(160a) Heterogeneous Chlorine Chemistry in the Atmosphere: Insights From Modeling and Experiments

Heterogeneous
Chlorine Chemistry in the Atmosphere: Insights from Modeling and Experiments.

Background

           
To date, numerous studies using both computational and experimental methods
have shown that the presence of gas phase atomic chlorine radicals (Cl^?)
can influence tropospheric photochemistry.  Specifically, Cl^?
participates in the ozone production cycle in a manner analogous to the
hydroxyl radical (OH^?), and elevated concentrations can lead to
increases in ozone production rates.  Cl^? radicals are produced
from the photolysis of a gas phase precursor such as Cl₂ or
HOCl.  Impacts on ozone production resulting from anthropogenic point
source emissions of these gas phase Cl^? precursors tend to be highly
localized due to rapid rates of photolysis during the day.

           
Aside from direct emission from anthropogenic sources, gas phase chlorine
species can be produced through heterogeneous mechanisms involving chloride
aerosols.  One such species is nitryl chloride (ClNO₂), which
was initially observed in ambient air in 2006 during a study along the Gulf
Coast of the U.S.  Additional observations have also been made in Boulder,
CO,  Alberta, Canada and off the coast of California, near the Los Angeles
Basin.  Multiple studies investigating ClNO₂ production have
determined that the responsible mechanism is the reactive uptake of N₂O₅
onto aqueous chloride aerosols.  N₂O₅, which is
formed from the gas phase reaction between NO₂ and NO₃,
photolyzes rapidly and therefore only builds up to significant concentrations
at night.  The nocturnal conversion of N₂O₅ into
ClNO₂ results in the rapid production of Cl^? when ClNO₂
photolyzes at sunrise.  Through gas phase reactions initiated by Cl^?,
this can increase the rate of oxidation for volatile organic compounds (VOCs)
and thus increase the rate of ozone production.  The heterogeneous uptake and
dissociation of N₂O₅ has been determined to be the rate
limiting step, and the full mechanism is shown below.

NO_2,g + NO_3,g              
↔       N₂O_5,g                                     (1)

N₂O_5,g                         
 ↔      
N₂O_5,aq                                    (2)

N₂O_5,aq                        
 ↔      
NO₂⁺_aq + NO₃^-_aq                     (3)

NO₂⁺_aq + H₂O_aq            →       
H₃O⁺_aq + HNO_3aq                   
(4a)

NO₂⁺_aq + Cl^-                 
→        ClNO₂ + H₂O_aq                        (4b)

NO₃^-_aq + H⁺                  
↔       HNO₃⁺_aq                                  (5)

Another heterogeneous mechanism of interest involves the
production of Cl₂ from chloride aerosol particles.  Previous
studies have suggested a direct reaction between O₃ and particulate
chloride.  Other studies have suggested a surface reaction between particulate
chloride and OH^?.  These mechanisms were formulated in response
to experimental observations of gas phase Cl₂ when aqueous NaCl and
O₃ were present together.  The reaction has been reported to
occur both in the presence of light as well as in the dark.  The mechanism
for the reaction between OH^? and particulate chloride can be
represented as follows, where 2OH???Cl^- represents an intermediate
species that forms at the particle surface.

OH_g + Cl^?-_aq,surface         
→        OH???Cl^?-_aq,surface        
        (6)

2OH???Cl^-_aq,surface           →       
Cl_2,g + 2OH^-_aq               
         (7)

Cl₂ concentrations up to 150 ppt_v have
been detected in marine air at night, suggesting a nocturnal 330 ppt_v
day^-1 Cl₂ source.  Additionally, detection of Cl₂
in both coastal marine and coastal urban air has been reported. 
Therefore, the determination of an exact mechanism and quantification of the
rate would be useful in predicting the impact Cl₂ production from
chloride aerosols on tropospheric chemistry.  This is particularly
relevant in coastal regions where chloride particles are typically present in
large quantities.  The mechanism involved in reactions 6 ? 7 is the focus
of the experimental results presented here.

Methods
and Discussion

This work presents both computational and experimental
investigation of the heterogeneous processes described above.  An overview
of a sensitivity analysis of the physical parameters involved in the mechanism
described by reactions 1 ? 5 will be discussed.  The parameters
investigated in the analysis include the reactive uptake coefficient, ClNO₂
yield, particle surface area, and gas phase concentrations of VOCs and NO_x.
 The sensitivity analysis was carried out through photochemical box
modeling using the SAPRC software and the Carbon Bond 5 mechanism.  The
focus of this sensitivity analysis was on the effects of variations in
parameter values on the production of ClNO₂ and subsequent impacts
on ozone production.  The results obtained from simulations using various
parameter value combinations were compared to a base case modeling scenario in
which all heterogeneous reactions were absent.  In order to quantify
differences in O₃ production, the peak O₃ concentration
from each simulation was used as a point of comparison.  An analysis of
parameter values reaching the upper limits reported by the literature was
undertaken. 

Results from the sensitivity analysis indicate that ClNO₂
chemistry can potentially change peak O₃ concentrations by -10.5% ?
27%.  The availability of NO_x was found to play an important
role.  The presence of additional NO_x during nocturnal hours
resulted in greater production of N₂O₅ and thus higher
concentrations of ClNO₂.  However, NO_x emissions
after sunrise reduced the impact of N₂O₅-ClNO₂
chemistry on O₃ production.  Additionally, the presence of
large concentrations of VOCs that are highly reactive with the NO₃
radical significantly reduced the impact of N₂O₅-ClNO₂
chemistry on ozone production.  Figure 1 shows the combined influence of
variations in the reactive uptake (γ) and ClNO₂ yield (Y_ClNO2)
on peak ozone concentrations as compared to the base case scenario.
 Further discussion of details on the impacts of ClNO₂
chemistry with respect to O₃ production and NO_x cycling
within the atmosphere will be included in the presentation.

Figure 1: Percentage peak O₃
increases in over the base case scenario (no heterogeneous reactions) for various
combinations of the ClNO₂ yield and reactive uptake parameter
values.

Experimental work investigating the heterogeneous mechanism
described in reactions 6 ? 7 will also be covered.  Specifically, results
from environmental chamber experiments involving the exposure of NaCl aerosol
particles to various gas mixtures representative of conditions present in the
atmosphere, including HO_x, NO_x and O₃will
be presented.  Experiments were carried out in a 10 m³ reaction
chamber equipped with UV lights which induce photochemistry.  Chemical
Ionization Mass Spectrometry (CI-MS) utilizing iodide as the reagent ion was
used to quantify and identify gas phase species produced by the mechanism.
 Aerosol population size distributions were monitored using a Scanning Electrical
Mobility System (SEMS). 

Experiments were carried out by generating and injecting NaCl
aerosol particles into the chamber along with photolytic precursors of the
hydroxyl radical.  The mixture was then exposed to UV radiation in order
to initiate photochemistry and hydroxyl radical production.  Resulting gas
phase Cl₂ concentrations were monitored using iodide CI-MS. 
Various initial conditions were used to test the impact of several factors on
the overall efficiency of the mechanism in producing gas phase chlorine. 
One factor is the impact of variations in relative humidity levels on the
efficacy of the mechanism.  Another factor is the variation of the
composition of the bulk aerosol.  The impact of the presence or absence of
elevated O₃ concentrations is also examined.  Resulting
variations in overall Cl₂ production due to these factors will be
compared and discussed.