Evidence of formation of submicrometer water‐soluble organic aerosols at a deciduous forest site in northern Japan in summer

Semicontinuous measurements of submicrometer water‐soluble organic aerosols and particle size distributions were conducted at a deciduous forest site in northern Japan in August 2010. Increases in particle number concentration were frequently observed in daytime, accompanied by an increase in the concentrations of water‐soluble organic carbon (WSOC). We found that daily averaged WSOC concentrations positively correlated with gross primary production of CO 2 by the forest ecosystem (r 2  = 0.63) and ambient temperature during daytime. These relations suggest that the formation of WSOC is closely linked to photosynthetic activity by the forest ecosystem, which depends on both temperature and solar radiation. Off‐line chemical analysis of samples of particles with aerodynamic diameter smaller than 1  μ m collected during a 2 day event of elevated WSOC levels suggests that photochemical aging of both α ‐ and β ‐pinene and isoprene oxidation products contributes to the particle growth and the WSOC mass. Organic tracers of primary biological aerosol particles (PBAPs) showed distinct diurnal variations with a maximum around noontime, also indicating that higher temperature and light intensity induce emissions of PBAPs. However, their contribution to the submicrometer WSOC mass was likely insignificant. During the day, the concentrations of 3‐methyl‐1,2,3‐butanetricarboxylic acid (3‐MBTCA) showed a strong dependence on temperature, and the ratios of WSOC to particle volume concentration increased with an increase in the concentration ratios of 3‐MBTCA to pinonic acid (PA). This result supports a previous proposal that the 3‐MBTCA/PA ratios in submicrometer particles can be a useful tracer for chemical aging of biogenic secondary organic aerosol from forest vegetation.