minimum and maximum at each location.
Baton Rouge, Louisiana (1) 1989 (854–4721) 2992 (1467–4962)
Riverside, California (2)
(715–3115)
(7050–28900)
Pasadena, California (3)
(240–472)
(1220–3520)
Corvallis, Oregon (4)
56 (16–182)
102 (47–216)
Dubendorf, Switzerland (5) (196–778)
(286–1293)
Po Valley, Italy (6)
(230–8100)
(165–9250)
Strasbourg, France (7)
(2150–5020)
(1240–3410)
New Delhi, India (8)
(125–300)
(10–60)
Geographic location
TABLE IV
Comparison of ion compositions (µM) for fogwater from Baton Rouge and other parts of the United States and the world
NH+
4
206 (38–451) 5764 (194–14,118)
–
(8340–25,800)
–
(1290–2380)
–
361 (201–528)
–
1160 (336–2793)
–
(560, 12,750)
–
(630–12,640)
–
(300–550)
PO3−
4
108
S. RAJA ET AL.
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
109
The high concentration of sulfate results from the gaseous SO2 in the atmosphere
that converts to sulfate in aerosols and fogwater. However, because of the proximity
with the Gulf of Mexico, it is also possible that sulfate has a marine origin. If that
were the case, the ratio of sulfate to sodium should be 0.12 as for seawater. Absence
of such a correlation between the sulfate and sodium concentrations corroborates
the lack of marine input. Mean SO2 air concentrations of 8 µg/m3 (year 2002),
16 µg/m3 (year 2003) and 35 µg/m3 (year 2004) have been reported in the Baton
Rouge area. The predicted fogwater S(IV) concentration will then be given by
K1 K2
K1
[S(IV)]fog = [SO2 ]gas RT K H 1 + + + + 2
(1)
[H ] [H ]
where KH (=1.23 M/atm) represents the gas–liquid equilibrium Henry’s constant
for SO2 , K1 (= 1.3 × 10−2 M) and K2 (= 6.6 × 10−8 M2 ) are the dissociation
constants for SO2 (Seinfeld and Pandis, 1998; Johnson et al., 1987). [SO2 ]gas in the
earlier equation is given by
[SO2 ]gas = [SO2 ]tot 1 + RT K H θL
K1 K2
K1
1+ + + + 2
[H ] [H ]
−1
(2)
At the average pH of 6.45 for the Baton Rouge fogwater [H+ ] is 3.57 × 10−7 M.
−8 3
3
The
average fogwater content, θ L is 2 × 10 m /m .3Hence, the predicted value of
[S(IV)]fog for the 2003 mean [SO2 ]tot of 16 µg/m will be 217 µM. The earlier
prediction is based on the fact that only direct gas transfer of SO2 to the droplet is
considered. The observed value (854–4721 µM) of sulfate concentration is higher
than the expected value by more than 1 order of magnitude. SO2 is a prevalent
pollutant in the five-parish area, which includes East Baton Rouge parish, where
fog sampling was conducted (US EPA, 2004). Sulfur dioxide emissions in Louisiana
result largely from electric power plants, refineries, sulfuric acid production, and
carbon black production [La DEQ, 1999]. As indicated earlier, the parish industries
emitted about 44,924 tons of SO2 to the atmosphere in 1999. Oxidation of S(IV) to
S(VI) in the atmosphere is a well-known process and can be promoted by ozone in
the atmosphere. As stated earlier, the five-parish area (including Baton Rouge) is a
highly oxidative atmosphere. Hence, we conclude that gas-to-particle conversion of
SO2 to SO2−
4 occurs within the fog in the atmosphere to account for the discrepancy
in the predicted and observed sulfate concentration.
The high concentrations of nitrate in fogwater represent the direct gaseous input of NOx and nitrate from aerosols in the Baton Rouge atmosphere. Oxides of
nitrogen are known inputs to rivers and streams in the Gulf Coast as reported by
a recent USGS survey (ESA, 1999). NOx enters the atmosphere not only from
agricultural sources in the predominantly agricultural areas of the North Louisiana,
but also from industrial emissions in Louisiana largely from compressor stations,
refineries, nitric acid plants and utilities. The total emission of NOx in the parish
110
S. RAJA ET AL.
was estimated at 47,792 tons in 1999. As much as 15–18% of the input of nitrogen
into streams in Louisiana have been attributed to atmospheric sources (Alexander
et al., 2000). The LaDEQ has detected nitrogen oxides in the Baton Rouge air and
annual mean concentrations of 33 and 30 µg/m3 have been reported in 2002 and
2003, respectively (US EPA, 2004). Since dissolved nitric acid exists solely as nitrate in fogwater, the total concentration [N(V)]fog is given by (Seinfeld and Pandis,
1998).
[N(V)]fog = [NOx ]tot
[H+ ]
K H RT K 1
+ θL
(3)
For HNO3 , K H = 2.1 × 105 M/atm and K 1 = 15.4 M. Therefore, the predicted
concentration of nitrate in fogwater for a [NOx ] of 30 µg/m3 is 24,193 µM. Total
nitrate and nitrite concentrations in fogwaters sampled during this period varied
from 1543 to 6366 µM. The much higher predicted value implies that not all of
the NOx converted to NO−
3 exist as dissolved species. The oxides of nitrogen have
been known to play a dominant role in the ozone problem that the five-parish area
of Louisiana has experienced in the last several years.
The chloride content in the fogwater can also be of marine origin, in which
case the chloride to sodium ration should be approximately 1.17. In the present
case, this ratio was close to 2.26. The larger ratio shows that chloride is not only of
marine origin, but also of anthropogenic origin. The total concentration of HCl in
air that gives rise to the measured chloride in fogwater can be ascertained from the
following equation:
[H+ ]
[HCl]tot = θL +
(4)
[Cl− ]fog
K H RT K 1
where KH is 1.1 M/atm and K1 is 1.7 × 106 M (Pruppacher and Klett, 2003).
For the average water content of 2 × 10−8 m3 /m3 and average pH of 6.45, the
total HCl in air is estimated to be 1.3 ppb. Chlorine is an essential feedstock for
many manufacturing plants in the Baton Rouge area that produce polyvinylchloride,
plastics and chlorinated solvents. For example, the emission of one of the most
volatile organic compounds, vinyl chloride from stationary sources in the industrial
corridor exceed 16 tons with atmospheric concentrations measured in Louisiana
ranging from 0.05 to 0.25 ppbv in 2003 (Ford et al., 2000; US EPA, 2004). It
has been shown to degrade to HCl in the air with a mean atmospheric residence
time of approximately 4 h (Ford et al., 2000). There are several other chlorinated
compounds (chlorinated alkanes, chlorinated benzenes, hexachlorobutadiene, etc.)
apart from direct chlorine discharges that have also been observed in Baton Rouge
(LaDEQ, 2000). These emissions can also potentially contribute to the evolution
of chloride in aerosols. Refuse incinerators can also be a source of chloride in air,
but they are not a major source in this area.
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
111
As per the literature, sulfate and nitrate are strongly involved in the acidity
of fogwater (Johnson et al., 1987; Sigg et al., 1987). A computation of the ratio
2−
[NO−
3 ]/[SO4 ] on an equivalent basis shows that the values vary over the range 0.5–
3.2 with the lower values (0.5–1.4) for fogwater samples of pH 6.0–6.4, whereas the
higher value (3.2) was for the fogwater of pH 4.7. This implies that fogwater free
acidity is mostly due to HNO3 either generated within the fog droplet or advected
from external sources. Similar conclusions have been observed for the Po Valley
fog (Fuzzi et al., 1992). Nitrate acidity in Baton Rouge fogwater is also supported
by the frequent ozone exceedances in this area, which is caused by NOx pollution.
In contrast, HCl is less involved in fogwater acidity, even though its concentration
may be comparable (Colin et al., 1989). Overall sea-salt contribution to inorganic
composition of fogwater was minor. The ratios of K, Cl− , SO2−
4 and Ca to Na
were higher than those found for sea-salt [mean ratios = 0.27, 2.26, 1.64 and 1.1,
respectively, compared to sea-salt ratios of 0.02, 1.17, 0.12 and 0.04, respectively].
3.4. M ETALS
SPECIES CONCENTRATIONS
A large number of metals species were detected in the fogwaters sampled during
this period. As this was a preliminary work, only total metal concentrations were
deduced. Table V summarizes these results. The mean of the values along with the
minimum and maximum values observed are given. These concentrations are also
similar in magnitude to those reported in urban areas elsewhere around the world
(Table V). The only difference is that for the samples from Corvallis, Oregon, which
is not an urban industrialized area. The major ions observed in fogwater were Na,
K, Ca, Fe, Al, Mg and Zn. Of these Na, K, Ca and Mg were predominant with mean
concentrations >100 µM. The fact that the sediment and soil compositions from
the Baton Rouge area also contain predominantly Ca and Mg lend support to the
idea that the metals in fogwater result primarily from the aerosol particles derived
from the surface soil in the area (Valsaraj, 2000). Further corroborating data is also
available from the direct analysis of aerosols captured from the Baton Rouge air
(Subramanyam, 1992). Fe and Al are the two other predominant metals in Baton
Rouge soils and sediments, and also found in the fogwater. Al, Fe and Zn were
present in the samples, at mean concentrations <100 µM.
Small concentrations of Mn, Pb, and As were also observed in the fogwaters.
Arsenic in fogwater at a mean concentration of 0.32 µM (0.13 µM is the WHO
drinking water standard) was somewhat surprising. The likely origin is fossil-fuel
combustion and long-range transport (WHO, 2001). Pb was observed in 7 of the
14 samples analyzed at a mean concentration of 0.07 µM. The origin of lead in
air is traditionally traced to leaded gasoline usage in the early 1970s. The primary
production of tetraethyl lead used as an additive in gasoline was in the Baton Rouge
area and known soil contamination of the same have been reported. Moreover, in
the New Orleans area lead in soil and paints have been shown to be the main
cause of higher levels of lead in children. This region of the country has the largest
Fe
Al
Mg
Mn
(53–198)
(9–265)
14 (3.5–89)
(11–975)
(5.5–242)
(54–3543)
(100–300)
–
(1.7–31)
2.4 (0.1–18)
–
(0.5–42)
(ND–304)
–
(46–206)
(3.5–180)
14 (4–40)
(1–66)
(2–120)
–
(25–80)
References as in Table III. Values within brackets are minimum and maximum reported at each location.
(4–53)
3 (2–20)
(4–63)
(11–145)
(45–1260)
(25–50)
–
Pb
0.07
(0.03–0.16)
–
–
–
–
–
–
1.1 (0.2–4) 0.2 (0.07–0.47) <0.12
(0.6–28.1) –
(0.1–5.6)
–
(0.1–11)
–
–
(ND–4.1)
(ND–24)
(ND–174) –
–
Zn
–
–
–
–
–
–
–
Ca
Riverside, California (2)
(30–188)
Pasadena, California (3)
(12–500)
Corvallis, Oregon (4)
28 (5–85)
Dubendorf, Switzerland (5)
(1–35)
Po Valley, Italy (6)
(30–765)
Strasbourg, France (7)
(120–3150)
New Delhi, India (8)
(50–200)
K
61 (1.8–429) 319 (33–1875) 93 (4–815) 8 (0.3–72)
Na
Baton Rouge, Louisiana (1) 1212 (304–3913) 328 (23–2359) 1342 (2–3425) 84 (2–223)
Geographic location
TABLE V
Comparison of the total metal concentrations (µM) in fogwater in Baton Rouge and other parts of the United States and the world
As
0.32
(0.03–1.8)
–
–
–
–
–
–
–
Cu
2.6
(0.26–20)
–
–
–
(0.3–10.6)
–
–
–
112
S. RAJA ET AL.
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
113
bauxite-ore–processing operation, which also produces Pb. The observed low fogwater concentration indicates that this is a minor pollutant in this area. Pb as the
primary pollutant in the air has been declining throughout the last 25 years over the
United States since the phase-out of leaded gasoline. US EPA has classified As, Pb
and Mn as hazardous air pollutants. As, Pb and Mn have been observed at mean air
concentrations of 0.0011, 0.0037 and 0.0012 µg/m3 , respectively, in PM2.5 particulate samples collected as part of the urban air pollutant monitoring program during
82 separate occasions in East Baton Rouge parish during the year 2003 (US EPA,
2004). Assuming that all of these metals will result in dissolved species in water
and converting the mean air concentrations of the metals to fogwater concentrations
for a mean water content of 0.02 g/m3 provides predicted fogwater concentrations
of As, Pb and Mn at 0.73, 0.89 and 1.1 µM, respectively. The agreement between
observed (0.32 µM for As, 0.07 µM for Pb and 7.8 µM for Mn) and predicted
values indicates that aerosols are the likely sources of these metals in fogwaters in
Baton Rouge.
Concentrations of copper observed were small (2 µM) but similar to those
reported elsewhere (Siefert et al., 1998). The main sources of copper are anthropogenic and involve varied sources such as fossil-fuel combustion, iron and steel
manufacturing, and other nonferrous metal-production industries (Keiber et al.,
2004). The presence of heavy metal ions such as Mn and Fe in fogwater can potentially alter the uptake of gaseous species such as SO2 in the droplet (Barrie
and Georgii, 1976; Behr and Sigg, 1990). Zn is one of those metals not frequently
observed in fogwaters from other parts of the country. However, it is observed in
the present study at a mean concentration of 93 µM, and its source in this area is
unknown.
Throughout the duration of our study, we noticed that in general, higher concentrations for the principal ions and metals were observed for short duration, light,
dissipating fog events. This indicates that if fog persists for a lengthy period, a
dilution effect will reduce the concentrations of ions and metals. This behavior
has previously been noticed for California fogs (Munger et al., 1983). Time-series
sampling of fog, is therefore warranted to further understand the evolution of ion
and metal concentrations in the Baton Rouge fogwater.
3.5. COMPARISON
TO RAINWATER
The US Geological Survey has carried out rainwater analyses for a decade or more
through the National Atmospheric Deposition Program (2004) over several parts
of the United States. In Louisiana, the site selected was the Iberia Research Station
located in Jeanrette in the New Iberia parish of the State. Data for the years 2001
through 2003 are summarized in Figure 3a. The values reported were weighted to
the mean precipitation for the year. Four dominant cations (Na, K, Ca and Mg)
2−
−
−
and four anions (NH+
4 , SO4 , NO3 and Cl ) were monitored through the different
seasons and the mean values for each was reported. The average concentrations in
114
S. RAJA ET AL.
Figure 3. Concentrations of ions and metals in (a) rainwater and (b) fogwater.
rainwater varied little during the 3 years. The concentrations reported for rainwater
should be compared to those in fogwater (Figure 3b). The concentrations in the
fogwater observed were much higher than those for the rainwater for all compounds.
The ratio of fogwater to rainwater ranged from 86 for Na to 613 for ammonium.
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
115
However, there are some similarities in the fact that both fogwater and rainwater
−
+
−
contain the same predominant ions, viz., SO2−
4 , NO3 , Cl and NH4 . Whereas
+
−
Cl concentration is higher in rainwater, NH4 is the dominant ion in fogwater.
This reflects the different aerosol sources, viz., upper atmosphere for rainwater and
the near-surface local atmosphere for fogwater. This is not surprising because such
trends have been observed at a number of other locations (Dasch, 1988; Millet et al.,
1993; Collett et al., 1993; Schemenauer et al., 1992; Capel et al., 1991). Raindrops
generally fall faster through the atmosphere and are constantly in contact with a
different fresh parcel of air with lifetimes of minutes. On the other hand, fog droplets
travel horizontally and slowly with atmospheric lifetimes of hours. Moreover, the
surface areas of fog droplets are much higher than raindrops, owing to the smaller
size of fog droplets. The aforementioned two effects combine to produce a higher
concentration in fogwater.
3.6. O RGANIC
SPECIES IN FOGWATER
The average total organic carbon in the Baton Rouge fogwater samples was 97 mg/L,
but varying between 22 and 548 mg/L depending on the day of sampling. These
values are slightly higher than other urban areas (Dubendorf and Po Valley), but
closer to the non-urban, agricultural area of Corvallis, indicating that the organic
aerosols arise from both industrial and anthropogenic sources in the Baton Rouge
atmosphere. There have been several reports of the determination of the organic
carbon composition of fogwaters, most notably by Decesari et al. (2000b). These
investigators reported that there exist four main classes of organic compounds, viz.,
neutral compounds, mono- and di-acids, polyacids and an uncharacterized fraction.
Further recent work by Herckes et al. (2002) showed that the neutral compounds
can be classified into linear alkanes, polycyclic aromatic hydrocarbons (PAHs),
oxygenated PAHs, n-alkanoic acids, resin acids and n-alkenoic acids. Earlier work
by Glotfelty et al. (1987) had shown considerable concentrations of pesticides in
fogwater collected near agricultural areas.
A number of organic compounds were observed in the fogwater samples collected in this work using GC/MS analysis. These are present both as water-soluble
organic compounds (WSOC) and particulate-associated organic compounds (POC).
Because of the survey nature of this investigation, no distinction was made between
WSOC and POC in this work and the total species was analyzed. Specifically two
classes of organic compounds were identified in our fogwater samples, viz., linear
alkanes and pesticides. GC/MS traces depicting the two are shown in Figures 4(a)
and 4(c). Several linear alkane species ranging from C6 to C44 were detected,
which were also observed in the interstitial air samples obtained simultaneously
(Figure 4(b)). These linear alkanes have been established as markers for aerosols
in the atmosphere derived from petroleum combustion (Schauer and Cass, 2000).
As such, we conclude that aerosols are the predominant source of linear alkanes
in the fogwater. In addition to this, we also observed several alkanoic acids and
116
S. RAJA ET AL.
Figure 4. GC/MS traces of fogwater and air samples collected.
pesticides in the fogwater. Alkanoic acids have been known to result from the
oxidation of alkanes. The presence of pesticides (atrazine, metolachlor) in the fogwater sample was not surprising as the sample collection was near the LSU Ag
Farm, which uses pesticides periodically during the year. Our initial aim was to
MONITORING OF FOGWATER CHEMISTRY IN THE URBAN INDUSTRIAL CORRIDOR
117
quantify each of these compounds in greater detail. However, due to the limited
nature of sample volume and analytical work-up, we were only able to quantify
very few of the compounds. For example, the pesticide atrazine, and two linear
alkanes (pentadecane and tetratetracontane) were observed in the fog samples at
quantifiable concentrations. The observed fogwater concentration varied from 0.39
to 0.89 µg/mL for atrazine (three samples), 0.08 µg/mL for pentadecane (one sample), and varied from 0.02 to 0.79 µg/mL for n-tetratetracontane (two samples).
The respective aqueous solubilities of the compounds are 30 µg/mL for atrazine,
and 8 × 10−5 µg/mL for pentadecane at 298 K, whereas for tetratracontane there is
no reported aqueous solubility. For atrazine the observed fogwater concentrations
are much less than the aqueous solubility, whereas for pentadecane it is 3 orders
of magnitude larger than the aqueous solubility. We note that there has been little
effort directed in this work towards properly characterizing the various organic
compounds constituting the dissolved fraction. There are several hypotheses for
the enrichment of organic compounds in the fog droplets (Valsaraj et al., 1993);
these require a more comprehensive analysis of droplet size-fractionated analysis
of fog. Our current efforts are directed towards resolving the earlier issues.
The total concentrations of the principal ions, soluble total metals and total
organic carbon were multiplied into the average water content in fog to obtain the
total mass per cubic meter of air sampled. These values are plotted in Figure 5
for a comparison of the respective mass contributions to the total soluble material
in fog. The major component (78.7%) is the principal ions, whereas the soluble
Figure 5. Distribution of various species in Baton Rouge fogwater.
118
S. RAJA ET AL.
metals and total organic carbon contribute 8.3 and 12.9%, respectively, of the total
dissolved mass in fogwater. The fogwater chemistry is therefore dominated by the
principal ions. Although a significant mass is organic carbon, only little is known
regarding the different compounds that make up the organic carbon fraction. A
large portion of this organic carbon is probably composed of large molecular weight
humic and fulvic-like components (Capiello et al., 2003), and the rest composed of
low molecular weight dissolved organic molecules such as linear alkanes, alkanoic
acids, esters, and PAHs (Herckes et al., 2002). Our future work is geared towards
understanding the organic composition of the Baton Rouge fogwater.
Acknowledgments
This research was supported by grants from the US National Science Foundation
(ATM-0082836 and ATM-0355291).
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