Gas-Phase Carboxylic Acids in a University Classroom: Abundance

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Gas-Phase Carboxylic Acids in a University Classroom: Abundance, Variability, and Sources SHANG LIU, Samantha L. Thompson, Harald Stark, Paul J. Ziemann, and Jose L. Jimenez Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01358 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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Environmental Science & Technology

Gas-phase carboxylic acids in a university classroom: abundance, variability, and sources Shang Liu1, Samantha L. Thompson1, Harald Stark1,2, Paul J. Ziemann1,* , Jose L. Jimenez1,* 1

Department of Chemistry & Biochemistry, and Cooperative Institute for Research in Environmental

Sciences (CIRES), University of Colorado, Boulder, CO, USA 2

Aerodyne Research Inc., Billerica, MA 01821, USA

*Corresponding author. E-mails: [email protected], [email protected]

Key words: Indoor air quality, indoor carboxylic acids, human source, chemical ionization mass spectrometry ABSTRACT Gas-phase carboxylic acids are ubiquitous in ambient air, yet their indoor occurrence and abundance are poorly characterized. To fill this gap, we measured gas-phase carboxylic acids in real-time inside and outside of a university classroom using a high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) equipped with an acetate ion source. A wide variety of carboxylic acids were identified indoors and outdoors, including monoacids, diacids, hydroxy acids, carbonyl acids, and aromatic acids. An empirical parameterization was derived to estimate the sensitivity (ion counts per ppt of the analytes) of the HRToF-CIMS to the acids. The campaign-average concentration of carboxylic acids measured outdoors was 1.0 ppb, with the peak concentration occurring in daytime. The average indoor concentration of carboxylic acids was 6.8 ppb, of which 87% was contributed by formic and lactic acid. While carboxylic acids measured outdoors displayed a single daytime peak, those measured indoors displayed a daytime and a nighttime peak. Besides indoor sources such as off-gassing of building materials, evidence for acid production from indoor chemical reactions with ozone was found. In addition, some carboxylic acids measured indoors correlated to CO2 in daytime, suggesting that human occupants may contribute to their abundance either through direct emissions or surface reactions. 1 ACS Paragon Plus Environment


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INTRODUCTION Volatile organic compounds (VOCs) are pervasive in indoor environments. A myriad of VOCs, including alkanes, aromatics, terpenes, alcohols, aldehydes, ketones, esters, and halogenated compounds, has been identified indoors, typically with elevated concentrations compared to outdoors.1 Indoor measurements of carboxylic acids, however, are scant. Carboxylic acids can be tracers of oxidation chemistry. However, there are missing sources for forming certain carboxylic acids in outdoor air, as measured concentrations of formic and acetic acids are typically several-fold what can be explained with the known sources.2–4 In addition, carboxylic acids are irritants for eyes, skin, and mucous membranes,5 and could be harmful for tissues and organs such as the optic nerve, brain, heart, and kidney6 during occupational exposure. The threshold levels for these effects are compound-specific. For example, the odor thresholds for formic, acetic, butyric, and hexanoic acid are 22, 6-148, 0.23, and 0.6–13 ppb, respectively,7,8 and the irritation thresholds of formic and acetic acid are suggested to be 14 and 1 ppm7,8 for occupational environments. Besides their health effects in occupational environments, carboxylic acids play an important role in corrosion of indoor facilities, for example, formic and acetic acids (at ppb and sub-ppm levels) were found to be responsible for the corrosion of organ pipes in churches.9 These negative effects of carboxylic acids provide additional motivation for their study. It is thus of significant interest to expand our knowledge of the type, concentration, variability, and sources of carboxylic acids in indoor environments, especially where large numbers of people congregate, such as in classrooms.

Previously reported indoor measurements of gas-phase carboxylic acids are limited to formic and acetic acids. These acids have been measured using long path Fourier transform infrared

2 ACS Paragon Plus Environment

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spectroscopy,10 high performance liquid chromatography,11,12 and ion chromatography.13,14 Even from this small body of measurements, the reported indoor concentration of formic and acetic acids varied considerably, from 1 to 120 ppb for formic acid and from 0 to 292 ppb for acetic acid, with their indoor-to-outdoor concentration ratio (I/O) ranging from 0.2 to 56 (Table 1). The large variability of the indoor concentration and I/O of formic and acetic acids reflect multiple and unique indoor sources, which have been suggested to be direct emission from commercial building materials, such as wood-based products or latex paint,10,12,15 and from indoor combustion, such as wood and candle burning,13,14 or formation from indoor chemical reactions involving O3.11,14,16

Carboxylic acids other than formic and acetic acid have not been measured indoors, in part because of the lack of reliable analytical methods. For example, highly polar carboxylic acids elute poorly on typical gas chromatography columns. With the recent development of acetate chemical ionization mass spectrometry (“acid CIMS”), carboxylic acids can be measured in situ with high time resolution,17 as discussed further below. Acid CIMS instruments have been deployed during outdoor measurements and laboratory reactor studies,18,19 but to our knowledge have not been applied indoors.

As a pilot study applying advanced techniques to study indoor air chemistry, we deployed a proton transfer reaction mass spectrometer (PTR-MS) to investigate the sources and chemical reactions of VOCs in a university classroom.20 Based on a statistical analysis of measured concentration time profiles of VOCs, CO2, O3, and relative humidity we determined that humans (through respiration and skin lipid reactions), background emissions and reactions, and



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ventilation contributed approximately equally to the measured VOCs indoors during daytime. In addition, we identified key reaction products from skin lipid ozonolysis, adding to the evidence that human occupants can alter indoor VOC composition and oxidative capacity by surface reactions. Indoor carboxylic acids, however, cannot be measured by PTR-MS.

In this work, we extend our previous study to measure gas-phase carboxylic acids in real-time in the same university classroom and in adjacent outdoor air using a high-resolution time-of-flight CIMS equipped with an acetate ion source. We present results of comprehensive analysis to identify and quantify a broad range of gas-phase carboxylic acids. We further compare indoor and outdoor carboxylic acids in their concentration, diurnal cycle, and likely sources. In addition, we evaluate the role that human occupants play in affecting the concentration of carboxylic acids.

EXPERIMENTAL SECTION Classroom sampling site. This study was part of the CLASsroom Study of Indoor Chemistry (CLASSIC) campaign, which has been described by Liu et al.20 Briefly, the measurements were conducted in the spring of 2014 in a ~700 m3 classroom in the Cristol Building at the University of Colorado Boulder. The classroom has stadium-style seating with capacity for 194 people. The walls of the classroom are covered with latex paint. The floor of the classroom is mostly made of concrete and is cleaned daily using water only. The classroom was ventilated in economizer mode using a dedicated air handler unit in daytime: from 07:00 to 21:00 during weekdays and from 09:00 to 19:00 during weekends. Ventilation was turned off at night. The outdoor air exchange rate (AER) was estimated to be 2–11 h-1 during the measurement period, with higher AER when outdoor air temperature was higher and vice versa.20


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High-resolution time-of-flight CIMS (HRToF-CIMS). The HRToF-CIMS instrument (hereinafter CIMS for short) has been described in earlier publications.18,21 Briefly, the CIMS consists of a series of differentially pumped chambers that ionize, transfer, and analyze the target molecules. The air flow first enters the ion-molecule reaction chamber where the target molecules are ionized through reactions with reagent ions. The analyte ions are transferred and accelerated through three chambers containing various ion optics, followed by mass analysis and detection with a time-of-flight mass spectrometer (HTOF, TOFWERK AG, Switzerland). Acetate ion was used as the reagent ion in our measurements, which was generated by flowing acetic anhydride through a 210Po ion source.17 As acetate ion has the lowest gas-phase acidity of the common atmospheric carboxylic acids,17 it reacts with gas-phase carboxylic acids causing them to deprotonate. This ion chemistry leads to little or no fragmentation during CIMS measurement,17 thereby retaining the molecular identity of the parent ions. Other functional groups, such as aldehydes and alcohols, do not react with acetate ions, making the ionization process selective for carboxylic acids. Recent studies show that some non-acid compounds can also react with acetate ion and hence be measured by the acetate CIMS. These compounds include nitrophenols,22 peroxy acids,23 and benzoyl peroxide.23 For this reason, care must be taken when interpreting the CIMS data. During measurements the mass spectrometer was operated at a maximum mass resolution of 4000 and was tuned in “declustering mode”, such that the acetate reagent ions are primarily in the form of bare ions.21,24

The indoor and outdoor CIMS measurements were conducted from 1–6 and 8–10 May 2014, respectively. Indoor and outdoor measurements could not be made on the same day because the



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CIMS is too large and the set-up time too long to be able to sample from different locations in short time intervals. Furthermore, outdoor air could not be sampled from the classroom location because the supply air was inaccessible and the long tubing required to reach the outdoors from the interior classroom would have led to unacceptably large sample losses. For indoor air sampling the CIMS instrument was located in a projection room adjacent to the classroom and operated at 25–30 °C. Air was sampled through a Teflon tube (ID = 6.35 mm, 1.5 m long) extending inside the classroom at a height of 2.4 m above the floor. The air flow rate was 2 l min1

, resulting in a residence time of 1.4 s. During the study, the classroom was heavily occupied for

50 min lecture periods followed by 10 min breaks. A typical class involved 50–150 students. For outdoor air sampling, the CIMS instrument was deployed in a laboratory near the Cristol building and the sampling height was ~5 m. Previous study has shown that the sensitivity of formic acid measured by acetate CIMS varied by ~50% during month-long campaigns.22 The variability of sensitivity was primarily driven by changes in trailer temperature.22 Since our CIMS instrument was located in stable environment for both indoor and outdoor measurements and the campaign was relatively short, the variability of our CIMS sensitivity during the measurement is conservatively estimated to be 20% based on our tests and experience. Indoor and outdoor measurement backgrounds were determined by sampling UHP N2 for 5 min with the same CIMS settings and inlet. Indoor background measurements were performed on 5 May and 6 May 2014, and the outdoor background measurement was conducted on 9 May 2014. Signals dropped sharply to nearly the same levels during the background tests for indoor and outdoor measurements, respectively (Fig. S1), indicating that the acid signals were above the backgrounds and that the latter were constant during the measurement period.


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Although indoor (1–6 May) and outdoor (8–10 May) concentrations of carboxylic acids could not be measured on the same days, the observation that outdoor O3 concentrations were similar during these two periods (Fig. S2) indicates that carboxylic acid concentrations were also similar. As shown by Veres et al.,25 outdoor concentrations of carboxylic acids and O3 are highly correlated due to their similar photochemical sources. According their results, a decrease in the O3 concentration of 10 ppb, which was the average decrease outdoors in Boulder between 1–6 and 8–10 May (Fig. S2), corresponds to only a 10–20% decrease in carboxylic acid concentrations. This small difference suggests that it is valid to compare concentrations of carboxylic acids measured indoors during 1–6 May with those measured outdoors during 8–10 May, although the reported indoor-to-outdoor ratios may be slight overestimates.

Sensitivity estimation of CIMS. It is impractical or even impossible to calibrate the sensitivity (ion counts per ppt) for all measured carboxylic acids. Previous applications of the acetate CIMS have typically assumed a constant sensitivity for bulk analysis. That constant sensitivity is either calculated as the average19,26 or assigned to be the maximum23 of the sensitivities measured for a subset of carboxylic acids. In this study, we derived an empirical formula to estimate the sensitivity of carboxylic acid measurements. We first measured the sensitivity of 13 carboxylic acids using a mobile organic carbon calibration (MOCCS) system.27 We then examined the dependence of the measured sensitivity on mass-to-charge ratio (m/z), molecule oxidation state (OSC), number of carbon atoms (nC), molecular oxygen-to-carbon atomic ratio (O:C), and molecular hydrogen-to-carbon atomic ratio (H:C). We found that the variability of the sensitivity was best described by OSC and nC together. An empirical parameterization of the sensitivity was derived as



log10(Sensitivity) = 0.68 x OSC + 0.23 x nC – 0.33

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(1)

Using Equation (1), the calculated and measured sensitivity values correlated with a Pearson’s r of 0.93 and generally agreed well. With the exception of pinonic and azelaic acid the ratios of the calculated and measured sensitivities are within about a factor of 2 (Fig. 1). We used this sensitivity equation to calculate mixing ratios for all measured carboxylic acids, except those that were specifically calibrated. It should be noted that the derived parameterization for the sensitivity is specific for the instrument used in this study with its particular tuning. However, the general approach to derive sensitivities based on a combination of elemental parameters could be applicable to other tuning settings for the acetate CIMS and possibly other reagent ions.

CIMS data processing. The 1-second CIMS data were averaged to 1-min intervals and then processed using the Tofware (version 2.5.1) software package coded with Igor Pro 6 (Wavemetrics Inc., Lake Oswego, OR, USA). The analysis algorithm is described by Stark et al.24 The peaks in the indoor CIMS spectra were fitted using the Tofware multi-peak fitting algorithm. An ion list for the fitted peaks was generated by assigning the peaks to possible ion elemental compositions. This list was combined with the peak list18,24 developed for outdoor measurements to produce a final peak list, which contained 2012 ions. The final peak list was used for both indoor and outdoor CIMS measurements. Using the same peak list enables direct comparison of the abundance of indoor and outdoor carboxylic acids with the same molecular formulas. Due to the limited mass resolution of the mass spectrometer, most measured peaks were fitted using multiple ions. Comparison of the fitted areas with the corresponding raw peak areas at the nominal masses of these acids suggested that the peaks were well fit, with an average fitting uncertainty of 10%. The precision of the fitted intensity was generally smaller than 10%,


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with the average value of 6%. Example peak fits of major ions during the study are shown in Fig. S3.

The signals retrieved for the ions in the peak list were screened for detailed analysis. We first excluded ions whose average signal was below the detection limit (DL), which was determined by two independent methods. In one, the DL for the average signal of each ion was estimated by scaling the DL of its background signal, with the scaling factor determined as the square root of the ratio of the duration of the background and field measurements. This approach is valid as long as the uncertainties in background count rate varies slowly,21 in which case the DL is inversely proportional to the square root of the sampling duration. The DL for the background signal was determined as three times the standard deviation (1σ) of the background measurement. In the second method the DL was determined using counting statistics, with the DL calculated as three times the 1σ uncertainty determined as the square root of the measured ion count.28,29 Fig. S4 shows that the 1σ uncertainty estimated from the standard deviation of the background signal was generally higher than that estimated from counting statistics. This is expected, since in addition to ion counting the standard deviation method includes uncertainties arising from other components of the instrument. The DL used for each ion was the larger of the values calculated from the two methods. In addition, only ions with elemental formulas containing only C, H, and O with 1 to 18 C atoms, 2 to 5 O atoms, and 1), (3) if no > 2, and no > 2 x DBE, the ion is counted as a hydroxyl acid, (4) in the case of no > 2 and no ≤ 2 x DBE (DBE ≥ 2), the ions is defined as a carbonyl acid if no is an odd number, otherwise the ion is referred to as a diacid or hydroxycarbonyl acid. Table 2 lists the DBE and no for the ions identified in this study.

Saturation vapor pressures of the assigned carboxylic acids were estimated according to their functional group composition using SIMPOL.30 We further excluded carboxylic acids whose saturation vapor pressures are smaller than 0.1 µg m-3. This procedure ensures that the fraction of the carboxylic acid in the gas-phase was greater than 10%, assuming that the classroom organic aerosol concentration was ~1 µg m-3, which is typical of outdoor concentrations in this region.31,32 This procedure excluded 25 of the 180 ions, leaving 155 for further analysis. The remaining ions accounted for 87% and 62% of the total signals (above background) for indoor and outdoor measurements, respectively. Each ion was assigned a name according to its elemental formula and functional group composition, but we stress that these assignments are


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tentative since other isomers could be present. The elemental formula, name, estimated sensitivity, concentration, and saturation vapor pressures of carboxylic acids assigned to each ion are given in Table 3 and Table S1.

Supporting measurements. Indoor CO2 was monitored using a LI-COR LI-840 CO2/H2O gas analyzer with 1-second time resolution. As discussed elsewhere,19 O3 was measured from 22 March – 10 April 2014 using cavity a ring-down spectrometer described in Wild et al.33 During the March-April measurement, we observed a very consistent nighttime pattern of O3 when the ventilation was turned off. Therefore it is expected that the nighttime O3 profile during 1-6 May is similar to that of the March-April measurement.

RESULTS AND DISCUSSION In this section, we report the concentrations of carboxylic acids identified indoors and outdoors. We then analyze and compare the temporal variations of indoor and outdoor carboxylic acids. By examining the relationship of indoor carboxylic acids and CO2, the influence of human subjects on indoor carboxylic acids is evaluated.

Concentration of indoor and outdoor carboxylic acids. The campaign-average indoor and outdoor CIMS spectra (background corrected) are shown in Fig. S5 (the background signals are shown in Fig. S6 for comparison). Signals for m/z 40–200 accounted for 98% of all backgroundcorrected signal. For the ions identified both indoors and outdoors, larger abundances were usually found indoors (Fig. S5a). Both indoor and outdoor measurements had the highest signals at m/z 45 and 89. The ion signal at m/z 45 was fitted with a single ion with a formula of CHO2–,



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which likely represents formic acid. The ion signal at m/z 89 showed two peaks that were fitted with C2HO4- and C3H5O3- (inset of Fig. S5c) and were assigned as oxalic and lactic acid, respectively. While the indoor m/z 89 peak was dominated by lactic acid, oxalic acid had a relatively larger signal outdoors, reflecting the strong outdoor source of oxalic acid that includes motor vehicle exhaust and photochemical production.34,35 Indoor concentrations of formic and lactic acid were 5 and 10 times their outdoor concentrations on campaign average, respectively, suggesting large indoor sources. The elevated indoor concentration of formic acid was likely due to direct emissions from wood-based products10 or latex paint,12 sources that are not present outdoors, and human perspiration36–38 was likely the major contributor to the elevated indoor concentration of lactic acid. As discussed in more detail below, surface reactions of O3 with unsaturated organic compounds present in skin and other materials, or similar gas-phase reactions could also form carboxylic acids.

The campaign-average concentration of carboxylic acids was 6.8 ppb in the classroom, which was small compared to the concentration of non-acid VOCs (100 ppb for m/z 20–200) measured using a proton transfer reaction mass spectrometer during the same campaign.20 We note that acetic acid is not included in our dataset due to interference with our reagent ion. Based on previous measurements (Table 1), the concentration of acetic acid is expected to be comparable or higher than the concentration of formic acid. Addition of acetic acid thus would likely make the total gas-phase organic acid concentration ~20% higher. For formic, propionic, butyric, and pentanoic acid, whose odor thresholds are available,7 their concentrations in the classroom were well below the threshold values. The indoor concentration of carboxylic acids, however, was larger than their outdoor concentration for each acid category (Table 4). The indoor/outdoor ratio


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(I/O) for the different acid categories ranged from ~2 to ~7, with an average of 6.5. This large I/O is consistent with previous studies indicating that the majority of indoor VOC classes have higher concentrations indoors compared to outdoors.1 Indoor and outdoor carboxylic acids both consisted largely of hydroxy acids, followed by monoacids and carbonyl acids. In particular, formic acid and lactic acid accounted for 77% and 99% of monoacids and hydroxy acids indoors and 97% and 59% outdoors, respectively.

Diurnal cycles of indoor and outdoor carboxylic acids. Distinct diurnal features were observed for indoor and outdoor carboxylic acids (Figs. 2, S7). Outdoor carboxylic acids showed a concentration maximum in daytime, which is consistent with the expectation that they are photochemically produced in sunlight3,39 whereas indoor carboxylic acids displayed a later daytime peak and a nighttime peak. The nighttime increase of carboxylic acids was facilitated by the near-zero AER at night (the ventilation was turned off from 21:00 to 07:00 and no lights were on) and may be due to direct off-gassing of these compounds from indoor building materials, such as latex paint12 that covers the classroom walls. Because this nighttime increase anti-correlated with indoor O3 concentrations (measured earlier in the campaign20), however, it is quite likely that reactions of O3 with unsaturated compounds on surfaces or in the gas phase11,40 also contributed to the increase in carboxylic acid concentrations. The surface and gas-phase reactions both occur through mechanisms that involve addition of O3 to the C=C double bond and can lead to the formation of carboxylic acids and other products with a range of yields.41,42 The maximum possible contribution of these reactions to nighttime formation of carboxylic acids is equal to the decrease in the O3 concentration during this period, which can be estimated by assuming that the average nighttime concentration profile of O3 in the classroom was similar to



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that measured from 22 March – 10 April 2014. As shown in Fig. S8, this corresponds to ~14 ppb between 23:00 and 05:00, about 5× the average increase in carboxylic acids of ~2.7 ppb. This result would therefore be consistent with a yield of carboxylic acids from these reactions of ~20%, although this should be an upper estimate because of contributions of off-gassing to the carboxylic acid concentrations.

Since the nighttime profiles of each of the carboxylic acid classes are approximately linear (Fig. 2a), the production rates (∆concentration/∆time) for each class were calculated for a series of time intervals from the concentration time profiles. The average nighttime production rates were 96, 3, 211, 19, and 1 ppt h-1 for monoacids, diacids, hydroxy acids, carbonyl acids, and aromatic acids, respectively, although values decreased during the night and co-varied with O3 concentration (Fig. 3a). These observations are consistent with O3 being an oxidant in the chemical production of carboxylic acids during nighttime, since the production rate should be proportional to the oxidant concentration if the source is chemical reactions but constant if the source is off-gassing of building materials. We note that the peak in the calculated production rate at 05:00 is probably due to a perturbation in the classroom. This is the time when the janitor starts to clean the classroom every day, and s/he may keep the classroom door open. This would cause transport of outdoor O3 to the classroom, thereby increasing the production rate and slowing the decay in O3.

The daytime peak of indoor carboxylic acids could arise from a combination of outdoor-toindoor transport, gas-phase or surface reactions, and direct emissions from indoor sources including materials and human occupants. The daytime peak of indoor carboxylic acids lagged


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that of outdoor carboxylic acids, but because the lag was independent of carboxylic acid vapor pressure (Fig. S9), it was probably not due to adsorption/desorption on the walls of the ventilation system. Interestingly, many of the carboxylic acids measured outdoors had two peaks during daytime, a morning peak and an afternoon peak. The more volatile acids had a relatively larger peak in the afternoon (Fig. S9). This suggests that the indoor carboxylic acids were likely produced indoors, rather than being transported from outdoors.

The I/O was greater than 1 for all carboxylic acids (Fig. 2b) except for benzenebutanoic and oxodecanoic acid, and was substantially larger at night when the ventilation was shut down. This indicates that indoor sources were the dominant contributor to carboxylic acids and that the ventilation rate had a major effect on the resulting concentrations. As discussed in the Experimental Section, the I/O reported in this study may represent upper limits since simultaneous indoor and outdoor measurements were not performed. Nonetheless, the effects on I/O and its diurnal pattern are likely to be small.

Special attention is given to formic acid, as it was abundant and its sensitivity was accurately measured with an authentic standard. Indoor formic acid concentrations ranged from 0.2 ppb to 3.5 ppb, with an average of 1.2 ppb. This concentration is within the range of previous indoor studies, but towards the lower end (Table 1), likely due to the lack of combustion sources in the classroom and/or different indoor materials.10,13,14 The average outdoor formic acid concentration was 0.25 ppb, which is similar to concentrations measured in a forested region,43 oil and gas production areas3, and urban areas.3,25,44 The measured I/O of formic acid was 5, comparable to I/O values in literature (Table 1). Similar to the other carboxylic acids, the indoor concentration



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of formic acid peaked during daytime and at night (Fig. 3b). It accumulated from the time the ventilation was turned off at night until the ventilation started again at 07:00 the next morning.

Human-related carboxylic acids. The concentrations of individual carboxylic acids were compared with CO2 to investigate possible association with human sources. Our measurements have shown that CO2 is a good indicator for the presence of human occupants20. We found that 12 carboxylic acids correlated with CO2 in daytime (Fig. 4; example time series are shown in Fig. 5), suggesting that human occupants contributed significantly to their indoor abundance. Of particular interest is lactic acid (Fig. 4h) — a major component in human perspiration36–38 that is also used in commercial health care products,45 which accounted for ~2/3 of the total indoor carboxylic acid concentration.

Many of the human-related acids, including saturated monoacids (octanoic, nonanoic, decanoic, undecanoic, and dodecanoic acid, Fig. 4a-e), unsaturated monoacids (hexenoic and nonenoic acid, Fig. 4f, g), a hydroxy acid (lactic acid, Fig. 4h), and a carbonyl acid (pyruvic acid, Fig. 4i) have been identified in human emanation,36,46,47 supporting their origin from human occupants. In addition, since human skin lipids contain a number of long-chain unsaturated fatty acids,48 ozonolysis of skin lipids via the Criegee mechanism can produce carbonyl acids.49 In the Criegee mechanism, O3 adds to the C=C double bond to form an ozonide, which rapidly decomposes to produce a carbonyl-containing compound and a Criegee intermediate. If the carbonyl group is added to the fragment containing the carboxylic acid group, then a carbonyl acid is formed. Since many unsaturated fatty acids identified in human skin lipids have a C=C double bond between the 6 and 7 carbons, such as sapienic acid that was observed in this study (Table S1), O3


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oxidation will lead to formation of carbonyl acids with 6 carbons.48 For example, 6-oxohexanoic acid was identified and observed to correlate with CO2 (Fig. 4j). O3 can also react with squalene, the most common compound in human skin lipids,48 via the Criegee mechanism. Squalene contains 6 C=C bonds so it is highly reactive. Zhou et al.50 found that levulinic acid, succinic acid, and 4-oxobutanoic are the major condensed-phase products from heterogeneous ozonolysis of squalene. These acids can partition into the gas phase and were identified in the gas phase during this study, with their indoor concentration substantially higher than outdoors (Table S1). Furthermore, levulinic acid, the only major acid identified under both dry and humid conditions from ozonolysis of squalene,50 was found to correlate with CO2 in this study (Fig. 4k). This observation supports the formation of levulinic acid via the Criegee mechanism. Aromatic acids, including benzoic and benzeneacetic acid, have been detected in human skin secretions.51 These compounds were identified in this study (Table S1), but did not correlate with CO2, suggesting that their concentrations were dominated by non-human sources such as outdoor vehicular emissions.52 Fig. 4l shows that the concentration of benzylsuccinic acid increased with increasing CO2. To our knowledge a human source of benzylsuccinic acid has not been reported, and thus this observation needs further study.

IMPLICATIONS AND OUTLOOK During our classroom measurements no evidence was found for any gas-phase carboxylic acids to exceed their odor or irritation thresholds, suggesting that the health effects of carboxylic acids are likely small in this well-ventilated classroom. Many classrooms, however, are poorly ventilated,53 and in these and other densely populated settings with poor ventilation (such as in aircraft cabins and subway carriages) the concentrations of human-derived carboxylic acids are



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expected to be substantially higher and might pose larger concerns. Additional measurements are needed to enrich the dataset for carboxylic acids in other indoor environments, especially those associated with potential sources, such as chemical laboratories and chemical manufacturing facilities. These measurements would facilitate assessment of human exposure and health effects of indoor carboxylic acids as well as their corrosive effects on building materials. Further development of instrumentation and measurement techniques, such as higher resolution mass spectrometry and ion mobility spectrometry for separating structural isomers,54 as well as greater availability of carboxylic acid standards for improved quantification, will allow for the development of a deeper understanding of the sources and abundance of indoor carboxylic acids.

ACKNOWLEDGEMENT We thank the Alfred P. Sloan Foundation (Grant No. G–2013–6–02) for funding this study. The authors acknowledge Denise Thomas and the Facilities Management Department at University of Colorado Boulder for supporting the use of the sampling site. SLT is grateful for a CIRES Fellowship. We thank the Aerodyne ToF-CIMS users community for useful discussions.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org Includes a full list of the carboxylic acids identified in this study and details of data quality, outdoor meteorological conditions, and diurnal cycles of indoor and outdoor carboxylic acids.

AUTHOR INFORMATION *Jose L. Jimenez: Phone: 303-492-3557; fax: 303-492-1149; e-mail: [email protected]. 18 ACS Paragon Plus Environment

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*Paul J. Ziemann: Phone: 303-492-9654; fax: 303-492-1149 ; e-mail: [email protected]



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Tables Table 1. Summary of indoor measurements of formic and acetic acid. Sampling site

Analytical method

Measurement resolution

Concentration I/O range range (ppb) Formic acid

Mobile office/home

FTIR

10–20 min

50–120

NA

Residential house

HPLC

6h

Residential house

HPLC

24 h

Office/restaurant/hotel Church

IC IC

6h 12 h

1.6–19.2 7.4–17.4 (w)2 8.6–33.1 (s)3 12.4–56.0 0.5-14

4–48 3.2 (w) 4.6 (s) 0.8–6 0.19–4.69

Classroom

CIMS

1s

0.2–3.5

5

Residential house

HPLC

6h

Residential house

HPLC

24 h

Office/restaurant/hotel Church

IC IC

6h 12 h

1

Acetic acid 3.6-81.0 4–56 11.1-19.9 (w) 8.6 (w) 9.2-88.0 (s) 14.4 (s) 33–292 1.6–15 0–1.8 0.40-5.68

Proposed sources

Reference

Direct emission from commercial 10 formaldehyde products 11 Indoor chemical reactions involving O3 Off-gassing of household products or O3 55 oxidation reactions 13 Wood burning for cooking 14 Candle burning and reactions involving O3 Off-gassing of building materials, outdoor-toThis study indoor transport, indoor chemical reactions Indoor chemical reactions involving O3 Off-gassing of household products or O3 oxidation reactions Wood burning for cooking Candle burning and reactions involving O3

11 55

13 14

“w”: winter. 2“s”: summer.


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Table 2. Classification of measured carboxylic acids using double bond equivalent (DBE) and number of oxygen atoms (no).

Carboxylic acid formula DBE = 1, no = 2 DBE = 2 or 3, no = 2 DBE = 2 or 3, no = 4 DBE = 1, no =3–5 or DBE = 2, no = 4–5 DBE = 2, no = 3 or DBE = 3, no = 5 DBE = 3, no = 3 DBE = 5 or 6, no = 2–5

Assigned acid category Monoacid (saturated) Monoacid (unsaturated) Diacid/hydroxycarbonyl acid Hydroxy acid Carbonyl acid (saturated) Carbonyl acid (unsaturated) Aromatic acid



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Table 3. Molecular weight (MW; g mol-1), saturation vapor pressure (Pv; pa), saturation mass concentration (C*; µg m-3), sensitivity (counts ppt-1), and average indoor (Cin) and outdoor (Cout) concentration (ppt) of the major carboxylic acids (based on concentration in each category) reported in this study. The saturation vapor pressures and mass concentrations were estimated at 25 °C. A full list of carboxylic acids is shown in Table S1. Formula

Name

MW

CH2O2 C3H6O2 C4H8O2 C5H10O2

Formic acid Propionic acid Butyric acid Pentanoic acid

46 74 88 102

C4H6O4 C5H8O4 C2H4O3 C3H6O3 C3H6O4 C4H8O3 C2H2O3 C3H4O3 C4H6O3 C5H8O3 C6H10O3

Butanedioic acid Pentanedioic acid Glycolic acid Lactic acid Glyceric acid Hydroxybutyric acid Glyoxylic acid Pyruvic acid Oxobutanoic acid Oxopentanoic acid Oxohexanoic acid

Pv (Pa) C* (µg m-3) Sensitivity (count ppt-1) Cin (ppt) Monoacid (saturated) 3.4E+03 6.3E+07 4.7E+02 1.4E+07 1.3E+02 4.6E+06 3.7E+01 1.5E+06

Cout (ppt)

1.5E+01 3.1E-01 5.8E-01 1.0E+00

1.2E+03 3.8E+01 1.1E+02 5.4E+01

2.3E+02 ND1 ND ND

Diacid/hydroxycarbonyl acid (saturated)2 118 1.6E-02 7.6E+02 8.6E+00 132 4.5E-03 2.4E+02 1.8E+01

1.3E+01 9.5E+00

1.2E+00 1.2E-01

3.2E+00 2.3E+00 6.5E+00 1.8E+00

9.1E+01 4.7E+03 3.6E+01 1.4E+01

ND 4.4E+02 2.9E+00 3.6E+00

3.1E+01 1.1E+01 3.9E+00 3.6E+00 4.0E+00

5.3E+00 3.1E+01 7.3E+01 1.9E+02 3.7E+01

2.3E+00 7.3E+00 1.9E+01 7.7E+00 4.6E+00

Aromatic acid 1.4E+00 7.0E+04

1.2E+01

2.7E-03

2.2E+01

1.5E+01 3.9E+00

ND 2.9E+00

76 90 106 104 74 88 102 116 130

C7H6O2

Benzoic acid

122

C8H8O3

Hydroxymethyl benzoic acid 152

Hydroxy acid 5.9E+00 1.8E+05 1.2E+00 4.4E+04 6.9E-03 2.9E+02 3.0E-01 1.3E+04 Carbonyl acid (saturated) 1.0E+02 3.1E+06 2.1E+01 7.5E+05 5.2E+00 2.2E+05 1.5E+00 6.8E+04 4.4E-01 2.3E+04

1.7E+02


Page 27 of 33


Table 4. Campaign-average concentration, percentage of total indoor (or outdoor) concentration, and indoor/outdoor ratios (I/O) for carboxylic acid categories identified in this study. Category Monoacids1 Diacids or hydroxycarbonyl acid Hydroxy acids Carbonyl acids Aromatic acids Total 1

Indoor Concentration (ppt) 1515.5

Outdoor %

Concentration (ppt)

%

22.3

230.1

21.9

6.6

24.2

0.4

14.9

1.0

1.7

4879.5 351.5 21.3 6791.9

71.8 5.2 0.3

749.2 51.7 3.7 1049.6

71.4 4.9 0.4

6.5 6.8 5.8 6.5

I/O

Monoacids do not include acetic acid.



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Figures

TOC art


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Fig. 1. Scatter plot of estimated (using Equation 1) and measured sensitivity of 13 carboxylic acid standards.



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Fig. 2. (a) Campaign-average diurnal cycle of identified carboxylic acids grouped into monoacid, diacid, hydroxy acid, carbonyl acid, and aromatic acid categories. The average diurnal cycle for each category was calculated as the average of the normalized (normalized to the maximum) diurnal profiles for the acids in that category. (b) The indoor-to-outdoor ratio of the carboxylic acids commonly identified indoors and outdoors (pink). The black line represents the average and the dashed line indicates a ratio of 1.


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Fig. 3. (a) Campaign-average production rate of the normalized signal of the carboxylic acid classes in Fig. 3a and nighttime O3 concentration as a function of time. The enhancement rate for each acid class at hour X was calculated as the normalized signal at hour X+0.5 less the normalized signal at hour X-0.5 divided by one hour. (b) Campaign-average diurnal cycle of indoor and outdoor formic acid. The error bars represent standard deviations.



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Fig. 4. Box and whisker plots of indoor carboxylic acids as a function of CO2 in daytime for (a) octanoic acid, (b) nonanoic acid, (c) decanoic acid, (d) undecylic acid, (e) dodecanoic acid, (f) hexenoic acid, (g) nonenoic acid, (h) lactic acid, (i) pyruvic acid, (j) oxohexanoic acid, (k) oxopentanoic acid, and (l) benzylsuccinic acid. The data were binned based on CO2 concentration, with the bin width being 150 ppm. The upper and lower bounds of the boxes represent the concentration quantiles, and the whiskers represent the 91st and 9th percentiles. The formulae shown represent the acid molecules, not the deprotonated ions detected.


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Fig. 5. Time series of lactic acid (C3H6O3), octanoic acid (C8H16O2), and CO2 measured indoors.


Gas-Phase Carboxylic Acids in a University Classroom: Abundance

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