ACCESS NUMBER: -36186 (M05AZ10667) 

STUDY TITLE: Determination of Net Flux of Reactive Volatile Organic Compounds at 

the Air-Water Interface in the Gulf of Mexico 

REPORT TITLE: Determination of Net Flux of Reactive Volatile Organic Compounds at 

the Air-Water Interface in the Gulf of Mexico 

CONTRACT NUMBER: 1435-01-04-CA-32806-36186 (M05AZ10667) 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Northern 

FISCAL YEARS OF PROJECT FUNDING: 2004; 2005; 2006; 2007; 2008 

COMPLETION DATE OF REPORT: December 2010 

COSTS: FY 2004: $82,548; FY 2005: $62,511 

CUMULATIVE PROJECT COST: $145,059. 

PROJECT MANAGERS: E.B. Overton and M.S. Miles 

AFFILIATION: Louisiana State University, Department of Environmental Sciences 

ADDRESS: 1285 Energy, Coast, and Environment Bldg, Baton Rouge, LA, 70803 

PRINCIPAL INVESTIGATOR(S)*

: E.B. Overton 

KEY WORDS: RVOC; reactive volatile organic compound; VOC; volatile organic 

compound; low-level ozone; air quality; hydrocarbons; microFAST GC; sorbent tube; 

thermal desorption; GC-TOFMS; gas chromatography; time of flight mass spectrometry; 

northern Gulf of Mexico; flux; net flux 

BACKGROUND: The downwind transport of primary pollutants leading to excessive 

secondary pollutant production has been gaining acceptance since the early 1990’s 

work of the Southern Oxidant Study (SOS). Initial SOS research suggested that 

transport of ozone and ozone precursors could have significant negative impacts on 

ozone and haze in downwind urban areas. Hence, the possibility that air quality 

problems in coastal urban centers, namely Texas and Louisiana, derive to some extent 

from downwind transport of natural volatile organic compounds (VOC’s) and, in 

particular, the reactive volatile organic compounds (RVOC’s) emitted in the gulf must be 

considered. 

OBJECTIVES: This project is designed to be a pilot study of a methodology that may 

yield better estimates of geogenic and biogenic sources of RVOC’s to the atmosphere. 

The project involves lab studies for the development of RVOC sampling and analysis 
ACCESS NUMBER: -36186 (M05AZ10667) 

methods, and field evaluation of the lab generated methodologies. Additional objectives 

of this project are to preliminarily examine the concentration of RVOC’s at the sea 

surface-atmosphere interface in the northern Gulf of Mexico, and, to provide a 

quantitative methodology for more accurate estimates of RVOC loads contributed by 

natural sources and/or processes. The objectives are based on providing a better 

understanding of factors specific to the GOM region that affect the RVOC load from 

non-production sources. 

DESCRIPTION: This study directly examined analytical methodology that can be used 

to study the net flux of reactive volatile organic compounds (RVOC) from the sea 

surface to the atmosphere in the Gulf of Mexico in order to provide a better 

understanding of factors specific to the Gulf of Mexico region that affect the RVOC load 

from non-production sources and provides quantitative information that will allow more 

accurate estimates of RVOC load contributed by natural processes. The goal of the 

project is to provide technology for estimating the specific net flux data in terms of 

composition and quantity for selected exemplar areas. Two area types were examined: 

(1) areas near-shore to the GOM with optimum wind conditions (e.g. winds out of the 

south, southeast, or southwest); and, (2) offshore in the Green Canyon area. 

Currently, the impact on the Gulf Coast environment of these natural emissions, 

especially in comparison with production related emissions, remains an open question. 

Because the emission areas are large and the emission time frame is long, it is critical 

to have a reasonably accurate characterization of the material flux from the sea surface 

to the atmosphere in order to estimate the effect these sources have on nearby (or not 

so nearby) land areas. Estimates based on generalized emission factors are useful 

starting points, but the use of unvalidated factors can lead to large, unknown errors in 

actual environmental impact. The specific contribution of RVOC’s to ozone formation 

potential depends not only on the amount of RVOC’s in the air mass but also on the 

specific RVOC’s present due to the fact that photochemical activity varies widely for 

individual RVOC’s. Thus, a speciated characterization of RVOC’s, including 

identification of highly reactive volatile organic compounds (HRVOC’s) is necessary for 

creating an accurate estimate of the expected impact of natural GOM emissions on 

coastal urban centers. 

Air sampling and analysis procedures were developed and validated for two different 

analytical instrument systems, the on-site microFAST GC and a lab based time-of-flight 

gas chromatograph mass spectrometer system using thermal desorption tube samples. 

Four field validation sampling trips were made to near-shore locations and to one deep 

sea location in the Northern GOM. The goal of the field validation trips was to establish 

sampling methodologies that could identify individual RVOC’s in a large number of air 

samples and in water samples. The data can then be used to establish whether or not 

RVOCs emitted from natural sources in the GOM represents potential air quality 

problems if transported to an environment containing high levels of NOx (e.g. a nearby 

urban environment), an active ingredient necessary in the formation of atmospheric 

ozone. 
ACCESS NUMBER: -36186 (M05AZ10667) 

SIGNIFICANT CONCLUSIONS: This project demonstrates the ability to detect low part 

per billion levels of RVOC’s, from compounds as volatile as ethane and ethylene to the 

less volatile BTEX compounds, with a relatively large number of both on-site analyses 

as well as conventional laboratory based analyses. Furthermore, it demonstrates the 

potential capacity of analyzing a relatively large number of air and water samples that 

are needed to accurately estimate flux from the OCS area. 

The analytical methodology used for this project produced comparable data to other 

similar research cited in this report. The analytical methodologies and instrumentation 

used for this project are relatively cost-effective in comparison to using more complex 

sampling strategies (e.g. collection of air samples by airplane). This project 

demonstrates the ability to readily go into the field with quantitative analytical equipment 

and perform on-site analyses of RVOC’s. 

STUDY RESULTS: Overall detection of RVOC’s was highly variable from site to site 

and day to day for both instrument systems. The variability in the analytical results of 

this study is related to two things: a non-homogeneous mixture of RVOC’s moving 

through the water column and into the air; and, the reactivity of the target RVOC 

compounds with hydroxyl radicals, NOx, and ozone. 

The microFAST GC data from five field validation trips shows that ethylene and 

hexane/isoprene dominated the RVOC profile due to the fact that these RVOC’s were 

detected at three out of five field validation trips. Furthermore, the majority of the 

targeted RVOC’s were detected above the detection limits at least one time out of five 

field validation sampling trips. The exceptions were propylene, pentane, and 1-hexene; 

these RVOC’s were not detected above the detection limit at any of the five sampling 

locations. Butane was detected at two of the five validation trips; and, ethane, propane, 

1-butene, 1-pentene, benzene, toluene, and the xylenes were detected at one of the 

five sampling locations. Ambient air samples taken by the on-site microFAST GC 

during the off-shore sampling trip to Green Canyon was dominated by hexane/isoprene 

and the alkyl benzenes. 

The sorbent tube trap/GC-TOFMS data from four field validation trips also shows that 

almost all the targeted RVOCs were detected, above detection limits, at each sampling 

event. Propane, propylene, butane, 1-butene, pentane, hexane, benzene, 1-hexene, 

toluene, and ethylbenzene were detected at three out of four sampling events. 1-

pentene and m+p-xylenes were detected at two of four sampling events, and o-xylene 

was detected during one sampling event. Ambient air samples collected on the sorbent 

tube traps during the off-shore sampling trip to Green Canyon was also dominated by 

the alkyl benzenes. The purged water samples contained all the targeted RVOCs 

except pentane and 1-pentene. The average hexane concentrations in the purged 

water sample were below the detection limits of 100 pmol/L. Of the targeted RVOC’s for 

the off-shore sampling trip to Green Canyon, the ambient air and water samples were 

mutually dominated by alkyl benzenes. 
ACCESS NUMBER: -36186 (M05AZ10667) 

 

The data from this project is comparable to the numerous other research projects cited 

in this report and is evidence of the importance of RVOC’s from marine environments. 

This project, along with similarly related projects, shows that the ultimate impact on 

nearby urban areas is likely a function of not only the quantity of RVOC’s emitted into 

the atmosphere, but also by their composition. The detection of the various alkyl 

benzenes, especially toluene and the xylenes, and the highly reactive olefins, ethylene, 

propylene, and isoprene, was anticipated prior to project method development. As a 

result, the analytical methodologies and instrumentation was optimized for a full range 

of RVOC’s. All of these RVOC’s were detected at one point or another during the field 

validation trips. 

Results from the Green Canyon sampling trip in the northern GOM indicate the flux 

between the seawater and air boundary layers are comparable to those of previous 

studies (Milne 1994 and Sauer 1980). If RVOC’s are assumed to be in equilibrium 

between the seawater and air boundary layer above it, their concentration in the 

seawater should be equivalent to the product of Henry’s Law constant and the partial 

pressure of the specific compound in the atmosphere. The average products (Pcmpd x 

KH) for the individual RVOC’s within the seawater exceeded the equilibrium product 

concentrations, indicating a net flux of RVOC’s from the seawater into the atmosphere. 

Analytical results of the air and water samples from the Green Canyon sampling trip 

indicated a large standard deviation among the individual gas components. The large 

standard deviation implies a wide, naturally occurring RVOC distribution throughout the 

sampling area, but actual analyses indicate localization of some individual components 

within the northern GOM. Given both biogenic and anthropogenic source contributions 

for the individual compounds and the patchiness of detectable quantities, extrapolation 

of these few flux rates to give estimates of northern GOM marine RVOC inputs to the 

atmosphere requires further investigation. 

STUDY PRODUCT: Overton, E.B; M.S. Miles; Ashton, B.M; and R.L. Wong. 2010. 

Determination of net flux of reactive volatile organic compounds at the air-water 

interface in the Gulf of Mexico. U.S. Dept. of the Interior, Bureau of Ocean Energy 

Management, Regulation and Enforcement, Gulf of Mexico OCS Region, New Orleans, 

LA. OCS Study BOEMRE 2010-043. 65 pp. 

*

P.I.’s affiliation may be different than that listed for Project Managers.