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The prevalence of ragweed (<i>Ambrosia artemisiifolia</i>) pollinosis has been increasing worldwide. This study focused on the behavior of autumn airborne pollen and the major ragweed allergen -<i>Amb a 1</i> particle in urban Saitama, Japan, in 2016. Burkard sampler results showed that the airborne ragweed pollen scattering season was from September 1st to October 9th. Over 83% of sampling events had pollen counts of over 13 grains/m³, indicating the high potential health risks of ragweed pollen in the atmosphere. The results of a surface plasmon resonance immunoassay (SPR) indicated that the average <i>Amb a 1</i> count was about 16.5 pg /pollen. The airborne <i>Amb a 1</i> concentration was up to 4.7 ng/m³, of which about 45% was accumulated in ultrafine particles, such as particulate matter with a diameter ≤1.1 µm (PM_1.1). Although ragweed pollen was hardly observed during the October 14th–17th sampling campaign, the concentration of ambient <i>Amb a 1</i> particles in PM_1.1 was also determined to be 4.59 ng/m³, which could be explained by the longer scattering of fine particles in the atmosphere. Pearson correlation coefficient analysis results showed that temperature (daily, r = 0.41; event, r = 0.87) could affect the behavior of the airborne pollen counts, and ambient water-soluble ions (such as Ca²⁺ and NO₃⁻) could affect <i>Amb a 1</i> in PM_1.1. Additionally, air mass trajectories and wind rose results indicated that air masses with long-range transportation could also influence the temporary behavior of <i>Amb a 1</i> and pollen counts via the wind. Mugwort and <i>Humulus japonicus</i> pollens were also observed to extend pollen scattering periods. Airborne pollen and allergenic particles could be considered air pollutants, as they pose health risks and are susceptible to environmental influences.