Seasonality of the Water-Soluble Inorganic Ion Composition and

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Characterization of Natural and Affected Environments

Seasonality of the Water-Soluble Inorganic Ion Composition and Water Uptake Behavior of Urban Grime Alyson Baergen, and D. James Donaldson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00532 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Seasonality of the Water-Soluble Inorganic Ion Composition and

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Water Uptake Behavior of Urban Grime

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Alyson M. Baergen1 and D. James Donaldson1,2 *

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 1Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada

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 2Department of Physical and Environmental Sciences, University of Toronto at

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Scarborough, Toronto, Ontario M1C 1A4, Canada * Corresponding author. Email: [email protected]

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Abstract

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Impervious surfaces, especially in urban environments, are coated with a film

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composed of a complex mixture of substances, referred to as urban grime. Despite its

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ubiquity, the factors that dictate urban grime composition are still not well

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understood. Here, we present the first study of the seasonal variation in composition

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of water-soluble inorganic ions present in urban grime, performed by analysing

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samples collected in Toronto for 4-week intervals over the course of a year. A clear

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seasonality in the composition is evident, with NaCl dominating in the winter months

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and Ca2+ and NO3 dominant in the summer. We compare the grime composition to

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the water-soluble ion composition of PM2.5 and PM10 in order to infer chemistry

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occurring within the grime, and find evidence that chemistry occurring within the

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urban grime matrix could provide a source of ClNO2 and NH3 to the urban

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atmosphere. The uptake of water by urban grime also shows a clear seasonality,

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which may be driven by the changing proportions of nitrate salts and/or oxidized

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organic compounds over the year.

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Introduction

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Impervious surfaces are wide-spread in urban centers with models estimating

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surface area up to ten times the geographical surface area of a location.1,2 Even in

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areas with high particle loading, it is predicted that this can make up two to three

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orders of magnitude more surface area than particles in the lower troposphere.3

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Since heterogeneous reactions are an important consideration in atmospheric

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chemistry, eg.4,5, the role that these impervious surfaces, and the surface films that

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form on them, may play should be considered.

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Sometimes referred to as urban grime, surface films are made up of a complex

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mixture of organic and inorganic species with the ratios of these dependent on many

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factors including film location and age.6-8 There is growing evidence of a variety of

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roles that these films play in the environment. For example, there is evidence of

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reactions including nitrate photochemistry forming nitrogen oxides9,10 and of

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organic species, such as PAHs, undergoing photochemical and oxidative processing

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within the film.11 Beyond heterogeneous reactions, it has also been suggested that

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these films could play a role in mediating atmospheric concentrations of certain

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species by contributing another compartment into which species can partition in

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and out off, as well as facilitating pollutant transfer during precipitation events due

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to film wash off.12-14 From another perspective, the formation of films is also of

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interest due to the negative aesthetics of the soiling of surfaces and the possibility

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that films contribute to the degradation of underlying building components.6,15,16

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In the current paper we seek to expand the understanding of urban grime by looking

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at the seasonality of grime composition and water uptake behavior. The variability

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in composition of grime in a particular location over time is largely unstudied, with

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the exception of a report by Favez et al,6 who looked at compositional changes

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occurring in a film grown over two years in various European cities. Although that

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work gives a useful starting point, during such a long growth period compositional

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changes due to seasonality and film aging become conflated. In the following, we

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will expand on Favez et al6 by presenting monthly composition measurements


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which we compare to reported particle compositions in order to better understand

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the processes that occur in the grime that cause particle and grime compositions to

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differ.17 In the present work, we concentrate only on the inorganic ion fraction of the

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grime, as we did in reference 17. Although this fraction is not the largest by mass

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(e.g., see references 6, 7), it is expected to play a strong role in renoxification9,10,17

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and possible chloride activation17 to the local atmosphere.

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Examining the water uptake characteristics of the film is important as the presence

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of water impacts heterogeneous chemistry;18 in particular it has been found that the

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photochemical recycling of nitrate to gas phase products from urban grime shows a

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strong relative humidity (RH) dependence.10 To date, there have been limited

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studies looking at grime-water interactions. Lam et al7 examined changes to the

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grime’s NMR spectrum when the film was exposed to water versus dimethyl

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sulfoxide, finding that the film showed dynamic behavior with the more polar

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groups being brought to the surface upon exposure to water. In a different study it

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was found that a grime-coated glass surface took up twice as much water as a clean

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glass surface at 100% RH.19 It is of interest to expand on these studies and begin to

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examine the variability in and the driving factors of water uptake onto the grime.

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Experimental

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Sample collection

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Urban grime samples were collected onto either 5 cm x 7.6 cm pieces of window

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glass or 0.550 inch diameter quartz crystal microbalance (QCM) crystals by placing

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them outside on a platform 0.5m above ground level in downtown Toronto, Canada

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for four week sampling periods between October 2014 to September 2015. The

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samples were clipped onto the underside of a solid piece of stainless steel sheet that

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was maintained at a 30° angle from the ground to shield the samples from light and

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precipitation. The sides of the holder were open enabling airflow through the

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system to facilitate grime growth. Grime was collected on both sides of the glass


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substrates, while the underside of QCM crystal was covered to prevent grime

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collection from impacting the QCM response. For each four-week sampling period

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three pieces of glass and two QCM crystals were exposed to the atmosphere.

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Sampling dates can be found in Table S1 in the Supplementary Information (SI).

93 94

Ion analysis

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Ion analysis was carried out as we have described previously.10 Briefly, each piece of

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glass was submerged in 25 mL of deionized water, sonicated for 1 minute and then

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rinsed twice on each side with 5 mL of water per rinse, for a total extraction volume

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of 45 mL. Solutions were filtered with 0.2 μm IC Millex®-LG syringe filters and then

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analyzed using a Dionex ICS-2000. Samples of volume 1.33 mL were injected onto a

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concentrator/analytical column system: Ionpac® TAC-ULP1/AS19 with KOH eluent

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for anion detection and Ionpac® TCC-ULP1/CS17 with methanesulfonic acid eluent

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for cation detection. Solutions were analyzed for chloride, nitrate, sulfate, oxalate,

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sodium, ammonium, potassium, magnesium and calcium. A second extraction

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resulted in ion concentrations of less than 10% of levels from the first extraction.

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For each sampling period, three separate grime samples were collected and the

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average composition was calculated. The ion concentrations and associated

107

uncertainties are listed in Table S3.

108 109

Water Uptake

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Water uptake experiments were performed using a quartz crystal microbalance

111

(QCM) as we have described previously.10,20 Grime-coated crystals were mounted in

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a plexiglass chamber and their mass changes were monitored as the relative

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humidity was varied, first up from a “dry” atmosphere, then down. The relative

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humidity was controlled by flowing different ratios of dry and humidified air

115

through the chamber to maintain a rate of change of RH of approximately 1% per

116

minute up to an RH of 80% and then -1% per minute back to dry conditions. The air

117

was humidified by flowing the air through a water bubbler at room temperature.

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The frequency change of the QCM crystal due to the change in water content of the

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film was converted to a mass using the Sauerbrey equation (Δm=CΔf) where Δm is


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the mass change, C is a proportionality constant and Δf is the frequency change from

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the deposited mass. The value of the constant C is reported to be 8.147x107 Hz cm2

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g-1 for the 0.550 inch, 6 MHz crystals used in this study.21 The reported grime water

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uptake curves were blank corrected by subtracting the average water uptake onto a

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clean QCM crystal from the water uptake onto the same crystal coated in grime.

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Relative humidity was measured using a traceable Traceable® Memory

126

Hygrometer/Thermometer. Water uptake results were only reported if both

127

samples from a sampling period were free of visible contamination, therefore

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October and February water uptake results are not reported.

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Particle Composition

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Composition data for water-soluble ions extracted from PM2.5 and PM2.5-10 were

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obtained from the National Air Pollution Surveillance Program (NAPS).22 The south

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Etobicoke sampling station (NAPS ID=60435), located about 15 km to the SW of the

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grime sampling location was used due to the availability of data for both particle

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types at this location. Particulate mass concentrations are reported for chloride,

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nitrate, sulfate, oxalate, sodium, ammonium, potassium, magnesium and calcium.

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The cumulative mass of each of these ions reported for each sample period was

137

calculated and used to calculate mole fractions and ion mole ratios for PM2.5, PM2.5-10

138

and PM10. The reported PM10 values are the sum of the masses from the other

139

particle categories.

140

Results

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Figure 1a shows the mole fractions of water extractable ions in grime for the nine

142

four-week long grime collection periods between October 2014 and August 2015.

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The major ions detected are NO3 and Ca2+ in the summer and Na+ and Cl in the

144

winter. In agreement with all previous grime composition reports, eg.,6,17 there was

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minimal detectable NH4+ in the collected grime throughout the year. Figures 1b and

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1c display the corresponding average particle compositions for PM2.5 and PM2.5-10

147

respectively, averaged over each of the grime collection periods. We note that


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particle samples were taken in a different location than where the grime was

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collected, so compositional differences may reflect this. Nevertheless, we anticipate

150

that they are each broadly reflective of the Toronto urban environment and will

151

compare the compositions based on this assumption. Figure 1 shows that the

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composition of PM2.5 is markedly different from those of both the grime and the

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PM2.5-10 throughout the year, despite PM2.5 making up greater than 50% of particle

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mass for most of the year (Figure S1d). The grime and PM2.5-10 compositions are

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broadly comparable, possibly reflecting the greater contribution of heavier particles

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to the grime. Most dramatically, the fine particles (PM2.5) consist predominantly of

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ammonium sulfate, with nitrate becoming significant only during the winter

158

months. By contrast, in both the coarse particle fraction (PM2.5-10) and in the grime,

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ammonium is almost undetectable throughout the year and nitrate makes up a

160

higher proportion of the ions than sulfate year-round. Sodium and chloride ions

161

constitute major fractions of both grime and coarse particles during the winter, but

162

are seen in only small amounts in the fine particles. Oxalate also shows a higher

163

mole fraction during the summer months in both particles and grime.

164

In our earlier work17 we showed that inorganic nitrate in the grime film has a

165

different chemistry than that contained in PM2.5. We may tease out such grime-

166

related chemistry by comparing the ratios of nitrate to other species present in the

167

grime and in particles. The nitrate to sulfate ratio over the year displayed in Figure

168

2a and reported in Table S2 is consistent with the report of Favez et al,6 where a

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maximum was seen in the summer during their two year study period. However,

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the NO3- to SO42- mole ratio found here is greater than 2 for most of the year, and so

171

is higher than the reported mole ratio of 1.3 found in a previous report of Toronto

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grime composition.7 This difference may reflect the dropping SO2 concentration in

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Toronto over the last decade since these previous measurements were taken.22


6

14

0.8

r2 D ec 01 em 4 be r 20 Fe 14 br ua ry 20 15 M ar ch 20 15 M ay 20 15 Ju ne 20 15 Ju ly 20 A 15 ug us t2 01 5

20

1.0

m be

er ov e

er

m be

O ct ob 20 14

r2 D ec 01 em 4 be r 20 Fe 14 br ua ry 20 15 M ar ch 20 15 M ay 20 15 Ju ne 20 15 Ju ly 20 A 15 ug us t2 01 5

N

175

ov e

176

c)

PM10-2.5 Mole Fraction

PM2.5 Mole Fraction em

ct o be r

be r 20 14

D 20 ec 14 em be r2 Fe 01 br 4 ua ry 20 15 M ar ch 20 15 M ay 20 15 Ju ne 20 15 Ju ly 20 A 15 ug us t2 01 5

N ov

O

174

N

O ct ob

Grime Mole Fraction


a) 1.0 Cl

0.8 NO3

SO4

0.6 C2O4

0.4 NH4

0.2

0.0 Ca

b) 1.0 Cl

0.8 NO3

0.6 SO4

C2O4

0.4 NH4

0.2

0.0 Ca

0.6

0.4

0.2

0.0


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-

22-

Na

+

+

+

K

Mg 2+

2+

-

2-

-

+

2-

Na +

+

K

Mg

2+ 2+

Cl -

NO3

SO4

NH4

Ca -

2-

C2O4 2-

Na +

+ +

K

Mg

2+ 2+

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Figure 1: Ion Mole Fraction of a) urban grime and b) PM2.5 c) PM2.5-10 during 2014-2015 where orange is chloride, nitrate is blue, sulfate is red, oxalate is green, sodium is light blue, ammonium is yellow, potassium is purple, magnesium is pink and calcium is brown.

O 2+ ct NO3 : SO4 Mole Ratio ob er N ov 2 e m 014 be D r2 ec e m 014 be r2 Fe 0 br ua 14 ry M 201 5 ar ch 20 15 M ay 20 15 Ju ne 20 J u 15 ly 2 A ug 015 us t2 01 5

a)

10 8 6 4 2 0

180

O 2+ ct NO3 : Ca Mole Ratio ob er N ov 2 e m 014 be D r2 ec e m 014 be r2 Fe 0 br ua 14 ry 20 M 15 ar ch 20 15 M ay 20 15 Ju ne 20 15 Ju ly 2 A ug 015 us t2 01 5

b)

5 4 3 2 1 0

181 182 183 184 185

Figure 2: a) Nitrate to sulfate mole ratio in grime (black circles), PM10 (red open triangles) and PM10-2.5

186

Despite the broad similarity between grime and PM2.5-10, there are also

187

compositional differences, suggesting that the grime is not simply made up of

188

deposited particles but that chemistry is also occurring within it. To highlight these

(red closed bowties) b)Nitrate to Calcium ratio for Grime samples (black circles), PM10 (hollow red triangles), PM10-2.5 (solid red bowties). In all cases, the uncertainties in the grime sample ratios are reported in Table S2.


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differences, in Figure 2 and 3 and Table S2 the ion ratios in grime are compared to

190

the corresponding ratios in PM10-2.5 and PM10, in order to compare the expected

191

grime composition in the scenario where all particles deposit (i.e. both PM2.5 and

192

PM10-2.5) versus that where the coarse particles dominate. It should be noted that

193

particle composition for particles larger than 10 microns is not available and these

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particles may also contribute to the chemical makeup of grime. Figure 2b shows the

195

nitrate to calcium ratio is consistently larger in the grime samples than in either

196

PM10-2.5 or PM10 for all but three sampling periods. This suggests that there is a

197

nitrate source to the grime above and beyond PM10 deposition. In addition to this

198

extra nitrate source, it is possible that ammonium nitrate is lost from the film, as

199

suggested by our Leipzig study.17 It may be that this process is dominating the

200

difference in nitrate to calcium ratio during the sampling periods where those ratios

201

in PM10 are higher than those observed in the grime.

1.0 0.8 0.6

+

Cl : Na Mole Ratio

1.2

-

0.4 0.2

O ct ob er N 2 ov e m 014 be D r2 ec e m 014 be r2 Fe 01 br 4 ua ry 20 M 15 ar ch 20 15 M ay 20 15 Ju ne 20 15 Ju ly 20 A 15 ug us t2 01 5

0.0

202 203 204

Figure 3: Chloride to Sodium Ratio for Grime samples (black circles), PM10 (hollow red triangles), PM10-

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Figure 3 shows the chloride to sodium ratio in grime and in particles. As noted

206

earlier, there is a large increase in sodium and chloride found in the grime during

2.5

(solid red bowties). The dashed line indicates the 1:1 ratio, corresponding to NaCl.


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the winter months. This dominance of NaCl in the winter tracks the increase in the

208

amounts of these ions in coarse particles and corresponds well to when there was

209

snowfall in Toronto (see Figure S1 in the SI) and therefore when road salt was being

210

applied. During the months of elevated chloride and sodium in the winter, the

211

chloride to sodium ratio in grime is essentially unity, consistent with NaCl

212

deposition taking place without further processing. This unity ratio of chloride to

213

sodium is also seen in the coarse particles year-round. However, during the

214

summer, the chloride to sodium ratio measured in grime is less than one and lower

215

than that seen in particles. This change from the NaCl stoichiometry may indicate a

216

chemical chloride loss mechanism from the film.

217

Figure 4 illustrates the water uptake as a function of RH for urban grime samples

218

collected during the summer, spring and winter periods. The films collected in

219

summer show a higher water uptake affinity than those collected in the winter, over

220

the whole RH range measured. As we noted previously,10 the uptake curves are

221

generally smooth and featureless, with no obvious phase changes or hysteresis

222

observed in any of the samples over the RH range measured.

-2

Mass Change (µg m )

0.7

Winter Spring Summer

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

223 224 225

10

20

30 40 50 Relative Humidity (%)

60

70

Figure 4: Water uptake onto grime as a function of relative humidity where lines below 15% are dashed because of lack of calibration of the RH meter in this region. Thick, solid lines show the average water


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226 227 228 229

uptake for each sample period and the dotted, fine lines are each an individual experiment. Uptake

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In order to investigate possible mechanisms for water uptake, in Figure 5 the

231

amount of water taken up into the grime at 50% RH is plotted as a function of the

232

grime concentration of the various ions, as measured from grime collected

233

simultaneously on glass. Since we do not measure the total mass of grime deposited,

234

this assumes that the ion concentrations give a good indication of the water-sorbing

235

ability of the grime. The amount of water for each sample was calculated from the

236

average mass of water on the film when it was exposed to increasing and when it

237

was exposed to decreasing RHs in the QCM experiments. The water uptake onto

238

each of the two grime samples collected for a given sample period is plotted as a

239

separate point, thus giving two water numbers for each ion concentration. Table S3

240

reports the measured ion concentrations and Table S4 reports the water content

241

measured at 50% RH. Although not shown here, the same trends are seen using

242

relative humidities other than 50%. We note that grime collection to measure the

243

ion composition and collection to determine the water uptake were performed on

244

different substrates and so assume that the substrate does not impact the grime

245

composition.

246

Though one might expect water uptake to scale with total amount of ions on the

247

film, the plot shown in Figure 5a does not indicate such a relationship. Indeed, if

248

anything, there appears to be a weak anticorrelation between these quantities.

249

Water uptake seems to be somewhat anticorrelated with the sulfate concentration

250

as well. Figures 5c and 5d suggest that calcium and nitrate might have some

251

hygroscopic effects, but this seems far from convincing. The large changes in Na+

252

and Cl between summer and winter drive the total seasonal ion mass change, but

253

Figures 5e and 5f make it clear that neither of these ions show a good correlation

254

with water uptake despite NaCl being hygroscopic.23 These observations may

255

indicate that other species present in the grime, not measured in this study, are

curves are coloured by season, with samples collected in the winter (November to March) shown in black, spring (May and June) in green and summer (July and August) in pink. Lines below 15% are dashed because of lack of calibration of the RH meter in this region.


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controlling the water uptake to some degree. Of all the ions measured here, the best

257

positive relationship to water uptake is with oxalate, which displays a positive

258

correlation with an r2 value of 0.59. Good agreement is also seen when the total

259

water uptake at 50% RH is plotted as a function of the nitrate to sulfate mole ratio,

260

having an r2 of 0.83 as shown in Figure 5h.

261 -4

a)

2.0x10

1.5 1.0 0.5 0.0 0.0

0.5

b)

1.0

1.5

2.0 -2

1.5 1.0 0.5 0.0

2.5 3.0x10-4

0.0

0.2 0.4 0.6 0.8 1.0x10-5 -2 Sulfate mole (mol m )

Total mole (mol m )

262 -4

2.0x10

Amount of H2O (mol m )

c)

-4

d)

-2

-2


2.0x10

1.5 1.0 0.5 0.0 0

1

2

3

4x10 Nitrate mole (mol m )

1.5 1.0 0.5 0.0 0.0

-5

0.5

-2

2.0

2.5 3.0x10-5 -2

2.0x10

e)

1.5

-4

f)

-2

-4

-2


2.0x10

1.0

Calcium mole (mol m ) Amount of H2O (mol m )

263

1.5 1.0 0.5 0.0 0.0

264

-4

-2


-2


2.0x10

0.2 0.4 0.6 0.8 1.0 1.2x10-4 -2 Chloride mole (mol m )

1.5 1.0 0.5 0.0 0.0

0.4

0.8

1.2x10 -2 Sodium mole (mol m )


-4

12


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265 2.0x10

g)


-4

-4

h)

-2

-2


2.0x10

1.5 1.0 0.5 0.0 0

1

2

3 -2

Oxalate mole (mol m )

266

4x10

-6

1.5 1.0 0.5 0.0 0

2

4 6 8 Nitrate:sulfate mole ratio

10

267 268 269 270

Figure 5: Amount of water on the film at 50% RH measured by the QCM (blank subtracted) plotted as a

271

Discussion

272

As seen in our Leipzig study,17 a comparison of grime and particle compositions

273

indicates that grime does not merely reflect particle deposition, but rather suggests

274

that there is chemistry taking place after deposition that dictates the chemical

275

makeup of the grime. In that study it was both the greater ageing time and the

276

mixing of different particles that was suggested to promote the particle and grime

277

differences. Such differences are again inferred to play a role in the current study.

278

The current study shows further evidence for the loss of ammonium from grime

279

samples: the small amount of ammonium present in Toronto urban grime is only

280

measurable in the winter, consistent with temperature-dependant evaporation of

281

ammonium nitrate facilitated by the long exposure time of the grime to the

282

atmosphere.6,17,24 Beyond seasonal temperature differences, in the winter the grime

283

also contains less calcium. Therefore, the higher winter ammonium concentrations

284

are also consistent with the mechanism proposed by Baergen et al17 where the Ca2+

285

rich coarse particles, which are expected to also be high in carbonates, mix with the

function of moles of a) total ions b) sulfate c) nitrate d) calcium e) chloride f) sodium g) oxalate and h) nitrate:sulfate ratio in corresponding glass grime sample. In all panels, the black symbols represent winter, green represents spring and red symbols illustrate summer.


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NH4+ rich fine particles, increasing the pH environment of the grime deposit and

287

shifting the equilibrium away from NH4+ to NH3, which is then lost to the gas phase.

288

Also in our previous work,17 nitrate loss from particles after deposition was inferred

289

via a comparison between the grime and particle compositions. The lower nitrate to

290

calcium ratio seen here in the winter months in comparison to that measured in

291

particles, where ammonium nitrate concentrations in PM10 are highest (see Figure

292

S2), is consistent with this earlier inference. However, for the remainder of the year,

293

a comparison of nitrate to calcium ratios in grime vs. PM10 indicates that there is a

294

process or processes occurring that provides a net additional nitrate source to the

295

grime. Numerous mechanisms, known to occur in particles, may be playing a role in

296

this phenomenon. For example, carbonate could be displaced by HNO3 to form

297

calcium nitrate, and/or chloride could be displaced by N2O5 to form nitrate salts and

298

gas phase ClNO2.25,26

299

Carbonate displacement is suggested by the observation that while the NO3: SO42-

300

ratio is highest in the summer, the NO3: Ca2+ ratio is slightly lower in summer than

301

in winter. This disparity may reflect an enhancement of carbonate displacement

302

reactions in the summertime due to the higher proportion of crustal species, and

303

therefore likely carbonate, in the film then. Support for the chloride displacement

304

mechanism taking place in the grime is given by the grime’s very low chloride to

305

sodium ratio coincident with higher nitrate amounts during summertime. These are

306

much different than what is observed in particles, consistent with chloride loss via

307

displacement by or reaction with nitrate. Both of these processes may be further

308

enhanced in the summer due to higher water content in the film promoting greater

309

N2O5 uptake to the film.27 ClNO2 production from chloride-containing particles has

310

been observed for both marine and continental particles.28 Further studies to

311

measure gas phase ClNO2 emissions from urban grime-coated surface should

312

investigate this process further.


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313

The formation of Ca(NO3)2 on urban grime is of interest because it is hygroscopic

314

and has been attributed to an increase in the water content of dust aerosols aged by

315

reaction with HNO3.29-31 Therefore it may also be contributing to the trends

316

observed for water uptake. Although a phase transition is typically observed for

317

Ca(NO3)2 between 11 and 7% RH,23 our observations show no hysteresis in water

318

uptake, consistent with what was observed by Liu et al for Ca(NO3)2 in

319

microparticles.32 The water uptake in grime measured in this study shows a

320

generally positive correlation with both Ca2+ and NO3, consistent with Ca(NO3)2

321

playing a role in driving water uptake. However, a better correlation is observed

322

between water uptake and the nitrate:sulphate ratio; this may indicate that sulphate

323

and other less hygroscopic salts29 act to dilute the more hygroscopic salts, dropping

324

the film’s water uptake affinity.

325

While it is expected that grime has a significant fraction of inorganic salts,6,7 the

326

positive correlation between oxalate concentration and water uptake suggests that

327

organics could also be playing a role. During the summer months, one expects there

328

to be more photochemical oxidation in the atmosphere than in the winter, leading to

329

higher concentrations of oxidized organic species, such as oxalate, which may

330

readily condense onto particles and urban grime surfaces. This simple picture may

331

be somewhat complicated by the temperature dependant partitioning between gas

332

and particle phase, as demonstrated by a spring time maximum of deposited oxalate

333

measured in a UK study.33,34 Nevertheless, given that there is more biological

334

activity in the summer months, contributing to higher organic precursor

335

concentrations,35 the seasonality observed for oxalate is expected to be consistent

336

with other oxidized organics on the film and therefore oxalate could be a marker for

337

aged organics generally.

338

As organics age, their hygroscopicity is often seen to increase (eg. Reference 36). If

339

the water uptake by grime is being at least partly driven by the hygroscopicity of the

340

organics present there, the nitrate-to-sulfate ratio may simply be indicating the

341

effective photochemical age of the grime sample, rather than being chemically


15

Page 17 of 21


342

significant. The presence of organics is also known to affect particle chemistry by

343

inducing organic-aqueous phase separation; this has been shown to impact water

344

uptake in mixed organic, inorganic particles (see eg. reference 37). More detailed

345

studies of the grime organic composition and morphology will be necessary to

346

provide insight into the water uptake mechanism.

347

While there is much more to be learnt about the seasonality of grime composition

348

and behavior, the current study gives the first look at how the composition and

349

water uptake of grime changes over the course of a year on a window glass

350

substrate. Future work should look at the impact of other substrates, including

351

porous ones, on the composition of grime. Comparing the composition in grime with

352

that in particles, as shown in Figures 2 and 3, suggests a strong potential for

353

chemistry taking pace in urban grime. Such chemistry might include nitrate

354

displacement of carbonates in the grime, releasing CO2, and the reaction of gas

355

phase nitric acid or N2O5 with grime-associated chloride, providing a possible urban

356

source of ClNO2. Our present observations provide further data to support our

357

hypothesis17 about the loss of ammonium from urban grime into the atmosphere as

358

NH3. As a result of the changes in grime composition there are corresponding

359

changes in the water uptake behavior of grime, which could alter the reactive

360

environment of the grime. The grime shows the highest water affinity in summer

361

when both the NO3: SO42- mole ratio and oxalate concentration are the highest

362

which could indicate water uptake mediated by hygroscopic nitrate salts, oxidized

363

organics or a combination of these factors. This behavior should be studied further.

364

Acknowledgements

365

This work was funded by NSERC Canada. A. M. Baergen thanks NSERC for a CGS-D

366

award and the government of Ontario for an Ontario Graduate Scholarship. The authors

367

thank Prof. Jennifer Murphy for the use of analytical instruments.

368 369 370

Supplementary on-line Information: Tables of sampling period, ion concentrations and mole ratios and water uptake amounts; figures showing snowfall amounts and relative ion mass concentrations.


16


371

Page 18 of 21

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Seasonality of the Water-Soluble Inorganic Ion Composition and

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