Spatial Variation and Relationship of Indoor / Outdoor PM 2 . 5 at Residential Homes in Guangzhou City , China

Fine particles (PM2.5) were measured using Airmetrics mini-volume portable samplers during 2nd July and 13th August, 2004 inside and outside nine homes located in generic urban area, roadside area and industrial plant area in Guangzhou city, China. The indoor and outdoor PM2.5 average concentrations (67.7 and 74.5 μg/m) were about two times higher than the new guideline of WHO, and higher than the levels in other studies (New York, London, Amsterdam, Brisbane, Osaka and Hong Kong, etc). Both indoor and outdoor average PM2.5 concentrations in roadside area (73.5 and 79.4 μg/m) and in industrial plant area (73.4 and 92.9 μg/m) were higher than those in generic urban area (56.2 and 51.2 μg/m). The average I/O ratio in generic urban area, roadside area and industrial plant area were 1.10, 0.93 and 0.79, respectively. The indoor/outdoor correlations of PM2.5 in generic urban area, roadside area and industrial area were from strong (R = 0.70, p < 0.001, n = 11) to moderate (R = 0.54, p = 0.007, n = 12) and to poor (R = 0.17, p = 0.088, n = 12). Strong indoor/outdoor PM2.5 correlation with I/O > 1 in generic urban area indicated excellent ventilation condition at there, good indoor/outdoor PM2.5 correlation with I/O < 1 in roadside area suggested that the indoor PM2.5 were mainly from the outdoor air, and poor indoor/outdoor PM2.5 correlation in industrial plant area was ascribed to seldom ventilation in one home (IS2).


INTRODUCTION
Particulate matter (PM), especially respirable PM 2.5 (particles with aerodynamic diameter less than 2.5 µm), is the most ubiquitous and most complicated air pollutant in urban area (Ho et al., 2003).There is now a large body of epidemiological evidences associating exposure to ambient particles with short and long-term effects on health (Anderson, 2000;Kim et al., 2000;Kulkarni, 2006).With improvement of measurement techniques, the effects became clearer when smaller particle sizes were considered (Lippmann and Ito, 2000;Wichmann and Peters, 2000).Besides health effects, atmospheric fine aerosol particles play an important role in controlling a number of atmospheric processes such as the deposition of different compounds, the optical properties, etc (Molnár et al., 1999).From these points of view, it is still crucial to know the concentration and distribution of the fine particles in order to develop effective strategy for the control of fine aerosol pollution and the remove of associated problems.PM is usually generated from many different outdoor sources, such as automobile exhaust, industrial production processes, secondary conversion from gaseous pollutants (Cao et al., 2003) and also indoor sources, such as smoking (Cao et al., 2005), cooking (Naeher et al., 2000) and other activities (Chan, 2002).Generally, people spend most of their time indoor, but the majority of particulate concentration data is based on measurements conducted outdoors.Outdoor particulate concentrations may not be reliable indictors of indoor and personal particulate exposures (Geller et al., 2002).During the past few decades, more and more researchers have been aware of that; accordingly researchers in different parts of the world have begun to conducted studies on indoor particle levels.
In Asia, Funasaka et al. (2000) from his study in Osaka, Japan concluded that the mean indoor PM 2.5 concentrations in roadside site A mainly effected by commuter vehicles, in roadside site B mainly effected by high volume of diesel traffic, and in background site C were 20, 22 and 18 µg/m 3 , respectively, and the corresponding outdoor values were 37, 23 and 18 µg/m 3 , respectively.Tsai et al. (2000) found that PM 2.5 levels in living room and in ambient were 137 and 140 µg/m 3 for shop houses in Bangkok, Thailand on moderately busy streets, 184 and 62 µg/m 3 for shop houses near busy streets, and 64 and 153 µg/m 3 for near less bust streets, respectively.In a study conducted by Cao et al. (2005) found that the average indoor and outdoor concentrations of 24 h PM 2.5 were 56.7 and 43.8 µg/m 3 , respectively, at residential homes in Hong Kong.It was found that the concentrations of air particulates of indoors sometimes were much higher than that of outdoors (Lee et al., 2000;Ramachandran et al., 2000;Adgate et al., 2002;Ramachandran et al., 2003;Cao et al., 2006).Therefore, studies on indoor particles characteristics are as important as the outdoors'.Some PM studies had been done in overseas cities, but seldom were performed in Mainland China and the database there is not comprehensive.In Guangzhou city, China, studies on fine particles are scarce, and no specific research on simultaneous indoor and outdoor PM 2.5 characteristics has been carried out.The main objective of this study is to provide quantitative information on the indoor and outdoor mass concentrations of PM 2.5 at 9 selected locations of three type areas (generic urban, roadside and industrial plant area) in Guangzhou city and to investigate spatial variation in different areas.

METHODS
Guangzhou city (22º36′-24º18′N, 112º33′-114º35′E) is one of most developed and industrialized city in south China (Cao et al., 2003), with high population density of 1600 person/km 2 now, where the amount of population and motorcar is increasing year by year, excessive air pollutants are released, and atmospheric haze happen more often.Urban development and economy growth along with the acutely increasing population, motor vehicles and industrial activities would result in constantly elevated pollution levels.
Nine sampling sites (indoor and outdoor simultaneously) classified in 3 kinds of areas (general urban area, roadside area, the area around industrial plant) of Guangzhou city were selected for this study.All indoor environments were located in domestic home.Three sampling sites (GS1, GS2 and GS3) were selected in general urban area without impacts of heavy vehicle traffic and industrial production, three sampling sites (RS1, RS2 and RS3) were selected by roadside with heavy motor vehicle traffic, and three sampling sites (IS1, IS2 and IS3) were selected around industrial plant.The nearest distance between two sites of all nine sites was about 1 kilometer, and the farthest distance between two sites was over 20 kilometers.The purpose of considering the distance was to avoid the interaction with each other sites.Detailed properties of nine sampling sites are given in Table 1.The locations of sites were shown in Fig. 1.Indoor and outdoor source characters please see later analysis in the paper.

Industrial fuel combustion emission
Both the indoor and outdoor PM 2.5 samplings were collected simultaneously from 2 July to 13 August 2004 by using two mini-volume portable samplers (Airmetrics, USA) with a PM 2.5 cyclone operating with the flow rates of 5 L/min for continuous 24 hours.Two samplers were checked and calibrated in Hong Kong Polytechnic University and then sent to Guangzhou for PM 2.5 sample collection.All the samples were collected on 47 mm Whatman quartz micro-fibre filters.After collection, loaded filters were stored in a refrigerator at about 4 o C before weighing and chemical analysis.The total particulate mass was determined by weighing on an electronic microbalance with 1 µg sensitivity (Mettler M3, Switzerland).Each of the measured residences had an indoor PM 2.5 sampler located in the center of living room and kept above 1 meter distance to any objects indoors to prevent the sampler from closing to possible source emissions points.
And the outdoor one located in the balcony/platform, similarly keeping above 1 meter distance to outside wall and any possible object outside the residence.The indoor sampling heights were in the range of 1-1.5 m above ground in order to simulate the breathing zone and to avoid potential interferences from excessive re-suspension of particles.During the summer sampling period, sampling in every site repeated 4 days.
Within the sampling period, the mode wind direction was dominantly southeast and south.And the direction of all of the residences is mainly to south.The wind speed of 10 m above the ground level fell into the range of 0.8-3.5 m/s, and the average wind speed being 2.2 m/s.The temperature ranged from 25 o C-37 o C and ambient pressure varied insignificantly between 998.3 and 1007.2 hPa.The relative humidity (RH) was between 50-90%.

Spatial distribution of PM 2.5 in generic urban, roadside and industrial plant area
The indoor and outdoor PM 2.5 mass concentrations for a total of 72 samples during summer sampling period at nine sites in generic urban area, roadside area and industrial plant area were summarized in Table 2.
The indoor 24 hours PM 2.5 concentrations in generic urban area, roadside area and industrial plant area ranged from 20.9-81.7 µg/m 3 , 28.7-123.1 µg/m 3 , and 22.6-110.9µg/m 3 , respectively, while the corresponding outdoor concentrations ranged from 24.9-75.1 µg/m 3 , 38.1-111.6 µg/m 3 and 45.8-128.7 µg/m 3 , respectively.The average ranges of indoor concentrations were found wider than that of corresponding outdoor concentrations in three type areas, which was consistent with the fact that the indoor PM 2.5 is not only from outdoor environment but also from indoor activities/sources.Wider ranges were also found in roadside area and industrial plant area than in generic urban area.The range of indoor and outdoor 24 hours PM 2.5 mass concentrations in roadside area were 1.8 times and 1.5 times of that in generic urban area, the range of indoor and outdoor 24 hours PM 2.5 mass concentrations in industrial plant area were 1.7 times and 1.7 times of that in generic urban area, respectively.According to our investigation, Road 1, Road 2 and  The indoor average PM 2.5 concentrations in generic urban area, roadside area and industrial plant area were 56.2, 73.5 and 73.4 µg/m 3 , respectively, while the corresponding outdoor concentrations were 51.2, 79.4 and 92.9 µg/m 3 , respectively.Both indoor and outdoor average PM 2.5 concentrations in roadside area and in industrial plant area were higher than those in generic urban area.It indicated that traffic exhaust and industrial exhaust were the major outdoor source for concentrations of PM 2.5 .
For the sites in generic urban area, the highest indoor and outdoor 24 hours average concentrations of PM 2.5 were 67.1 and 56.5 µg/m 3 in GS2, while the lowest indoor and outdoor 24 hours average concentrations of PM 2.5 were 43.6 and 44.7 µg/m 3 in GS3, the values in GS1 (58.0 and 52.4 µg/m 3 for indoor and outdoor environments) were the medial.Analyzing the characteristics of indoor and outdoor environment and sources (Table 2), approximately there was no special obvious difference in indoor environment and sources among the three sites, except that the building and decoration year of GS3 was the oldest, the number of occupancies was the fewest, and the frequency of cooking was the lowest, to some extent which explain the result that the levels in GS3 were the lowest.According to the further investigation, the reason why the levels in GS2 were the highest might be the influence from a road 500 m far from with moderate motor vehicle emission (4000 car/trucks per hour).
For the sites in roadside area, the outdoor 24 hours average concentrations of PM 2.5 were 88.6, 80.1 and 69.5 µg/m 3 in RS2, RS3 and RS1 respectively, presenting descending order.It was consistent with the traffic flow rate of three roads.Road 2 was the busiest road with above 7000 car/trucks passing per hour, Road 3 was the moderate busy road with about 4000 car/trucks per hour, and Road 1 was the sub-moderate busy road with about 2500 car/trucks per hour.However, the highest indoor 24 hours average concentrations of PM 2.5 (87.1 µg/m 3 ) were not in RS2, but in RS3.It demonstrated there were more indoor sources for PM 2.5 in RS3.The indoor sources in RS3 was suspected to be the high frequency of cooking and a child of 4 years old often playing in indoor environment.
For the sites in industrial plant area, the higher two outdoor 24 hours average concentrations of PM 2.5 were 111.2 and 101.5 µg/m 3 in IS2 and IS3, which were also the top two values in nine sites, the lowest outdoor 24 hours average concentrations of PM 2.5 were 66.0 µg/m 3 in IS1, while the highest indoor 24 hours average concentrations of PM 2.5 were 99.8 µg/m 3 in IS3 and the lowest indoor 24 hours average concentrations of PM 2.5 were 54.6 µg/m 3 in IS2.As shown in Table 1, IS2, IS3 and IS1 were adjacent to power plants (HuangPu and HengYun), Sinopec Guangzhou Petrochemical Complex and ethylene plant of Guangzhou, respectively.Thus it can be seen the industrial exhaust from power plant and petrochemical plant affected the PM 2.5 outdoor concentrations much more severely.The indoor concentration of PM 2.5 in IS2 was the lowest, though IS2 had highest outdoor concentration.The main reason is ventilation condition in IS2 was very poor, because the door and windows of IS2 residence were always closed.

PM 2.5 mass concentration and comparison
The average indoor and outdoor PM 2.5 mass concentrations of total 72 samples in this study were given in Table 3.Also the results of other studies on PM 2.5 were compared and summarized in Table 3.The average indoor and outdoor PM 2.5 concentrations in nine sites were 67.7 and 74.5 µg/m 3 , respectively, with the indoor PM 2.5 concentrations ranging from 20.9-123.1 µg/m 3 and the outdoor PM 2.5 concentrations ranging from 24.9-130.1 µg/m 3 .A significant difference was observed between the value in this study and that in other studies of some US and Europe cities.
The concentrations of PM 2.5 in Guangzhou were 0.1~3.9times and 3.4~6.2times more than that of US in indoors and outdoors, and 1.5~5.8times and 2.0~5.4 times more than that of Europe's, respectively.It indicated that both indoor and outdoor air qualities in Guangzhou were much worst than those US and Europe cities.By comparing with other Asia studies, the concentrations of PM 2.5 in this study were around 2.1~2.8 times and 1.0~3.1 times higher than Japan's for indoors and for outdoors, respectively, and were 0.2 times and 0.7 times higher than indoor and outdoor concentrations of Hong Kong.The outdoor concentration for PM 2.5 was 0.7 times higher than that of Korea, but was slightly lower than that in Kaohsiung, Taiwan.The ratio of indoor PM 2.5 concentration of Bangkok, Thailand to that of this study was 0.95~2.7,and the corresponding ratio for outdoors was 0.8~2.1.It indicated that the indoor and outdoor PM 2.5 concentrations in Asia were similar, the fine particulate pollution level in Guangzhou was moderate but on the high side, comparative to Taiwan, better than Thailand, but worse than Japan, Korea and Hong Kong.The high PM 2.5 concentrations in this study were due to the huge population and motor vehicles and industrial production.
The new guideline of WHO for suspended particles requires PM 2.5 daily averages to be less than 25 µg/m 3 (WHO, 2006).In this study, the sampling lasted over one month, but the average PM 2.5 concentration even exceeded the daily average limit value of PM 2.5 in NAAQS.The PM 2.5 levels in Guangzhou city are very high; so more efforts should be taken for the control of fine particulate pollution, and the constitution of associated strategy and regulations.Currently there is no PM 2.5 guideline in China; therefore, more efforts also should be taken towards the establishment of a PM 2.5 guideline.

I/O ratios, indoor/outdoor correlation of PM 2.5 and source implications
The I/O ratio is an indicator for evaluating the difference between indoor concentrations and the corresponding outdoor level (Li and Lin, 2003).The I/O ratios of PM 2.5 in nine sites were shown in Table 2.It was found that I/O ratios of PM 2.5 in this study ranged from 0.49-1.19, the I/O ratios in GS1, GS2 and GS3 were 1.11, 1.19 and 0.98 with the average I/O ratio in generic urban area larger than 1 (1.10).The I/O ratios in RS1, RS2, RS3 and in IS1, IS2, IS3 were 0.90, 0.80, 1.09 and 1.00, 0.49, 0.98, with the average I/O ratio in roadside area and in industrial plant area smaller than 1 (0.93 and 0.79), respectively.It indicated that the indoor PM 2.5 levels are not always better than outdoors', especially in generic urban area and implied that there were major sources of PM 2.5 , such as smoking, cooking and cleaning solvent in indoors, which should be concerned thoroughly.Table 2 also showed that with the outdoor fine particulate concentrations increasing in roadside area and in industrial plant area, the I/O ratios were smaller accordingly.It was consistent with the results reported by Funasaka et al. (2000) and Riain et al. (2003), who found that the I/O ratio of PM 2.5 in roadside area of Osaka, Japan in December 1997 was 0.54, and the I/O ratio of PM 2.5 near a busy road in London, UK was 0.78, respectively.
The value of the coefficient of determination (R 2 ) between the indoor and outdoor data was used as an indicator of the degree to which PM 2.5 measured indoors is attributed to infiltration from outdoors (Colome et al., 1992;Clayton et al., 1993).The correlations between indoor and outdoor PM 2.5 concentrations in three different environments were shown in Fig. 2 (a-d) and tested using Pearson paired t-test.It was found that R 2 were 0.7028 (p < 0.001, n = 11), 0.5391 (p = 0.007, n = 12), and 0.1749 (p = 0.088, n = 12) for samples in generic urban area, roadside area and industrial area, respectively.The indoor/outdoor correlations of PM 2.5 in generic urban area, roadside area and industrial area were from strong to moderate to poor.It indicated that the compositions of PM 2.5 in industrial plant area were the most abundant, and the compositions of PM 2.5 in roadside area were more abundant than that in generic urban area in both indoors and outdoors.In addition, it is important to note that generally strong indoor/outdoor correlation of PM 2.5 suggests that fine particles of indoor are mainly from the outdoor environment and/or good ventilation, and poor indoor/outdoor correlation suggests indoor sources existing or very poor ventilation.In this study, in the site of generic urban area it was suspected there were much indoor sources based on the I/O ratio analysis above, therefore the reason explaining the strong correlation between indoor and outdoor PM 2.5 in generic urban area must be excellent ventilation condition and high air exchange rate.In the site of roadside, good indoor/outdoor PM 2.5 correlation accompanying with I/O ratio smaller than 1 suggested that indoor PM 2.5 were mainly from the outdoor air.In the site of industrial plant area, poor correlation between indoor and outdoor PM 2.5 was ascribed to seldom ventilation in IS2, since the correlation between indoor and outdoor PM 2.5 for the samples in IS1 and in IS3, was very strong (R 2 = 0.9065, p < 0.001, n = 8) shown in Fig. 2 (d), which indicated that the indoor fine particulate in IS1 and in IS3 were mainly from the infiltration of outdoor air.

CONCLUSIONS
This study has firstly quantified the concentrations of PM 2.5 in both indoor and outdoor air at nine locations in Guangzhou city.The average indoor and outdoor PM 2.5 concentrations during one half of month were 67.7 and 74.5 μg/m 3 , respectively, much higher than some US, Europe cities, higher than several cities in Asian, and even exceeded the newly daily limit value of PM 2.5 in WHO.These results suggested more efforts should be taken for the control of fine particulate pollution in Guangzhou city and the constitution of associated regulations.The average I/O ratio in generic urban area, roadside area and industrial plant area were 1.10, 0.93 and 0.79, respectively.The indoor/outdoor correlations of PM 2.5 in generic urban area, roadside area and industrial area were from strong to moderate to poor, which indicating the compositions of PM 2.5 in industrial plant area were most complicated.In generic urban area, the reason explaining the strong indoor/outdoor PM 2.5 correlation with I/O > 1, was excellent ventilation condition and high air exchange rate.In roadside area, good indoor/outdoor PM 2.5 correlation with I/O < 1, suggested that indoor PM 2.5 were mainly from the outdoor air.In industrial plant area, poor indoor/outdoor PM 2.5 correlation was ascribed to seldom ventilation in IS2.
Road 3 have 2500, 7000 and 4000 car/trucks per hour flux, respectively.HuangPu power plant and HengYun power plant, Sinopec Guangzhou Petrochemical Complex and ethylene plant of Guangzhou around the sites of IS1 IS2 and IS3, contributed a great lot of fuel combustion emission and industrial production techniques emission.Therefore, above PM 2.5 comparison result in different areas represented a significant amount variation of PM 2.5 emission source in roadside and industrial environments.Different traffic flow rate during daily different period might be the main reason of large range in roadside environment.Intermittent industrial exhaust emission and production differences between daytime and nighttime might be the main reasons of large range in industrial environment.

Fig. 2 .
Fig. 2. Relationships between indoor and outdoor concentrations of PM 2.5 in generic urban area (a), in roadside area (b), in industrial plant area (c) and in both IS1 and IS3 (d).

Table 1 .
Properties of indoor/outdoor sampling locations in Guangzhou city.

Table 3 .
Indoor and outdoor PM 2.5 mass concentrations in this study and comparison with other urban.