Environmental Engineering Reference
In-Depth Information
2.23 Influence of Chlorine Emissions on Ozone Levels
in the Troposphere
Golam Sarwar
1
, Ravi Joseph
2
, and Rohit Mathur
1
1
United States Environmental Protection Agency
2
Austin Energy, Austin, TX, USA
Abstract
Chlorine emissions from cooling towers are emitted mainly as hypo-
chlorous acid, not as molecular chlorine. Chlorine emissions from cooling towers
in electric utilities in the U.S. are estimated to be 4,400 t/year. On a molar basis,
molecular chlorine results in a greater increase in tropospheric ozone than hypo-
chlorous acid. However, hypochlorous acid produces more ozone than molecular
chlorine when an equal amount of chlorine is present on a mass basis.
Keywords
Emissions, molecular chlorine, hypochlorus acid, chlorine chemistry,
ozone
1. Introduction
Recent studies suggest that chlorine chemistry can increase ozone (O
3
) in the
troposphere in some areas of the U.S. (Chang et al., 2002; Knipping and Dabdub,
2003; Chang and Allen, 2006; Sarwar and Bhave, 2007). While these studies sug-
gest that chlorine may affect O
3
concentrations in the troposphere, information
about chlorine emissions is sparse. Molecular chlorine (Cl
2
) is a hazardous air
pollutant; thus, the National Emissions Inventory (NEI) for hazardous air pollutants
includes estimates of anthropogenic Cl
2
emissions in the U.S. However, both Cl
2
and hypochlorous acid (HOCl) can undergo photolysis to produce chlorine radical
(Cl) which can enhance chemical production of O
3
. Thus, emissions inventories
for air quality study need to include estimates of both Cl
2
and HOCl emissions.
This study develops an estimate of chlorine emissions from cooling towers in
electric utilities and evaluates the relative impact of Cl
2
and HOCl on tropospheric
O
3
formation.
