ACCESS NUMBER: 30660-19925 

STUDY TITLE: Air Quality and Dispersion Meteorology Over the Northern Gulf of 

Mexico: Measurements, Analyses, and Syntheses 

REPORT TITLE: Air Quality and Dispersion Meteorology Over the Northeastern Gulf of 

Mexico: Measurements, Analyses, and Syntheses 

CONTRACT NUMBER: 14-35-0001-30660-19925 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Northeastern Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1997, 1998 

COMPLETION DATE OF REPORT: March 2000 

COST(S): FY 1997: $139,823, FY 1999: $89,519: CUMULATIVE PROJECT COSTS: 

$229,342 

PROJECT MANAGER(S): S. A. Hsu 

AFFILIATION: Coastal Studies Institute 

ADDRESS: Howe-Russell Geoscience Bldg., Louisiana State University, Baton Rouge, 

Louisiana, 70803 

PRINCIPAL INVESTIGATOR(S)*

: S. A. Hsu 

KEY WORDS: Nitrous oxides, sulphur dioxide, dispersion, meteorology, Gulf of Mexico, 

mixing height, atmospheric stability, Wildlife Refuges, NAAQS, Class 1, air pollution 

BACKGROUND: Several areas in the coastal reaches of the northeast Gulf of Mexico 

have been designated as Wildlife Refuges and National preserves. In recent years, 

there has been notable growth in population (and hence increased power production, 

traffic, etc.) landward of the coast. At the same time, energy exploration and production 

remains very active in the offshore waters of the northeast Gulf. Therefore, considering 

their location, it may be inevitable that the air quality in these protected areas is being 

influenced by emissions from these various sources. However, since some of the area 

is assigned Class I and II status by the U.S. EPA, it is subject to the most rigorous 

requirements for air quality. Monitoring of ambient air quality and meteorological 

conditions at multiple near-shore stations was performed to investigate the pollutant 

concentrations impacting these areas and the meteorological characteristics which 

affect dispersion and transport over the OCS region. This project report presents the 

findings from these observations. 

OBJECTIVES: (1) To establish and maintain two monitoring stations for ambient air 
ACCESS NUMBER: 30660-19925 

quality and surface meteorological conditions in the near coast area of the northeast 

Gulf; (2) To collect data from other sources in the area including NOAA buoys and 

coastal stations; (3) To derive and compare annual pollutant statistics between stations; 

(4) To define and determine meteorological parameters pertinent to dispersion and 

transport in the OCS region; and (5) To associate pollutant fluctuations with prevailing 

meteorological conditions. 

DESCRIPTION: In order to study spatial and temporal variations over the northeast 

Gulf, one monitoring station was deployed on Breton Island, located at the southern 

extent of the Chandeleur Island chain off the southeast coast of Louisiana. This island 

is part of the Breton National Wildlife Area, and has Class I designation. The second 

station was installed on the eastern end of Dauphin Island, Alabama, near the 

southwest entrance of Mobile Bay. Dauphin Island is part of the Gulf Islands National 

Seashore. The stations were similarly equipped for measurement of ambient SO2 and 

NOx using EPA approved monitors. At Breton, a surface meteorological monitoring 

system was also installed, and included wind speed and direction, relative humidity, air 

and water temperatures, and turbulence intensity (from uvw values). No meteorological 

measurements were made at Dauphin since the site was located adjacent to the NOAA 

C-MAN station DPIA1. All data was stored on site and offloaded during regular 

servicings, at which time the air quality monitors were inspected and calibrated in 

accordance with methods described in the Code of Federal Regulations. The period of 

record at Breton extended from October 1994 through August 1998 and at Dauphin 

from March 1996 to May 1998. Air quality data was recorded at 5-minute intervals; 

most meteorological values were recorded hourly. All data was quality controlled 

through calibration reports and station operator notes. The data was reduced and 

analyzed to determine significant characteristics of ambient air quality and meteorology 

affecting these locations. 

SIGNIFICANT CONCLUSIONS: Hourly concentrations of SO2 and NO2 at both stations 

were less than 6 ppb for over half the study period. Annual and maximum 

concentrations of both pollutants were well below the NAAQS. Mean concentrations 

are generally higher in the fall and winter months at both stations. No significant diurnal 

trends are evident. Maximums at Dauphin are mostly associated with winds from northnortheast. SO2 maximums at Breton are primarily seen with northerly winds while NO2

maximums are generally from south-southwest. On average, SO2 concentrations are 

higher at Dauphin than Breton while NO2 is nearly equivalent. Higher maximums are 

seen at Breton. Good agreement is noted between time series of 24 hour averaged 

SO2 at Breton and Dauphin; poorer correlation for NO2, suggesting this pollutant is more 

locally influenced. High SO2 at Breton is mostly seen under Continental High synoptic 

conditions (northerly winds); high NO2 observed predominantly under Gulf Return 

(southerly winds). Turbulence intensities over Breton are in reasonable agreement with 

those in the literature, but are influenced by terrain effects. Monthly mean stability over 

the Breton are is typically unstable, with free convection observed up to 15% of the 

time. Under unstable conditions offshore, the mixing height is equivalent to the lifting 

condensation level, and on average is below 750 m. Stable conditions offshore can 

produce mixed heights near 100 m. 
ACCESS NUMBER: 30660-19925 

STUDY RESULTS: The difficult logistics involved in maintaining a monitoring station at 

the remote location of Breton Island is reflected in the relatively poor data return 

achieved. Power on the island was produced by generators, and frequently fluctuated 

or failed. When combined with lightning strikes, hurricanes, and other natural 

phenomena, many long data gaps resulted due mainly to damaged electronics. 

Hurricane Danny (July 1997) caused complete removal of the station and re-installation 

in an adjacent building. These facts, along with a lack of baseline measurements, 

invalidate comparison of concentration values to PSD increments. Dauphin Island 

mean annual SO2 concentration for year 1 (March 1996 - April 1997) was 1.86 ppb and 

for year 2 (May 1997 - April 1998) was 2.28 ppb. Mean annual NO2 for year 1 was 4.58 

ppb and for year 2, 4.38 ppb. Seasonal trends show consistent SO2 values, however 

NO2 clearly peaks in fall and winter. Observed maximum concentrations were strongly 

associated with north-northeast winds. NO2 / NOx ratios were typically high, suggesting 

transport of an aged air mass from the mainland. With the exception of the final 

reporting period (May 1997 - May 1998), annual statistics at Breton were not considered 

due to the incomplete record. On the other hand, consistent values were obtained from 

existing samples. For SO2, mean values were 1.2, 1.3, 1.0, and 1.6 ppb and for NO2, 

3.0, 5.3, 3.8, and 5.4 ppb for years 1994, 1995, 1996, and 1997, respectively. A 

“composite” year assembled from the most complete months during those years yielded 

an “annual” mean SO2 and NO2 of 1.14 ppb and 4.58 ppb, respectively. All of these 

values are in good agreement with the annual mean values during May 1997 - April 

1998 for SO2 of 1.2 ppb and during June 1997 - May 1998 for NO2 of 6.2 ppb. The 

slightly higher NO2 is attributed to re-location of the monitoring station. Similar seasonal 

trends are evident as at Dauphin. SO2 maximums are clearly associated with northerly 

winds, while NO2 maximums are more distributed but mostly seen with southerly winds. 

 It should be noted that the generator housing was located to the south of the monitoring 

site, and NO2 / NOx values were often low suggesting a nearby source. SO2 maximums 

were most often recorded under Continental High synoptic conditions, while NO2

maximums were primarily seen with the Gulf Return pattern. 

The low terrain elevation at Breton Island influences the turbulence intensities as 

crosswind values were found to be higher than those in the literature (North Sea), 

although downwind and vertical values were in reasonable agreement. Unstable 

conditions prevail over the area throughout the year. A relationship between the mixed 

height, or lifting condensation level, under unstable conditions and the surface dewpoint 

depression shows mean mixed heights over the area to range from 250 m to 750 m. 

Much lower heights can occur under stable conditions. Methods for determining the 

proper values of drag coefficient, stability parameter, mixed height, turbulence intensity, 

and vertical eddy diffusivity from routine measurements are provided and demonstrated, 

and recommended for use in offshore modeling efforts. 

STUDY PRODUCTS: Hsu, S. A. and B. W. Blanchard. 1998. Long-term 

measurements of SO2 and NO2 concentrations and related meteorological conditions in 

the northeastern Gulf of Mexico region. OCS Study MMS 98-0020. U.S. Dept. of the 
ACCESS NUMBER: 30660-19925 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 

308 pp. 

Hsu, S. A. and B. W. Blanchard. 2000. Air quality and dispersion meteorology over the 

northeast Gulf of Mexico: measurements, analyses, and syntheses. OCS Study MMS 

2000-014. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico 

OCS Region, New Orleans, LA. 210 pp.