Air quality index pattern of particulate around a haulage vehicle park

Abstract This study investigated the air quality index patterns for PM2.5 and PM10 in the airshed of a haulage vehicle park located around a major highway connecting Lagos, the commercial centre of Nigeria to its other parts. Measurements of PM2.5 and PM10 were done at five different sub-parks using the aerosol mass monitor GT-331 by Met one instrument. The measured concentrations ranged between16.07–29.95 μg m−3 and 125.95–433.08 μg m−3 for PM2.5 and PM10, respectively. The air quality index (AQI) within the park with respect to PM2.5 could be described as moderate but unhealthy within at least two sub parks when PM10 is considered; hence, the health of vulnerable people could be at risk. This study establishes that vehicular activities at the park could have significant impact on the park’s ambient air quality, thus calls for appropriate regulatory measure to protect commuters plying the major highway around the park.


PUBLIC INTEREST STATEMENT
This study investigated the air quality index patterns for PM 2.5 and PM 10 in the airshed of a haulage vehicle park located around a major highway connecting Lagos, the commercial centre of Nigeria to its other parts. It showed that vehicular activities are contributory factor to impaired air quality, also it showed that ambient particulate levels are high enough to pose serious health concerns to human health, especially people with history of respiratory diseases.
The air quality index (AQI) is an index for reporting daily air quality. It informs the public on how clean or unhealthy the ambient air is with all the associated health effects which might be of concern (Sonibare, Adebiyi, Obanijesu, & Okelana, 2010). The focus of the AQI is the heath effect that can be experienced within a few hours or days after breathing unhealthy air. It has been established for six major pollutants which are ground-level ozone, PM, carbon monoxide, sulphur dioxide, volatile organic compound and nitrogen dioxide. For each of these pollutants US EPA has an established national air quality standard to protect public health. The computation of AQI requires an air pollutant concentration from a study (EPA, 2006). Table 1 summarizes the AQI, descriptor and colour codes, while Table 2 summarizes health effects for AQI values above 100.

Pollutant index above 100 Sensitive groups
Ozone People with lung disease, children, older adults and people who are active outdoors are the groups most at risk PM 2.5 People with heart or lung disease, older adults and children are the groups most at risk PM 10 People with heart or lung disease, older adults and children and the groups most at risk CO People with heart disease are the group most at risk In the study, the air quality index (AQI) around a haulage vehicle park was investigated using PM with aerodynamic diameter of 2.5 μm and 10 μm (PM 2.5 and PM 10 , respectively) as indicators. This will guide in setting emission standards for these PM fractions in Nigeria since till date the national air quality regulation only takes care of total suspended PM.

Study area description
The study area was a haulage vehicle park located in Ogere, Ogun State, Nigeria. It is located at latitude 6° 55′ 0 ″ North and longitude 3° 38′ 0 ″ East. Ogere is characterized by two types of landforms; sparsely distributed low hills and knolls of granite, other rocks of the basement complex and a flat topography. Two main climate conditions exit, the rainy season lasting between April and October with an interruption in August, while the dry season runs through November till February. The park (0.6 km 2 ) is located along Lagos-Ibadan express way in Nigeria, a dual carriage road. The park is located between 56 and 59 kilometres from Lagos on the express way. The presence of the park has led to an increase in commercial activities and an increase in the population of the studied area. Five sampling points which are representative of the park and the residential environments within the park were chosen for the study (Figure 1).

Sampling PM 2.5 and PM 10
PM with aerodynamic diameter of 2.5 μm and 10 μm (PM 2.5 and PM 10 , respectively) were measured at five designated locations within the park and its environs using the GT-331 particle dust monitor. It is a unit of equipment from Met-One instruments, handheld, battery-operated and completely portable units measuring five mass ranges of particulate: PM 1 , PM 2.5 , PM 7 , PM 10 and total suspended solids (TSS). It has a concentration range of 0-1 mg m −3 , a sampling time of 5 min, a flow rate of 2.83 l/min and measures in μg m −3 . To measure, it is switched on in the environment of interest and the measured concentration is read directly on the LCD display. During this field study, the dust monitor was positioned 1 m above ground level in order to prevent measurement of fugitive dust mobilized by tides. Measurement was carried out during the dry season at each of the designated location for three different days consecutively at an averaging time of 8 h per day. The measured concentrations were extrapolated to obtain their 24-h averaging time concentrations using the atmospheric stability formula (Bashar, Kamel, & Khaldoun, 2009) given in Equation (1). where C 0 = The concentration at the averaging period t 0 , C 1 = The concentration at the averaging period t 1 , F = Factor to convert from the averaging period t 1 to the averaging period t 0 = t 1 t 0 n n = 0.28, the stability dependent exponent. (1)

Air quality index
The AQI around the haulage park was calculated using the EPA (2006) method in Equation (2). where I p = The index for pollutant p, C p = The rounded concentration of pollutant p, BP Hi = The breakpoint that is greater than or equal to C p as given in Table 3, BP Lo = The breakpoint that is less than or equal to C p as given in Table 3, I Hi = The AQI value corresponding to BP Hi , I Lo = The AQI value corresponding to BP Lo .

Results and discussion
The mean 8-h measured ambient PM 2.5 and PM 10 concentrations were 16.07-29.95 μg m −3 (Table 4) and 125.95-433.08 μg m −3 (Table 5), respectively, which on extrapolation to 24-h averaging period became 11.81-22.02 μg m −3 for PM 2.5 (Table 6) and 92.60-318.4 μg m −3 for PM 10 ( Table 7). The mean minimum and maximum PM 2.5 and PM 10 were at S2 and S1, respectively. Sampling point S2 where the mean minimum concentration of PM 10 was recorded also had the least 8-h daily concentration for day 1 and day 3 and these could be attributed to lesser haulage vehicles as compared to other sampling points. Sampling point S1 had the highest value for 8-h daily concentration all through the 3 consecutive days and this could be attributed to its being located on the middle of the dual carriageway where there were higher volumes of traffic as compared to other sampling points. Sampling points S2, S5 and S4 are sub-parks within the haulage vehicle park with varying number of vehicles.
The ambient temperature, wind speed and relative humidity during the field campaign were ranged 28.4-35.7°C with average of 32.3°C, 0.3-1.0 m s −1 with average of 0.8 m s −1 and 34.5-68.4% with average of 52.2%, respectively. The fairly moderate wind speed and temperature could aid in re-suspension of particulate from the dried unpaved floor of the studied region. Also, relative humidity could aid in the retention of these particulates in the air shed of the host environment. Likewise, the flat topography of the terrain has little or no effect on the air quality of the study area in terms of particulate concentration.
When compared with standards, the 24-h extrapolated ambient concentrations of PM 2.5 were within the guidelines of 35 μg m −3 and 25 μg m −3 by environmental protection agency (EPA) and World Health Organization, respectively, in all the sampling points. The extrapolated values of PM 10 exceeded the accepted value of 50 μg m −3 by WHO (2005) in all the sampling locations, while the EPA 150 μg m −3 for PM 10 was breached only at S1 and S4. The high level of particulates in S1 and S4 could be attributed to the high level of the haulage vehicular activities at these points compared to other sampling points.
(2)      Table 8 are the AQI and AQI ratings at each of the sampling locations for PM 2.5 and PM 10 . AQI is an indicator of air quality based on the pollution levels for the criteria air pollutant that have adverse effects on human health and the environment. The air quality with respect to PM 2.5 could largely be described as moderate though a good AQI rating was obtained at S2. The ambient air quality at sampling points S1 and S4 were unhealthy due to the level of PM 10 in their airshed, while the air quality in S2, S3 and S5 are moderate due to less vehicular activities in these areas.

Conclusion
This study investigated the air quality index patterns for PM 2.5 and PM 10 in the airshed of a haulage vehicle park located around a major highway connecting Lagos, the commercial centre of Nigeria to its other parts. Measurements of PM 2.5 and PM 10 were done at five different sub-parks using the aerosol mass monitor GT-331 by Met-one instrument. For the purpose of assessment, the 24-h concentrations of the measured particulates were extrapolated. The result observed from this study shows that ambient particulate levels are high enough to pose serious health concerns to human health, especially people with history of respiratory diseases. The 24-h values of PM 10 exceeded the accepted value of 50 μg m −3 by WHO in all the sampling locations, while the EPA 150 μg m −3 for PM 10 was breached at two locations. Going by the AQI pattern for this area, the study concluded that it is imperative that Nigeria sets ambient standard for PM 2.5 and PM 10 as the size fractions poses deleterious effects to human, vegetation and the environment.