Simultaneous measurements of new particle formation in 1-second time resolution at a street site and a rooftop site
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Abstract. This study is the first time to use two identical Fast Mobility Particle Sizers for simultaneously measuring particle number size distributions (PNSD) at a street site and a rooftop site within 500 m distance in winter and spring times in Beijing. The obtained datasets in 1-second time resolution allow reasonably deducting the freshly emitted traffic particle signal from the measurements at the street site and thereby pave the way to study reduced or enhanced effects on new particle formation (NPF) in urban atmospheres through the site-by-site comparison. The number concentration of newly formed particles, i.e., smaller than 20nm, and the NPF rate in the springtime were smaller at the street site than at the rooftop site. In contrast, NPF was enhanced in the wintertime at the street site with NPF rates increased by 35 times, characterized by a shorter NPF time and higher new particle yields than those at the rooftop site. Our results imply that the street canyon likely exerts distinct seasonal effects on NPF because of on-road vehicle emissions, i.e., stronger condensation sinks that may be responsible for reduced NPF in the springtime but efficient nucleation and partitioning of gaseous species that contribute to the enhanced NPF in the wintertime. We also analyzed the occurrence or absence of apparent growth for >10nm new particles. The oxidization of biogenic organics in the presence of strong photochemical reactions was argued to play an important role in growing >10nm new particles, but sulfuric acid was unlikely the crucial specie for the apparent growth.
