Control of reactive distillation process for production of ethyl acetate

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ELSEVIER The Science of the Total Environment 153 (1994) 267-273

the Science of the Total ~ m e n t

Short communication

The indoor/outdoor relationship of acid aerosols in Taipei

**Chang-Chuan Chan*, Hsiu-Fen Hung, Li-Fan Fu**

Institute of Occupational Medicine and Industrial H2,giens, College of Public Health, National Taiwan University, IOn. 1447, No. L Jen-Ai Rd., 1st See., TavTe~ Taiwan, RO.C

Received 29 September 1993; accepted 12 October 1993

Abstract

In order to estimate personal exposure to acid aerosols indirectly, we designed this study to characterize the relationship between indoor and outdoor acid aerosol concentrations in Taipei. The acid aerosols were collected using a Harvard-EPA annular denuder and analyzed by ion chromatography. Samples were collected on 4 sampling days at two sampling sites in May, 1992, and on 39 sampling days at four sampling sites from January to April, 1993. At each sampling site, both indoor and outdoor samples were collected concurrently. On each sampling day, we collected two 12-h samples in the summer and one 24-h sample in the winter. We found that acid aerosols in Taipei are rich in SO2, NI-I3, ammonium nitrate and sulfate. We also found that indoor/outdoor ratios w e r e > 1 for H N O 2, NI-I 3, NO~, NH~ and H +, but < 1 for SO 2, SO 2- and HNO3. The outdoor SO 2- levels were correlated with outdoor SO 2 levels, while the indoor SO 2- levels were correlated with indoor NH~ levels. The indoor SO 2- may originate from outdoor SO 2 emissions and then penetrate indoors in the form of (NH4)2SO 4 or (]~I-I4)HSO 4 in Taipei. We conclude that mobile sources can be one important source of acid aerosols in Taipei. The concentrations of acid aerosols, indoors and outdoors, are possibly controlled by factors such as hot and humid weather and crowded living space in Taipei.

Keywords: Acid aerosols; Indoor/outdoor ratio; Denuders; Ion chromatography; Taipei

1. Introduction

Many epidemiological and clinical studies have indicated that exposure t o acid aerosols, i.e. SO 2-, NO~-, NO 2, SO 2 and NO 2, may cause respiratory health effects [1-4]. In order to accurately quan- tify personal exposure and identify the sources of

* Corresponding author.

acid aerosols, some studies were designed to investigate the relationship between indoor and outdoor measurements of acid aerosols [5,6]. The prevalence of asthma in children aged from 7 to 15 years has increased 4.75 times for males and 3.78 times for females between 1974 and 1986 in Taipei, Taiwan [7]. The deterioration of air qual- ity has been considered as an important con- tributing factor to such a rising trend in asthmatics in the Taipei metropolitan areas. According to

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the ambient air monitoring data of the Taiwan Environmental Protection Agency (EPA), PM10, NO 2 and SO2 have been the three most important pollutants in urban areas for years. In 1991, the annual average concentrations of PM10, SO 2 and NO 2 were ~ 77-119/zg m -3, 21-27 ppb and 22-37 ppb in Taipei, respectively [8]. In addition to these primary air pollutants, we think that various secondary air pollutants, such as acid aerosols, can be formed in the climate of Taiwan, which is usually warm, humid and with much sunshine. This study is, therefore, designed to construct a database of acid aerosols in ambient air in Taipei, and to compare the relationship between indoor and outdoor acid aerosol levels in non-asthmatic and asthmatic homes. These measurements will serve as a database of exposure in a large follow-up study on investigating the causal relationship between asthma and acid aerosols in Taiwan.

the filters downstream. In analysis, all denuders and filters are first extracted with either deionized water or extraction solution and then analyzed by ion chromatography (Dionex 4500i). Anions, such as, CI-, NO 2, NO 3 and SO 2-, were analyzed using a Dionex AS4A column (eluent, 1.8 m M Na2CO 3 and 1.7 m M NaHCO3; regenerent, 0.025 N H2SO4). Cations, such as, NH~, Na ÷ and K ÷, are analyzed using a Dionex CS10 column (eluent, 20 mM HC1 and 2 m M DL-2,3-diaminopropionic acid monohydrochloride; regenerent, 100 m M te- trabutyl ammonium hydroxide). An aliquot of the Teflon filter extract was analyzed for aerosol acidity (H +) by a pH method which considered the influence of the extraction solution and the exis- tence of the filter [11]. Among the analyzed anions and cations, only five species, HNO2, HNO3, SO 2, NO 3 and SO 2-, were measured both in summer and winter. The data on CI-, Na ÷ and K ÷ will not be discussed in this paper.

2. Materials and m e t h o d s

2.1. Sampling and analysis

We used Harvard-EPA annular denuder sys- tems to collect acid aerosols outdoors in two seasons and indoors in the summer only [9,10]. Briefly, the air enters the system at a flow rate of 10 1 min -1 through a glass inlet-impactor which removes coarse particles (d a < 2.5/zm). The air then passes through a sodium chloride coated denuder which collects the acidic gas H N O 3, two sodium carbonate coated denuders which collect the acidic gases SO 2 and HNO2, and a citric acid coated denuder which traps N H 3. Downstream of the denuders a Teflon filter is mounted to collect fine particles and a nylon filter to trap volatilized vapors from the previous filter. In the winter, samples inside the homes of asthmatic children were collected with a personal annular denuder system, which is a miniaturization of the previous one and operates at a flow rate of 2 1 min-1. The personal annular denuder system consists of two short denuders. The first one is coated with sodium carbonate to collect gaseous HNO2,

H N O 3 and SO 2, and the second one is coated with citric acid to collect N H 3. The particulate nitrate, sulfate and ammonium are collected on

2.2. Sampling site and period

We conducted our field sampling in two seasons. The summer season was in May, 1992, while the winter season was from January to April, 1993. In the summer, we monitored indoor and outdoor acid aerosols in two homes of non-asthmatics for 4 days. In each sampling day, two 12-h samples were taken to compare the difference between the day time and the night time. In the winter, the sampling scale was expanded. We monitored 2 days a week in four outdoor sites near the residence of 18 asthmatic children re- cruited from the pediatric clinic of the National Taiwan University Hospital. In each sampling day, one 24-h sample was taken. The outdoor samplers were placed ~ 3 m above the ground, while the indoor samplers were ~ 1.5 m above the ground and ~ 1 m away from the walls.

3. Results and discussion

The results of the summer and winter sampling are summarized in Table 1. Among the gaseous components, the outdoor acid aerosols in Taipei are rich in SO2, while poor in H N O 3. In the summer, the average outdoor concentrations of gaseous HNO2, H N O 3 and SO 2 were 1.65, 0.52

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CC. Chan et al. / Sci. Total Environ. 153 (1994) 267--273 269

Table 1

Summary statistics for indoor and outdoor concentrations of particulate (nmol m - 3 ) and gas (ppb) compounds measured during the summer and winter sampling seasons

Compound Season Location N Mean S.D.

HNO2 Summer Indoor 16 6.6 3.7

Outdoor 15 1.7 0.9

Winter Indoor 96 8.0 4.8

Outdoor 36 2.7 1.5

HNO 3 Summer Indoor 16 0.7 1.2

Outdoor 15 0.5 0.5 Winter Indoor 90 0.3 0.2 Outdoor 38 0.5 0.6 SO 2 Summer Indoor 16 2.5 1.9 Outdoor 15 7.6 4.6 Winter Indoor 100 2.4 2.9 Outdoor 37 8.2 4.6 N i l 3 Summer Indoor NA NA NA Outdoor NA NA NA Winter Indoor 98 43.7 18.5 Outdoor 38 8.0 5.5

NO~- Summer Indoor 15 7.8 8.1

Outdoor 15 17.5 12.0

Winter Indoor 98 76.3 62.8

Outdoor 38 52.3 43.3

SO~- Summer Indoor 15 58.8 38.8

Outdoor 15 68.7 40.8 Winter Indoor 94 98.3 67.0 Outdoor 38 113.5 193.7 NH~ Summer Indoor NA NA NA Outdoor NA NA NA Winter Indoor 90 250.4 187.9 Outdoor 37 176.7 116.3 H ÷ Summer Indoor NA NA NA Outdoor NA NA NA Winter Indoor 101 6.0 13.1 Outdoor 39 4.6 11.6

N, number of observations; NA, samples not analyzed. Samples in which the filter extract was clearly alkaline were excluded for n + .

and 7.6 ppb, respectively. In the winter, the aver- age outdoor concentrations of gaseous

HNO2,

HNO 3 and SO 2 were 2.7, 0.5 and 8.2 ppb, respec- tively. The average concentrations of NH 3 out- doors was ~ 8.0 ppb in the winter. Among the particulate matter, although SO42- is the domi- nant component outdoors, the concentrations of NO 3 are also relatively high. In the summer, the average outdoor concentrations of particulate NO~- and SO42- were 17.5 and 68.7 nmol m -3, respectively. In the winter, the averaging outdoor concentrations of particulate NO 3 and SO4 z-

were 52.3 and 113.5 nmol m - 3 , respectively. The average concentrations of NH~- outdoors was ~ 176.7 nmol m - 3 in the winter. The nitrogen-related sources seemed to play a relatively important role in the formation of acid aerosols in Taipei. One major local source of nitrogen oxides is the tail-pipe exhausts of ~ 10 million vehicles and motorcycles in Taipei metropolitan areas. The N H 3 is possibly emitted from sources such as densely populated residential areas in Taipei and landfills and rice fields in the suburban areas. The relatively low aerosol acidity in Taipei, conse-

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270 C C Chan et al. / ScL Total Enciron. 153 (1994) 267-273

quently, is a result of the neutralization effects by these ammonia-related sources. The indoor acid aerosols are rich in H N O 2, NH3, SO 2- and NH~-. In the summer, the average indoor concentrations of H N O 2 and SO 2- were 6.60 ppb and 58.8 nmol m -3, respectively. In the winter, the average indoor concentrations of H O N O , N H 3, SO 2- and NH~ were 8.0 ppb, 43.7 ppb, 98.3 nmol m -3 and 250.4 nmol m -3, respectively. In addition to the outdoor sources discussed above, indoor H N O 2 can also be partially produced by some indoor sources, such as gas stoves in kitchens. We found no significant difference between the acid aerosol concentrations during day and night in the summer sampling. However, a seasonal difference was significant for some species. The o u t d o o r concentrations of SO2 and H N O 3 in the summer were significantly higher than those in the winter, while SO 2- and NO~- were significantly higher in the winter ( P < 0.05). T h e indoor concentrations of SO 2- and NO 3 in the winter were significantly higher than those in the summer ( P < 0.05) (Table 2).

In o r d e r to investigate the relationship between indoor and o u t d o o r acid aerosols, we first divided indoor measurements by matched outdoor measurements and then calculated geometric means and standard deviations of the ratios (Table 3). We found that i n d o o r / o u t d o o r ratios were > 1 for HNO2, NH~-, H + and

NH3,

but < 1 for SO 2 and H N O 3. For SO 2-, the i n d o o r / o u t d o o r ratios were ~ 1. The higher N H 3 concentrations indoors are probably due to greater emissions from humans during their indoor activities. The con- tribution of indoor sources and the lower decay rate of H N O 2 indoors are two possible reasons for the higher indoor H N O 2 concentrations. The commonly used gas stoves in kitchens and living rooms are believed to be the main indoor sources of H N O 2 emissions in Taipei. Additionally, the rate of H N O 2 photolysis indoors is much slower than outdoors due to the availability of sunlight. In contrast, N O 2 and SO 2 are mainly from outdoor sources, such as gasoline-powered motorcycles and cars, and diesel-powered trucks and buses in Taipei. This may explain the p h e n o m e n o n of higher NO 2 and SO 2 concentrations outdoors. In the urban atmosphere, H N O 3 is mainly formed through the photochemical reactions of N O 2 , 0 3

Table 2

The P-value of the t-test for the comparison of particulate and gas concentrations in s u m m e r and winter

Compound Indoor Outdoor

P-value P-value HNO 2 0.504 0.124 H N O 3 0.220 0.023 S O 2 0.016 0.000 NO~- 0.000 0.000 S042 - 0.000 0.002

and volatile organic compounds. T h e r e f o r e the rate of H N O 3 formation is higher in o u t d o o r environments [12]. A n o t h e r possibility is that the deposition velocity of H N O 3 is greater in indoor environments where more surfaces are available for the adsorption of H N O 3 [13].

We also found a significant seasonal change of i n d o o r / o u t d o o r ratios for H N O 2 which de- creased from 4.29 to 2.59 from summer to winter. This indicated that the source strength for gener- ating H N O 2 was greater in the summer in these participant's houses. Moreover, the p o o r regres- sion slopes between indoor and o u t d o o r measurements for acid aerosols (Table 4) indicated that the concentrations of indoor acid aerosols were influenced by some mechanisms in forming and removing acid aerosols indoors. In contrast, the i n d o o r / o u t d o o r ratios for N O 3 increased from 0.44 to 1.32 from summer to winter. The increase of i n d o o r / o u t d o o r ratios for N O 3 is possibly due

Table 3

Geometric mean (and geometric S.D.) of indoor/outdoor concentration ratios for particulate and gaseous compounds of acid aerosols

Compound Summer I/O ratio Winter I/O ratio

(geomeric m e a n , (geometric mean,

(G.S.D.)) (G.S.D.)) HNO 2 4.29 (2.85) 2.59 (1.89) HNO 3 0.98 (1.65) 0.69 (2.44) S O 2 0.24 (2.46) 0.23 (2.30) NO 2 NA 0.91 (1.11) NH 3 NA 5.65 (1.90) NO 3 0.44 (1.73) 1.32 (1.51) SO 2- 0.74 (2.11) 0.96 (1.87) NH~" NA 1.28 (1.70) H + NA 1.24 (4.17)

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Table 4

R e g r e s s i o n of indoor concentrations o n t h e outdoor concentrations

Gas, aerosol N Slope (SE) Intercept (SE) R'2

w H N O 2 78 0.56 (0.10) 1.37 (0.12) 0.28 w HNO3 76 0.35 (0.06) - 1.99 (0.09) 0.28 w s o 2 82 0.77 (0.13) - 1.02 (0.28) 0.30 WNH3 81 0.26 (0.06) 3.20 (0.13) 0.19 WNO 2 66 0.72 (0.15) NS 0.25 WNO~ 82 0.83 (0.05) 0.93 (0.19) 0.79 wSO42- 79 0.61 (0.08) 1.71 (0.34) 0.46 " N H ~ 73 0.81 (0.07) 1.20 (0.36) 0.65 s H N O 2 7 0.72 (0.15) NS 0.59 NS, n o t significant at P < 0.05; N, n u m b e r o f observations. SSummer samples. w W i n t e r samples.

Relationships not included in t h e table had non-significant slopes ( P < 0.05). Table 5

Correlation coefficients for particulate a n d gaseous c o m p o u n d s

H N O 2 H N O 3 S O 2 N H 3 NO~- SO 2 - NH~" H + H N O 2 H N O 3 S O 2 S l - I 3

NO7

so42-

Nn~

H + a _ _ b ¢__ d - - 0.83 N A - - - - N A N A N A - - 0.72 N A N A 0.73 0.84 - - 0.74 - - . . . . N A . . . . N A - - 0.73 - - - - - --- -- -- N A N A - - - - 0.73 N A N A 0.70 . . . . N A N A N A N A N A N A N A N A 0.72 - - 0.74 - - -- N A N A -- N A N A - - 0.77 - - 0.72 0.75 - - N A N A N A N A 0.79 N A N A

aIndoors during s u m m e r sampling period. b O u t d o o r s during s u m m e r sampling period. CIndoors during winter sampling period. d O u t d o o r s during winter sampling period.

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Table 6

Summary statistics for indoor and outdoor concentrations of aerosol (nmol m -a) and gas (ppb) species measured in Taipei, Boston and Chicago

Compound Location Mean Concentrations

Taipei N Boston N Chicago N

H N O 2 Indoor 8.0 (4.8) 96 5.4 (3.4) 29 Outdoor 2.7 (1.5) 36 0.7 (0.6) 24 0.5 (0.3) 81 HNO 3 Indoor 0.3 (0.2) 90 0.03 (0.06) 29 Outdoor 0.5 (0.6) 38 0.6 (0.5) 24 0.3 (0.4) 81 SO 2 Indoor 2.4 (2.9) 100 0.4 (0.4) 23 Outdoor 8.2 (4.6) 37 4.7 (2.5) 24 8.1 (7.4) 81 NO 2 Indoor 45.2 (1.6) 100 Outdoor 67.4 (1.4) 37 NH 3 Indoor 43.7 (18.5) 98 19.3 (6.4) 18 Outdoor 8.0 (5.5) 38 1.1 (0.9) 18 2.3 (2.4) 81 NO 3 Indoor 76.3 (62.8) 98 5.5 (5.6) 30 Outdoor 52.3 (43.3) 38 11.8 (11.3) 24 67.9 (61.3) 81 SO 2- Indoor 98.3 (67.0) 94 31.5 (17.0) 24 Outdoor 113.5 (193.7) 38 42.6 (15.3) 24 57.8 (60.5) 81 NH~" Indoor 250.4 (187.9) 90 66.4 (34.6) 18 Outdoor 176.7 (116.3) 37 66.8 (32.2) 18 152.2 (127.2) 78 H + Indoor 6.0 (13.1) 101 2.4 (1.8) 11 Outdoor 4.6 (11.6) 39 11.1 (8.6) 18 7.7 (11.6) 81 N, number of observations.

to the different formation rates of

NH4NO 3

indoors during the two seasons. The correlation coefficients for different species of acid aerosols can be used to investigate the mechanisms of aerosol formation (Table 5). The outdoor SO2-1evels were correlated with outdoor SO2 levels in the summer (r = 0.73). The indoor SO2-measurements, however, were correlated with indoor NH~- measurements in the winter (r = 0.79). The molar concentration ratios between H ÷ and SO 2- averaged at 0.2 indoors and 0.1 outdoors. It has been reported that the dominant species of acid aerosols collected on the filters are

mainly (NH4)2SO4, (NH4)HSO 4

a n d (NH4)3H(SO4) 2 when the ratio was < 1 [14]. Therefore, indoor SO 2- in Taipei is believed to originate from outdoor SO 2 emissions, and then penetrate indoors in the form of (NI-I4)2SO4,

( N H 4 ) H S O 4 o r ( N H 4 ) 3 H ( S O 4 ) 2. Apparently, the acidity of both indoor and outdoor aerosols has been largely neutralized by ammonium in Taipei. In comparison with the components of acid

aerosols in Boston and Chicago (Table 6), the acid aerosols in Taipei are similar to those in Chicago [15]. Both sulfate and nitrate are the main sources of acidity in the aerosols in Taipei and Chicago. In contrast, the main cause of acidity in the aerosols in Boston is sulfate. The aerosols in Taipei and Chicago also contained more ammonium. The ammonium concentrations in Taipei and Chicago are approximately twice as high as the concentrations in Boston. Accord- ingly, the aerosols in these two cities show less acidity. The relatively high H N O 2 concentrations in the aerosols in Taipei is a significant feature compared with the other two cities. The emissions from a large number of cars and motorcycles without catalysts in the congested traffic provide stocks of NO x in the atmosphere in Taipei. The NOx is then quickly transformed into H N O 2 through photochemical reactions in the hot (25-30°C) and humid (80-90% RH) weather in Taipei. Indoors, the H N O 2 can also be emitted directly from gas stoves, which are commonly

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C C Chan et al. /Sci. Total EnMron. 153 (1994) 267--273 273

used for cooking and making tea in Taiwanese homes.

4. Conclusion

We have identified that ammonium, nitrate and sulfate are important components of acid aerosols in Taipei. The main sources of acid aerosols are vehicles outdoors and gas stoves indoors. In most cases, the outdoor sources play a more important role in forming the acidity in aerosols. The formation, removal and penetration of acid aerosols indoors and outdoors are strongly influenced by climatic conditions and house settings. The year- round high temperature and high humidity are favorable meteorological conditions in forming acid aerosols, such as H N O 2. The crowded living spaces provide a strong source of NH 3 emissions as well as a large surface for the deposition of acid aerosols [16]. However, more studies are needed to understand the kinetics and mechanisms in controlling the apportionment of various species in acid aerosols in Taipei.

Acknowledgements

We would like to thank the support of the Taiwan Environmental Protection Agency (con- tract No. EPA-82-E3F1-09-01) in funding this study. We would especially like to thank all the participants who allowed us to measure acid aerosols in their homes.

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