**2. Experimental**

## **2.1. Aerosol sampling site**

Aerosol samples were collected at a rural site in Morogoro (300,000 inhabitants) between 26 April and 10 May 2011. This site is located at about 200 km west of the Indian Ocean and the city of Dar es Salaam, a business capital in Tanzania (Fig. 1). The samples were collected at Solomon Mahlangu Campus of Sokoine University of Agriculture (06°47'41"S, 37°37'44"E, altitude 504 m a.s.l.). This site is located about 5 km from Morogoro central area and major road systems and possible aerosol sources include biomass burning, agriculture, livestock and soil dust. Approximately 70% of this area is covered by vegetation and about 15% with pasture field. Conversely, tropical savannah is the most important land cover in large part of the sampling site.

## **2.2. Aerosol collection**

Two samplers were used in parallel to collect aerosol particles: a "Gent" PM2.5 and PM10 filter holder each with two quartz fibre filters (Whatman QM-A) in series. Quartz fibre filters can adsorb volatile organic compounds (VOCs) causing positive artifacts when measuring PM and particulate OC. On the other hand, semi-volatile organic compounds (SVOCs) in aerosols may partially evaporate during sampling resulting in negative artifacts (Turpin et al., 2000; Mader et al., 2003; Hitzenberger et al., 2004). The quartz fibre filters were pre-fired at 550 °C during 24 h before use. Samplers operated at a flow rate of 17 L/min and were mounted on grass survey at SMC synoptic station approximately 2.7 m above ground level. The sampling was carried out approximately in 24 h intervals and exchange of filters during sampling periods was done at 7:30 am. A total of 11 sets of actual filed samples and 2 blanks were collected for each sampler and used in this chapter. After sampling the exposed filters were folded in half face to face, placed in polyethylene plastic bags and kept frozen at -4 °C during storage and transported cool to the laboratory of research and development in chemistry (LPQ) at the Institute of chemistry, Federal University of Bahia (UFBA). The samples were stored in a freezer at −20 °C prior to analysis. All the procedures were strictly quality-controlled to avoid any possible contamination of the samples.

**Figure 1.** Location of the sampling site in Morogoro, Tanzania

During the sampling period meteorological data were collected from the site. The daily winds were predominantly south-easterly with an average speed of 6.8 m/s. Average temperature was 26.8 oC and average relative humidity was 73%. The recorded maximum temperature and relative humidity were 29.8 °C and 79.5%, while minimum values were 23.7 °C and 63.5%, respectively. During the campaigns 5 days hand rainfall of a total 19.9 mm.

## **2.3. Aerosol analyses**

204 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

extensively been reported.

**2. Experimental** 

the sampling site.

**2.2. Aerosol collection** 

**2.1. Aerosol sampling site** 

Chemical composition of PM2.5 and even that of PM10 aerosols is important to gain insights into sources and of their toxicity and to evaluate effectiveness of abatement strategies for relevant emission sectors. Particulate matter (PM) with aerodynamic diameter less than 2.5 μm (PM2.5) exhibited stronger relation with health than those with aerodynamic diameter less than 10 μm (PM10), but other studies have reported a strong potential of PM10 to human health (Salma et al., 2002; Kappos et al., 2004). Most studies on low molecular weight carboxylic acids and their related compounds (Limbeck et al., 2001; Limon-Sanchez et al., 2002; Kawamura & Yasui, 2005) and major ions (Harrison et al., 2004; Karthikeyan & Balasubramanian, 2006; Mariani et al., 2007; Kundu et al., 2010; Mkoma et al., 2010) have

In Africa similar aerosols measurements especially of organic components are missing. Therefore, a full scenario of air quality is far from being revealed because some pollutants including carboxylic acids have not been measured. The knowledge of elucidating chemical composition, levels, and source profiles of aerosols in the Tanzania atmosphere remains a challenge and is needed for both scientific and policy reasons. The continuous changes in socioeconomic and political environments in Tanzania result in changes in development, particularly in transport, industry, energy, and construction sectors. This chapter reports for the first time in Tanzania, composition of low molecular weight carboxylic acids in PM2.5 and PM10 aerosol samples collected from a rural background atmosphere in Morogoro. An insight of characteristics of water-soluble inorganic ions is also discussed in this chapter.

Aerosol samples were collected at a rural site in Morogoro (300,000 inhabitants) between 26 April and 10 May 2011. This site is located at about 200 km west of the Indian Ocean and the city of Dar es Salaam, a business capital in Tanzania (Fig. 1). The samples were collected at Solomon Mahlangu Campus of Sokoine University of Agriculture (06°47'41"S, 37°37'44"E, altitude 504 m a.s.l.). This site is located about 5 km from Morogoro central area and major road systems and possible aerosol sources include biomass burning, agriculture, livestock and soil dust. Approximately 70% of this area is covered by vegetation and about 15% with pasture field. Conversely, tropical savannah is the most important land cover in large part of

Two samplers were used in parallel to collect aerosol particles: a "Gent" PM2.5 and PM10 filter holder each with two quartz fibre filters (Whatman QM-A) in series. Quartz fibre filters can adsorb volatile organic compounds (VOCs) causing positive artifacts when measuring PM and particulate OC. On the other hand, semi-volatile organic compounds (SVOCs) in aerosols may partially evaporate during sampling resulting in negative artifacts (Turpin et al., 2000; Mader et al., 2003; Hitzenberger et al., 2004). The quartz fibre filters were pre-fired

For particulate mass measurements, the filter samples were weighed before and after sampling with an analytical microbalance balance Mettler Toledo MX5 (reading precision 1 μg). Before weighing, the filters were conditioned in a chamber equipped with hydro-

thermometer clock at a temperature of 20 °C and relative humidity of 40% for 48 h and the weightings were done under these conditions.

Characteristics of Low-Molecular Weight

Carboxylic Acids in PM2.5 and PM10 Ambient Aerosols From Tanzania 207

site in Tanzania are in line with few available other data sets for rural sites in Southern Africa (Nyanganyura et al., 2007). They are also comparable to or lower to other sites in Europe and Asia (Van Dingenen et al., 2004; Gu et al., 2010; Maenhaut et al., 2011; Ram &

Table 1 present mean total concentrations and range of carboxylates (TCAs) which were 23.7±6.5 ng/m3 (range: 13.3-36.5 ng/m3) in PM2.5 and 36.4±12 ng/m3 (range: 10.7-58.2 ng/m3) in PM10 aerosols. Oxalate and malonate were most abundant carboxylates in PM2.5 accounting for 32.5% and 31.85% of total carboxylates, respectively, whereas in PM10 acetate was most abundant accounted for 62.5% of total carboxylates followed by oxalate which accounted for 32.6% of total carboxylates. Other studies have also reported oxalates to be most abundant carboxylate in aerosol samples (Mochida et al., 2003; Warneck, 2003). Pyruvate was also found in substantial amount and formate the least abundant counting on average 3% of total carboxylates in each of the aerosol fractions. Succinate and malonate were below detection limit in PM2.5 and PM10 aerosols, respectively. The total carboxylates accounted for 0.18% to total PM2.5 mass and 0.22% to PM10 mass. In comparison with other studies, the mean concentrations of all measured carboxylates in Tanzania were lower to those reported in urban and rural sites around the world (Souza et al., 1999; Kerminen et al.,

Chemical characteristics of water-soluble inorganic ions and their relative abundances in PM2.5 and PM10 aerosols are also shown in Table 1. In both aerosol fractions, water-soluble Mg2+ was the most important cation and SO42– the main anionic species. On average Mg2+ accounted for 44.4% of total water-soluble ions in PM2.5 and 24.7% in PM10 whereas SO42– accounted for 22.8% and 35.2% of total ions in PM2.5 and PM10, respectively. High levels of crustal element Mg2+ together with Ca2+ are essentially attributable to soil/mineral dust dispersal. As to reasonable NH4+ levels (8% of total ions) in PM2.5, this may be due to presence of ammonia gas from biomass burning especially during smoldering combustion (Andreae & Merlet, 2001) and from agricultural activities in particular cattle raising (Street et al., 2003; Stone et al., 2010). Water-soluble K+, a good indicator for biomass burning, was second most abundant cation in PM2.5 accounted for 10.6% of total water-soluble ions.

For SO42– the higher levels could be attributed to its efficient formation by in-cloud processing of SO2 (Yao et al., 2003) and from secondary formation processes (Allen et al., 2004). As to low NO3– levels, this is likely due to the fact that the site is rural with little or no traffic and undoubtedly there are less anthropogenic emissions of precursor gas NOx. Also as to low concentrations of Na+ which is mainly derived from sea-salt, this is presumably due to long distance (about 200 km) from the Indian Ocean to our sampling site. The observed levels for water-soluble ions are comparable with those reported in our previous work in Morogoro (Mkoma et al., 2009a; Mkoma et al., 2010). It appears that the levels of

Sarin, 2011).

**3.2. Concentrations of carboxylates ions** 

2000; Yao et al., 2003; Kawamura & Yasui, 2005).

**3.3. Water-soluble inorganic ions and ratios** 

For determination of carboxylic acids and water-soluble ions one-half of 12.88 cm2 portions punched from of each PTFE filter was extracted using 5 ml Milli-Q ultrapure water (resistivity of 18.2 MΩcm, Barnstead International, USA) in a shaker tubes Model AT56 (Fanem Ltd, Sao Paulo, Brazil) for 5 minutes, followed by filtering through Polytetrafluoroethylene (PTFE) filter (0.45 μm pore size, Sartorius Stedim, Germany). The concentrations of aqueous extracts were determined by Dionex ion chromatography ICS 1100 and ICS 2100 for acids/anions and cations respectively which was equipped with an auto sampler (Dionex ICS Series AS-DV). An analytical column AS16 (3 x 50 mm) with AG16 guard column (3 x 50 mm) and CSRS-300 I (2 mm) suppressor in ion-exchange mode was used to determine carboxylates (monocarboxylates: formate and acetate; dicarboxylates: oxalate, malonate, succinate, and maleate; ketocarboxylate: pyruvate) and water-soluble anions (chloride Cl−, nitrate NO3− and sulphate SO42−). The eluent gradient programme was sweeping from 6.0 to 8.0 mmol/L KOH in 35 minutes under flow rate of 0.38 μL/min, except for acetic acid which was determined in another run, reducing injection time to avoid overlap of peaks. For determination of water-soluble cations (NH4+, Na+, K+, Mg2+ and Ca2+) an analytical column CS16 and Guard column CG16 (both 3 x 50 mm) and CSRS-I (2 mm) suppressor in a chemical mode were used. An eluent of 17.5 mmol/L H2SO4 was used at flow rate of 0.35 μL/min. The injection volume was 25 μL for all detection. Peak identification was confirmed based on a match of ion chromatograph retention times and standard samples. Limit of detection determined as mean equal to 3 times standard deviation of the field blank value corresponded to a range of 0.008 to 0.017 ng/L for carboxylates, 0.008 to 0.023 ng/L for anions and 0.021 to 0.083 ng/L for cations. Limits of quantification were between 0.026 and 0.058 ng/L for carboxylates, 0.028 and 0.078 ng/L for anions and 0.063 and 0.252 ng/L for cations**.**
