Observation of ClNO2 in a Mid-Continental Urban Environment

ARTICLE pubs.acs.org/est

Observation of ClNO2 in a Mid-Continental Urban Environment Levi H. Mielke,† Amanda Furgeson, and Hans D. Osthoff* Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada

bS Supporting Information ABSTRACT: In the troposphere, nitryl chloride (ClNO2), produced from uptake of dinitrogen pentoxide (N2O5) on chloride containing aerosol, can be an important nocturnal reservoir of NOx (= NO + NO2) and a source of atomic Cl, particularly in polluted coastal environments. Here, we present measurements of ClNO2 mixing ratios by chemical ionization mass spectrometry (CIMS) in Calgary, Alberta, Canada over a 3-day period. The observed ClNO2 mixing ratios exhibited a strong diurnal profile, with nocturnal maxima in the range of 80 to 250 parts-per-trillion by volume (pptv) and minima below the detection limit of 5 pptv in the early afternoon. At night, ClNO2 constituted up to 2% of odd nitrogen, or NOy. The peak mixing ratios of ClNO2 observed were considerably below the modeled integrated heterogeneous losses of N2O5, indicating that ClNO2 production was a minor pathway compared to heterogeneous hydrolysis of N2O5. Back trajectory analysis using HYSPLIT showed that the study region was not influenced by fresh injections of marine aerosol, implying that aerosol chloride was derived from anthropogenic sources. Molecular chlorine (Cl2), derived from local anthropogenic sources, was observed at mixing ratios of up to 65 pptv and possibly contributed to formation of aerosol chloride and hence the formation of ClNO2.

’ INTRODUCTION The oxides of nitrogen are important trace gas constituents of the troposphere. In urban areas, NOx (= NO + NO2) is emitted mainly in the form of NO from combustion engines used in automobiles and often concentrates near the surface due to inversions, especially in wintertime. High mixing ratios of NOx are of concern because there are links between NOx exposure and adverse health effects (e.g., ref 1), and because NOx promotes formation of secondary pollutants such as ozone (O3) and the peroxycarboxylic nitric anhydrides (PANs, general formula RC(O)O2NO2). Nitrogen oxides are removed from the atmosphere mainly by conversion to nitric acid (HNO3), which deposits onto surfaces and aerosol particulate matter. During daytime, the main HNO3 production (and NOx removal) pathway is the reaction of NO2 with the hydroxyl radical (OH). Nocturnal nitrogen oxide chemistry is dominated by reactions of the photolabile nitrate radical (NO3), produced from reaction of NO2 with O3, and of dinitrogen pentoxide (N2O5), which is in a temperature dependent equilibrium with NO3 and NO2.2,3 NO2 ðgÞ þ O3 ðgÞ f NO3 ðgÞ þ O2 ðgÞ, k1 ¼ 1:2 1013 expð 2450=TÞcm3 molecule1 s1

ð1Þ

NO2 ðgÞ þ NO3 ðgÞ h N2 O5 ðgÞ, k2 ¼ 2:7 1027 expð11000=TÞcm3 molecule1

ð2Þ

Here, temperature (T) is in units of Kelvin. In urban areas, the chemistry of NO3 and N2O5 is often suppressed because NO3 is r 2011 American Chemical Society

rapidly titrated by (freshly emitted) NO, but may still be significant aloft in the residual layer4 or during times of the night when traffic and hence NO emissions are at a minimum. NO3 ðgÞ þ NOðgÞ f 2NO2 ðgÞ, k3 ¼ 1:5 1011 expð170=TÞcm3 molecule1 s1

ð3Þ

At low temperatures and high NOx levels, equilibrium (2) is shifted toward N2O5, and nocturnal HNO3 production (and NOx loss) proceeds mainly by heterogeneous reactions of N2O5.5 het

N2 O5 ðgÞ þ H2 OðaqÞ sf 2HNO3 ðaqÞ

ð4aÞ

In the presence of chloride containing aerosol, reaction 4b competes with NOx loss by reaction 4a: het

N2 O5 ðgÞ þ Cl ðaqÞ sf ClNO2 ðgÞ þ NO3 ðaqÞ

ð4bÞ

The product of reaction 4b, nitryl chloride (ClNO2), was first observed in laboratory experiments by Finlayson-Pitts et al.6 and was initially believed to be of significance only in areas with marine influence, i.e., in the presence of sea salt aerosol.7,8 Formation of ClNO2 can affect air quality in polluted coastal areas,9 because of the lower rate of nocturnal NOx loss relative to reaction 4a and because ClNO2 is sufficiently long-lived at night to photodissociate in the morning to yield NO2 and highly Received: June 8, 2011 Accepted: August 30, 2011 Revised: August 12, 2011 Published: August 30, 2011 8889

dx.doi.org/10.1021/es201955u | Environ. Sci. Technol. 2011, 45, 8889–8896

Environmental Science & Technology

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reactive Cl atoms,10,11 which accelerate photochemical ozone production.12 hv

ClNO2 ðgÞ sf ClðgÞ þ NO2 ðgÞ, jnoon ≈ð40 minÞ1

ð5Þ

In 2006, ClNO2 mixing ratios in ambient air were quantified for the first time along the Texas coast using chemical ionization mass spectrometry (CIMS) with I reagent ion.12 Mixing ratios of ClNO2 were found to exceed 1 part-per-billion by volume (ppbv) in plumes, which suggested that ClNO2 is long-lived at night and that the yield of ClNO2 from reaction 4b was considerably larger than had been predicted. These high yields have since been rationalized by efficient ClNO2 production on aerosol with only partial chloride content8,13,14 and by surfacecatalyzed reaction of N2O5 with HCl.15 The efficiency of reaction 4b observed in coastal areas also suggested that the production of ClNO2 should extend inland and occur wherever nitrogen oxides and sources of aerosol chloride coexist, such as urban areas.12 Recently, Thornton et al.16 observed ClNO2 mixing ratios in excess of 400 parts-per-trillion by volume (pptv) in Boulder, CO. The observed production of ClNO2 1400 km from the nearest coastline implied that a considerable amount of the aerosol chloride had been derived from nonmarine sources.16 Many such sources are known, e.g., biomass burning,17 emissions from cooling towers,18 coal combustion, and waste incineration,19 and the use of road salt in winter.20 In this work, we report measurements of ClNO2 mixing ratios in Calgary, Alberta, Canada, in April 2010. At this location and time of year, production of ClNO2 was favored by low nocturnal temperatures, which shifted equilibrium (2) toward N2O5 and probably increased the N2O5 uptake probability,21 absence of biogenic VOCs that could have consumed NO3, reduced NO3 photolysis rates due to long nights and high daytime solar zenith angles, large NOx emission rates by cars and furnaces, and because the city had applied salt to its roads over the course of the winter. Mixing ratios of ClNO2 were quantified in parallel with those of peroxyacetic and peroxypropanoic nitric anhydride (PAN and PPN) and molecular chlorine (Cl2) by I-CIMS. The observations of ClNO2 reported here are only the second reported at a nonmarine site and the first in Canada. Potential sources of aerosol chloride in the region are discussed.

’ EXPERIMENTAL SECTION Overview of Measurement Locations and Instruments. Calgary is a city of more than 1 million people and is located approximately 70 km east of the Rocky Mountains, 680 km east of Vancouver, and 800 km east of the open ocean (Figure 1) at an elevation of 1100 m above sea level. Measurements were conducted at the University of Calgary (UC), which is located in the northwest quadrant of the city and is surrounded by suburban communities and residential roadways. Auxiliary data were collected at a routine monitoring station (operated by Alberta Environment) located 900 m west of the main measurement site. We sampled from the afternoon of April 16 to the morning of April 17 and during a 2-day consecutive period from the afternoon of April 19 to the early afternoon of April 21, 2010. Two instruments were deployed at the University, the I-CIMS (described below) and a NO/O3 chemiluminescence (CL) “NOx” monitor (Thermo 42TL). The latter was equipped with an internal heated Mo converter to reduce nitrogen oxides to NO and was found in laboratory experiments to not only convert NO2 but also

Figure 1. Map of the sampling region showing the location of the City of Calgary with respect to the ocean. The insert shows the main sample location on the University of Calgary campus. The AB Environment routine monitoring site is located 900 m to the west. The images are from Google Earth.

components of NOz (= NOy NOx = PANs + HONO + HNO3 + NO3 + 2N2O5 + ClNO2 + ...) to NO with high efficiency. The two instruments were housed in the “Penthouse” laboratory located on the roof of the Science B building (lat. 51.07933°N, long. 114.12950°W) and sampled from a common inlet, constructed from Kynar with dimensions of 1/2” outer diameter (o.d.), 3/8” inner diameter (i.d.), and ∼10 m length, which was passed through an open window. ClNO2 has relatively small uptake coefficients22,23 and thus transmits through inlets efficiently. We also did not observe production within the inlet, as we occasionally sampled plumes of NO (in which N2O5 would have been titrated) without observing any attenuation of the ClNO2 signal. The passing efficiency of Cl2 was not assessed during this work and was assumed to be unity; since Cl2 can be scrubbed in inlets,24 the Cl2 data presented in this manuscript are likely lower limits. The tip of the inlet was secured, facing downward, at a height of ∼3 m above the flat rooftop. The total sample flow rate through the common inlet was 4.0 standard liters per minute (slpm), of which 1.0 slpm were sampled by the CL instrument. Both instruments were equipped with polycarbonate filter holders and inline Teflon membrane filters with 2 μm pore size (Pall). Hourly data of meteorology parameters, i.e., wind speed and direction, temperature, relative humidity (RH), and of O3, CO, NO, and NOy mixing ratios collected at the Northwest station during the time of this study were downloaded from the “Clean Air Strategic Alliance” (CASA) Web site.25 Comparison of NO and NOy mixing ratios measured at both locations (Figure S-1 of the Supporting Information, SI) showed that both stations usually sampled the same air mass. Chemical Ionization Mass Spectrometry. The I-CIMS (Figure 2) was purchased from THS instruments and is a compact (width depth height: 21” 40” 52”; weight: 200 lbs) version of, and operated similarly to, the instrument described by Slusher et al.26 and has been partially described elsewhere.27,28 Following sampling of ambient air drawn through the Kynar inlet tubing, the sample flow was split using a PFA Teflon Tee and sampled in part by the CL analyzer and in part by the I-CIMS. A small flow of approximately 38 standard cubic centimeters per minute (sccm) containing photochemically generated 13C-labeled PAN in ultrapure, or zero, air was continuously added to the CIMS sample flow to track PAN matrix 8890

dx.doi.org/10.1021/es201955u |Environ. Sci. Technol. 2011, 45, 8889–8896


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Figure 2. Schematic of the University of Calgary I-CIMS and sampling setup. For a detailed description, see text.

effects.26,27,29 The stability of the photochemical source was monitored by periodic flooding of the inlet with N2. The output of the source stabilized over the course of several hours and remained stable (variability 5%) for the duration of the experiments. The sample air was then passed through a section of 1/2” o.d. Teflon tubing heated to 190 °C. This temperature is sufficient to quantitatively dissociate PANs to create peroxyacyl radicals (RC(O)O2) and NO2,30 but is too low to dissociate ClNO2.28 A fraction of the sample air (1.32 slpm) entered the ion molecule reaction region (IMR) through a pinhole, with the remainder being pumped away by a diaphragm pump whose flow was set to 1.68 slpm using a mass flow controller (MFC). In the IMR, the sample air was diluted with N2 passed through a bubbler containing deionized water, which was set to a flow rate of ∼480 sccm. I reagent ions were formed from dissociative electron transfer of CH3I in a 210Po ion source (NRD P-20311000). CH3I (0.5% in N2, ScottMarrin) was delivered at a flow rate of 0.68 sccm and diluted with 1.2 slpm of N2. The pressure in the reaction chamber during ambient measurements presented in this manuscript was 21 Torr. The main reactions in the IMR leading to quantifiable ions were as follows:31,26,32 H2 O

ClNO2 þ I sfðClNO2 ÞI ðm=z 208 and m=z 210Þ

ð6Þ

H2 O

RCðOÞO2 þ I sf IO þ RCO2 ðm=z 59, m=z 61, m=z 73, etc:Þ

ð7Þ

H2 O

Cl2 þ I sfðCl2 ÞI ðm=z 197, m=z 199, and m=z 201Þ

ð8Þ The anions generated in the IMR were transferred (via a pinhole) to the collisional dissociation chamber (CDC), which was operated at 0.5 Torr. The function of the CDC was to break up water cluster ions.26 The anions were then passed through a second octopole ion guide region, operated at 103 Torr, to improve the signal-to-noise ratio, mass-selected by a quadrupole mass filter (range m/z 30 - m/z 300) operated in selected ion mode, and detected using a channeltron detector. The ions

monitored and dwell times are given in Table S-1 of the SI. Each ion was monitored once in a 25 s period. Mass counts were normalized to 1 106 counts of I. Calibration. Response factors of the I-CIMS to PAN, PPN, ClNO2, and Cl2 were determined off-line in a similar fashion as described earlier.27,28 The PAN and PPN response factors were assumed to be equal during this campaign.26,27,29 PPN was eluted from a passive diffusion source and sampled simultaneously by CIMS and thermal dissociation cavity ring-down spectroscopy (TD-CRDS)27 to determine a response factor of 18 counts/pptv. ClNO2 was generated by passing a dilute flow of Cl2 in N2 past an aqueous bed containing sodium nitrite and quantified simultaneously by CIMS and TD-CRDS as described in ref 28. The Cl2 response factor was determined by successive dilutions of the output from a standard cylinder (9.85 ( 5% ppmv Cl2 in N2, Praxair, certified). The ClNO2 response factor at m/z 208, determined using humidified calibration gas (for reasons that are discussed below), was 0.31 counts/pptv, whereas the Cl2 response factor was 0.33 counts/pptv. The accuracy of the PAN, ClNO2 and Cl2 mixing ratios reported in this manuscript was approximately (15%, ( 30%, and (30%, respectively. For 25 s data, the limits of detection (LOD) were 3, 5, and 22 pptv for PAN, ClNO2, and Cl2, respectively; the difference between ClNO2 and Cl2 was caused because we selected a higher dwell time at m/z 208 compared to m/z 197 (Table S-1 of the SI). The Cl2 data presented in this manuscript were averaged to 10 min to improve the LOD to approximately 6 pptv. Humidity Dependence. Reactions 68 are promoted by water vapor within the IMR.26,29,33 The water dependence of the CIMS response was determined in off-line experiments in which the CIMS bubbler (Figure 2) was bypassed, and the sample flow was humidified using two 30 slpm MFCs delivering zero air. The effluent of the first MFC was connected in series to a second bubbler and combined with the effluent of the second MFC which delivered dry zero air. Relative humidity and air temperature of the combined effluent was monitored with a NIST-traceable digital hygrometer (VWR 35519041) and converted to partial pressure of water vapor using the Goff and Gratch equation.34 To the humidified sampled flow were added the output of the 13C-labeled PAN photochemical source27 and either the output of the ClNO2 source 8891



Figure 3. Relative signal intensities of 13CH313CO2 (m/z 61, shown as green triangles), generated from 13C-labeled PAN, (ClNO2)I (m/z 208, shown as blue circles), and (Cl2)I (m/z 197, shown as downwardfacing triangles) as a function of humidity in parts-per-thousand by volume (ppthv) in the ionmolecule reaction (IMR) region (after dilution with reagent ion and “dry” bubbler flows). The trend lines are hyperbolic expressions 29 fitted to the data. Superimposed are data from Zheng et al. 29 at m/z 61 (shown as diamonds, )) and from Kercher et al. 33 at m/z 208 (shown as red squares).

or the Cl2 gas cylinder. Mixing ratios of ClNO2 were monitored in parallel by TD-CRDS to verify the source stability (data not shown). Figure 3 shows the relative response of the CIMS to 13C-labeled PAN (monitored at m/z 61), ClNO2 (monitored at m/z 208), and Cl2 (monitored at m/z 197) plotted as a function of water vapor mixing ratio in the IMR. The trend lines are hyperbolic expressions29 fitted to the data and are shown here merely as visual guidelines. The response of the CIMS is humidity-dependent at low humidity and is constant above a threshold; this threshold is higher for ClNO2 and Cl2 than for 13C-labeled PAN, consistent with previous reports for PAN and ClNO2 in similar instruments.29,33 With the flow settings as described in the previous section and assuming that the CIMS bubbler achieves saturation with respect to water vapor, we calculate a water vapor mixing ratio of 4 parts-per-thousand by volume (ppthv) in the IMR if dry air is sampled. This amount of water suffices to ensure measurements are RH-independent for the PANs, but not for ClNO2 and Cl2. We found that ClNO2 and Cl2 calibrations conducted with dry zero air (and with the CIMS bubbler operated normally) yielded a response factor 27% lower than those conducted with humidified calibration gases. This implies that the actual humidity achieved with the CIMS bubbler is lower than the calculated value. The ambient air sampled in this campaign contained a sufficiently high amount of moisture to ensure that the CIMS response was independent of RH.

’ RESULTS AND DISCUSSION General Observations. During the three days of measurements, the meteorological conditions (Figure S-2 of the SI) were typical of early spring, i.e., sunny with daytime temperature highs just below +20 °C and the nights frost-free with nocturnal lows of +5 °C. There were no leaves on the deciduous trees, which suggests that biogenic emissions of volatile organic compounds (VOCs) were relatively small and limited to those from grasses and conifers. The dominant wind directions were from the W to NW and from the SE, where downtown and most of city’s people and roadways including a large industrial park and railroad yards are located. An overview of NO, NOy, O3, and CO mixing ratios collected at the routine monitoring site and of PAN

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Figure 4. Time series of ClNO2 and Cl2 mixing ratios observed on the University of Calgary campus.

and PPN mixing ratios collected at UC is provided in the SI section (Figure S-3). ClNO2. Figure 4 presents a time series of ClNO2 mixing ratios derived from m/z 208. ClNO2 exhibited a clear diurnal profile, similar to previous measurements of this compound at other locations.12,16,33 ClNO2 was observed every night during which measurements were made with maxima between 80 and 250 pptv, corresponding to a maximum of 2% of NOy (Figure S-4 of the SI). The observed range of mixing ratios is lower than those observed in coastal areas12,33 and similar to what was observed in Boulder, Colorado.16 Measurable quantities of ClNO2 were observed until noon, consistent with the expected slow photolysis of ClNO2. Mixing ratios of ClNO2 were at a minimum and below the instrumental detection limit of 5 pptv in the early afternoon. Figure 5 shows a pair of partial mass scans during the campaign (the full spectra are shown as Figure S-5 of the SI). The first spectrum was taken on April 16 at 2 p.m., the other on April 17 at 3 a.m. local time. The nocturnal spectrum contains peaks at all masses expected for ClNO2 with the expected 35Cl:37Cl isotope ratio, i.e., m/z 208 and m/z 210 (ClNO2)I, m/z 162 and m/z 164 (Cl)I, and m/z 35 and m/z 37 (Cl). The insert of Figure 5 shows a scatter plot of m/z 210 to m/z 208 for the entire campaign with the expected chloride isotope ratio. Cl2. We observed statistically relevant counts at m/z 197, indicating the presence of Cl2 (Figure 4). Only m/z 197 (35Cl2)I was continuously monitored. The other Cl isotopomers were observed in mass spectra recorded when m/z 197 counts were elevated (see Figure S-6 of the SI for two examples). Concentrations of Cl2 were highest at night, with maxima in the range of 30 to 65 pptv and were sometimes correlated with those of ClNO2, and sometimes anticorrelated (Figure 4). It is not likely that Cl2 was produced in the atmosphere, e.g., by heterogeneous conversion of ClNO2,32 as the aerosol would have to contain high levels of chloride and be very acidic. Further, elevated Cl2 levels were observed during daytime in spite of Cl2 loss due to photolysis. Hence, the observed Cl2 most likely originated from one or more local anthropogenic sources, perhaps from evaporation of chlorinated municipal tap water or fugitive emissions from industrial sources18 located in SE Calgary. Other Chlorine Containing Species. The daytime spectrum, shown in Figures 5 and S-5 of the SI, contains elevated counts at m/z 35 and m/z 37 but at no other Cl containing anions. The absence of counts at m/z 208, 210, 197, 199, and 201 suggests that fragmentation of iodide ion cluster ions was not contributing to the enhanced ion counts at m/z 35 and 37 and suggests the 8892



Figure 5. Mass spectra of ambient air sampled on April 16, at 13:47 MDT (orange trace) and on April 17, 2010, at 02:47 MDT (blue trace). The mass spectrum acquired at night contains ion counts at m/z 208 and m/z 210 ((35ClNO2)I) and (37ClNO2)I, m/z 162 and m/z 164 ((35Cl)I and (37Cl)I), and m/z 35 and m/z 37 (35Cl and 37Cl). The mass spectrum acquired during the day only contains m/z 35 and m/z 37, suggesting that species other than ClNO2 contribute to counts at these masses. The insert shows ion counts observed at m/z 210 plotted against counts m/z 208 for 25 s and 5 min averaged data, respectively. The observed ratio of 1:(3.17 ( 0.03) is very close to the range in natural variation of the Cl isotopes (1:(3.13 ( 0.01)).42

presence of a Cl containing trace gas other than ClNO2 or Cl2 during daytime in Calgary air. For HCl, we would expect to see adduct ions at m/z 163 (H35Cl)I and m/z 165 (H37Cl)I. However, m/z 165 was not present in any of the mass spectra. We speculate that the unidentified daytime species is hypochlorous acid (HOCl).24 Directional (In)dependence of ClNO2 and Cl2 Production. The night of April 1617 (Figure S-7 of the SI) was an interesting case as we experienced three distinct wind flow regimes. At the beginning of the night (period I), the winds were from the SE. We observed moderate production of ClNO2 and some Cl2 in this air mass. The PPN to PAN ratio, which can be used to gain insight into the chemical history of the air mass sampled,35 was approximately 0.113 ( 0.003 (where the error indicates the precision of the measurement). Halfway through the night, the wind-direction switched to NW (period II). At that time, a sharp increase in ClNO2 mixing ratios and a decrease in Cl2 abundance were observed. The PPN to PAN ratio increased to a maximum of 0.128 ( 0.003), slightly higher than earlier. Further, NOy mixing ratios reached 40 ppbv, the largest of that night. The elevated levels of ClNO2, high NOy abundance, and greater PPN/PAN ratio are indicative of a more polluted and processed air mass of anthropogenic origin. One plausible explanation is that the same air mass was observed twice, once leaving the city and once again coming back. An alternative explanation is subsidance of an “aged” air mass from aloft, coinciding with the change of wind direction. The greater ClNO2 abundance in an “older” and more NOx-polluted air mass is consistent with its production from heterogeneous uptake of N2O5 and implies that this particular air mass was not depleted of aerosol chloride to sustain production of ClNO2. After the initial peak in period II between 1:002:00 a.m., ClNO2, PPN/PAN, and NOy decrease with mixing ratios of NOy reaching 9 ppbv by 3:00 a.m. At the end of

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the night (period III in Figure S-7 of the SI), the winds were still from the NW and the PPN/PAN ratio dropped to about 0.088 ( 0.003. A ratio of 85° for the four cases. The results are presented in Figure 6. Depending on input parameters, the total amount of N2O5 lost via reaction (4) was in the range of 0.1 ppbv to 1.2 ppbv, which is small compared to the amount of NOx present and suggests that most of the chemical processing of NOx in this air mass occurs 8893



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Table 1. Overview of Model Input Parameters case 1

case 2

(high N2O5 reactivity)

(low N2O5 reactivity)

γ(N2O5)

0.03

0.005

0.02

0.02

ϕ(ClNO2)

1

1

1

15%

500

200

250

250

parameter

SA (μm2 cm3)

case 3

case 4

(intermediate (fit to case) data)

Figure 6. Comparison of estimated integrated N2O5 losses with observed ClNO2 mixing ratios for the four scenarios outlined in Table 1. The amount of ClNO2 is less than the total integrated loss of N2O5.

downwind from the city, aloft, and/or during daytime. We obtain reasonable agreement of observed and modeled ClNO2 if a ϕ(ClNO2) value of 0.15 is used in the calculation (case 4). Assuming that the aerosol was internally mixed and deliquesced, a ϕ(ClNO2) value of 0.15 would imply an aerosol chloride concentration in the range of 10 to 50 mM.13 The yield of 15% calculated above is likely too high if there is significant production of ClNO2 aloft, where NO mixing ratios are typically lower,4 and if the long-lived ClNO2 mixes down to the surface. We repeated the model runs assuming NO mixing ratio of zero ppbv, i.e., with kNO3 = 0. Figure S-8 of the SI shows the results. In this scenario, the amount of N2O5 consumed is in the range of 3.3 to 4.7 ppbv, and ϕ(ClNO2) values of 3% (2nd night) and 1.5% (3rd night) reproduce the observed ClNO2 mixing ratios. This yield would imply an aerosol chloride concentration in the range of 1 to 10 mM.13 However, given that the aerosol chloride is most likely derived from a surface source, the first scenario is probably more realistic. In either case, the amount of N2O5 consumed by heterogeneous reactions is considerably larger than the amount of ClNO2 observed. This suggests that the observed ClNO2 can be rationalized by production via reaction 4b alone, and additional ClNO2 source chemistry is not needed to explain the observations.

Sources of Aerosol Chloride. The observed production of ClNO2 in Calgary requires the presence of a substantial amount of aerosol chloride dispersed over a large area. High levels of reactive chlorine, in particular HCl and aerosol chloride, are not uncommon in urban areas.36 Further, chloride is ubiquitous in the environment, as evident from observed chloride levels in rainwater samples across the continent, which have been interpreted as being mainly sea-salt derived.37 However, we do not think sea salt played a major role in the observed formation of ClNO2 because its lifetime is short,7 in particular since sea salt concentration drops rapidly with distance from the coast38 and the western slopes of the coastal and Rocky Mountain ranges favor deposition. We used the online HYSPLIT model39,40 to calculate 2-day back-trajectories (see Figure S-10 of the SI), which indicated that the air originated from the south to east, i.e., the continental U.S. Thus, transport of marine aerosol to the study area was unlikely. Anthropogenic activities within the city of Calgary were the most likely the main source of aerosol chloride and of ClNO2 during the time of these measurements. Coal-fired power plants and cooling towers are insignificant sources of Cl species in the Calgary area, since there are no coal-fired power plants in the vicinity of the city and there are few cooling towers. Some of the aerosol chloride was in all likelihood derived from photolysis of the anthropogenically released Cl2 and subsequent reaction of Cl with hydrocarbons to form HCl, which partitions to the aerosol phase. Another, likely major, source of aerosol chloride in the area was suspension of road dust, which in springtime contains residual road salt that accumulated over the course of the winter. During the time of this work, the city conducted its annual streetcleaning campaign, which generated a considerable amount of suspended dust particles. Ultimately, though, the injection of new anthropogenic Cl does not have to be large to sustain a considerable level of aerosol chloride and hence ClNO2 production because of cycling of Cl between various gas-phase species and the aerosol phase in the atmosphere.41 Implications. The high mixing ratios of ClNO2 and Cl2 observed in this work show that there can be active halogen chemistry attributable to anthropogenic sources in a midcontinental urban environment. The primary fate of ClNO2 and Cl2 is photo dissociation at sunrise to yield NO2 and Cl atoms, which oxidize hydrocarbons and initiate or accelerate photochemical O3 production.18 However, the amounts of NO2 generated in this data set are relatively small compared to the amounts of NOx emitted locally by cars and thus are unlikely to have significant impact on air quality within the city. It would be interesting to investigate how far the observed halogen chemistry extends downwind of the Calgary area, where the recycling of NOx could potentially have greater impact, or what impacts there are during stagnation periods when air quality rapidly deteriorates. The Cl atom source from ClNO2 photolysis observed here could make a substantial difference as it will generate organic peroxy radicals from otherwise fairly unreactive alkanes and thus should be included in atmospheric chemistry and air quality models of landlocked urban areas.

’ ASSOCIATED CONTENT

bS

Supporting Information. 11 Figures and 1 Table. This material is available free of charge via the Internet at http:// pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Phone: 403-220-8689; Fax: 403-289-9488; E-mail: hosthoff@ ucalgary.ca. Present Address †

School of Public and Environmental Affairs, Indiana University, Bloomington, IN 47405.

’ ACKNOWLEDGMENT This work was financially supported by an Alberta Ingenuity New Faculty Award, the National Science and Engineering Research Council of Canada (NSERC) in the form of Discovery and Research Tools and Instruments (RTI) grants, and the University of Calgary via startup funds and laboratory space. Amanda Furgeson acknowledges financial support by an NSERC Alexander Graham Bell Scholarship. ’ REFERENCES (1) Gauderman, W. J.; Avol, E.; Lurmann, F.; Kuenzli, N.; Gilliland, F.; Peters, J.; McConnell, R. Childhood asthma and exposure to traffic and nitrogen dioxide. Epidemiology 2005, 16 (6), 737–743. (2) Finlayson-Pitts, B. J.; Pitts, J. N. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, And Applications; Academic Press: San Diego, Calif., 2000. (3) Sander, S. P.; Friedl, R. R.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Keller-Rudek, H.; Finlayson-Pitts, B. J.; Wine, P. H.; Huie, R. E.; Orkin, V. L. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation Number 15. NASA/JPL: Pasadena, CA, 2006; Vol. 062. (4) Stutz, J.; Alicke, B.; Ackermann, R.; Geyer, A.; White, A.; Williams, E., Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 1. Observations during the Texas Air Quality Study 2000. J. Geophys. Res.-Atmos. 2004, 109, D12306; DOI 10.1029/ 2003JD004209. (5) Chang, W. L.; Bhave, P. V.; Brown, S. S.; Riemer, N.; Stutz, J.; Dabdub, D. Heterogeneous atmospheric chemistry, ambient measurements, and model calculations of N2O5: A review. Aerosol Sci. Technol. 2011, 45 (6), 655685; DOI 10.1080/02786826.2010.551672. (6) Finlayson-Pitts, B. J.; Ezell, M. J.; Pitts, J. N. Formation of chemically active chlorine compounds by reactions of atmospheric nacl particles with gaseous N2O5 and ClONO2. Nature 1989, 337 (6204), 241–244. (7) Erickson, D. J.; Seuzaret, C.; Keene, W. C.; Gong, S. L. A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive chlorine emissions inventory. J. Geophys. Res.-Atmos. 1999, 104 (D7), 8347–8372. (8) Behnke, W.; George, C.; Scheer, V.; Zetzsch, C. Production and decay of ClNO2, from the reaction of gaseous N2O5 with NaCl solution: Bulk and aerosol experiments. J. Geophys. Res.-Atmos. 1997, 102 (D3), 3795–3804. (9) Simon, H.; Kimura, Y.; McGaughey, G.; Allen, D. T.; Brown, S. S.; Osthoff, H. D.; Roberts, J. M.; Byun, D.; Lee, D., Modeling the impact of ClNO2 on ozone formation in the Houston area. J. Geophys. Res.-Atmos. 2009, 114, D00F03; DOI 10.1029/2008jd010732. (10) Ganske, J. A.; Berko, H. N.; Finlayson-Pitts, B. J. Absorption cross-sections for gaseous ClNO2 and Cl2 at 298 K—Potential organic oxidant source in the marine troposphere. J. Geophys. Res.-Atmos. 1992, 97 (D7), 7651–7656. (11) Behnke, W.; Kruger, H. U.; Scheer, V.; Zetzsch, C. Formation of atomic Cl from sea spray via photolysis of nitryl chloride— Determination of the sticking coefficient of N2O5 on NaCl aerosol. J. Aerosol Sci. 1991, 22, S609–S612.

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