A tracer gas technique has been developed by the Norwegian Institute for Air Research (NILU) to study emission rates, dilution and diffusion of gases. The technique has been applied in several countries to quantify diffuse VOC leakages from refineries, petrochemical plants and oil production platforms.

Battery operated integrated tracer sampler with timer.

The tracer gas technique
The dual tracer gas system developed by NILU has been applied in several field studies in Europe. Easy-to-use portable release systems, syringe-based samplers and portable gas chromatographs are developed for various applications. Several field studies linked to model development have been reported. The system has been designed to meet the following requirements:
• easy to modify,
• easy to handle,
• light weighted,
• samplers with preset timers,
• rapid analyses in field.

The release system
The release system can be modified for different scales of application to give concentrations of from 10 ppt to 105 ppt (parts per trillion). Normally the tracer gases are released from 10 litres cylinders containing the liquefied gas. Through a reduction valve and a flowmeter the gas release can be kept constant at the desired rate. The two tracers (SF6 and CBrF3) can be analysed simultaneously by the portable field gas chromatographs are.

Integrated samplers
The air samples are collected in inexpensive syringes integrated over short time intervals (grab samples) or over a preset time period. Normally the time period is 15 minutes. A number of battery-operated samplers have been developed with a timer system for presetting of start/stop. These samplers are small, light and handy. One person can easily carry 24 samplers in field.

Tracer sample analyses
The samples can be analysed in the field by portable gas chromatographs immediately after the collection. The technique thus allows the results of one experiment to be used in the design of the next.

Tracer gas technique experiment layout for studies of diffuse hydrocarbon leakages.

Fast response mobile SF6 detector
Also semi instantaneous sequential samplers and a continuous sampler have been developed by NILU. The latter instrument provide instantaneous tracer concentrations in ambient air with an accuracy of <10 ppt. It can be used as a “mobile plume detector” during the set-up of cross wind concentration measurements. The instrument can also be connected to a logging unit to provide time series of concentration fluctuations at fixed locations in the plume. Typical logging frequency would be around 1 Hz.

Data presentation 
Tracer gas data evaluation and presentation software has been developed at NILU. This enables tracer gas concentration mapping on a gridded coordinate system specified for the area studied. It also plots cross-wind concentration profiles and estimates cross-wind integrated concentrations. These results are used in the evaluation of sample representativeness, and support the further evaluation of emissions and emission estimates.

Several applications
The NILU tracer technique has been used in many different applications, e.g. to study:

• transport and dispersion,
• dispersion parameters,
• transport in complex terrain,
• building wakes,
• traffic tunnel ventilation rates,
• ventilation in street canyons, 
• building and room ventilation rates,
• industrial emission rates and 
• diffusive leakages of air pollutants.

Diffusive VOC leakages
One of the more interesting examples is the estimate of diffusive leakages of hydrocarbons (VOC) from petrochemical factory complexes. SF6 was released in different identified release areas at various heights above the surface in the factory area. The release rates were adjusted to match the rough emission assumptions based upon screening studies undertaken prior to the tracer gas experiments.

Measured 15 min average concentrations of SF6 and selected hydrocarbons downwind from a petrochemical factory area. The sampling arrays are indicated and the concentration distribution shows an area of SF6 concentrations with more than 10 µg /m3 extending 300 m downwind.

The crosswind concentration profile measured along traverse A-B show the SF6 concentration distribution and concentrations of ethene, propane and propene in selected points. In this case there is a close correlation between SF6 and HC concentrations, which means that emission estimates can be performed with a large degree of confidence.

Emission estimates
During near stationary wind conditions and with no significant atmospheric reactions or deposition of hydrocarbons (VOCs) or other release gases between the leakage points and the sampling points, the emission rates of specific hydrocarbons can be estimated from simple flux considerations.

A model for such estimates has been developed by NILU. The tracer gas technique thus provides a flexible tool for estimating emission rates and evaluating the downwind impact of these emissions on health and the environment.