Abstract. Semicontinuous PM 2.5 and black carbon (BC) concentrations, and 24 h integrated PM 2.5 filter samples were collected near roadways in the Kathmandu Valley, Nepal. Instruments were carried by a group of volunteer traffic police officers in the vicinity of six major roadway intersections in the Kathmandu Valley across two sampling periods in 2014. Daily PM 2.5 filter samples were analyzed for water-soluble inorganic ions, elemental carbon (EC) and organic carbon (OC), and 24 elements. Mean PM 2.5 and BC concentrations were 124.76 µg m −3 and 16.74 µgC m −3 during the drier spring sampling period, and 45.92 µg m −3 and 13.46 µgC m −3 during monsoonal sampling. Despite the lower monsoonal PM 2.5 concentrations, BC and several elements were not significantly lower during the monsoon, which indicates an important contribution of vehicle-related emissions throughout both seasons in this region. During the monsoon, there was an enhanced contribution of chemical species (elements and water-soluble inorganic ions), except secondary inorganic ions, and BC to PM 2.5 (crustal elements: 19 %; heavy metals: 5 %; and BC: 39 %) compared to those in spring (crustal elements: 9 %; heavy metals: 1 %; and BC: 18 %). Silica, calcium, aluminum, and iron were the most abundant elements during both spring and the monsoon, with total concentrations of 12.13 and 8.85 µg m −3 , respectively. PM 2.5 and BC showed less spatial variation compared to that for individual chemical species.

<p><strong>Abstract.</strong> Semi continuous PM<sub>2.5</sub> and BC concentrations, and 24-hour integrated PM<sub>2.5</sub> filter samples were collected near roadways in the Kathmandu Valley, Nepal. Instruments were carried by a group of volunteer traffic police officers in the vicinity of six major roadway intersections in the Kathmandu Valley across two sampling periods in 2014. Daily PM<sub>2.5</sub> filter samples were analyzed for water soluble inorganic ions, elemental carbon (EC) and organic carbon (OC), and 24 elements. Mean PM<sub>2.5</sub> and BC concentrations were 124.76 μg m<sup>−3</sup> and 16.74 μgC m<sup>−3</sup> during the drier spring sampling period, and 45.92 μg m<sup>−3</sup> and 13.46 μgC m<sup>−3</sup> during monsoonal sampling. Despite the lower monsoonal PM<sub>2.5</sub> concentrations, BC and several elements were not significantly lower during the monsoon, which indicates an important contribution of vehicle-related emissions throughout both seasons in this region. During the monsoon, there was an enhanced contribution of chemical species (elements and water soluble inorganic ions) except secondary inorganic ions, and BC to PM<sub>2.5</sub> (crustal elements: 19 %; heavy metals: 5 %; BC: 39 %) compared to those in spring (crustal elements: 9 %; heavy metals: 1 %; BC: 18 %). Silica, calcium, aluminum, and iron were the most abundant elements during both spring and the monsoon, with the total concentrations of 12.13 and 8.85 μg m<sup>−3</sup>, respectively. PM<sub>2.5</sub> and BC showed less spatial variation compared to that for individual chemical species.</p>

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