Local Air Pollution Versus Short-range Transported Dust Episodes: A Comparative Study for Submicron Particle Number Concentration

We measured the submicron particle number concentrations in the urban/suburban atmosphere of Amman-Jordan during the spring of 2009. The main objective was to distinguish the differences in the submicron particle number concentrations with/without dust episodes. In the absence of dust episodes the concentrations showed distinguished daily patterns, which were similar at both the urban and the urban atmosphere with lower concentrations (at least 1:2) at the suburban site. The daily pattern during the first five working days (Saturday-Wednesday) was different than that that on either Thursday or Friday. During the morning rush hours the number concentrations were as high as 120×10^3 and 75×10^3 1/cm^3 at the urban and suburban sites during those workdays, respectively. These concentrations were about 21 and 14 times what was observed during the background conditions (5.5×10^3 1/cm^3 between midnight and early morning) at both sites. This suggests that traffic emissions are one of the main sources in the urban/suburban atmosphere of Amman. The number concentration of submicron aerosol particles, which originated from the nearby highway, at the urban site decreased exponentially with the wind speed. During a dust episode the total number concentration of submicron particles was about 1/5 of what is typically observed during workdays without dust episodes. The lower concentrations were attributed here for two reasons: increased wind speed and coagulation of locally emitted urban particles with the regional dust particles. These observations in the absence of dust episodes agree well with other studies for highly populated cities. The limitation of this study is the lack of information about the particle number size distribution that can reveal the modal structure of aerosol particles in Amman.