Hygroscopic properties of ultrafine aerosol particles in the boreal forest: diurnal variation, solubility and the influence of sulfuric acid

[摘要] The hygroscopic growth of aerosol particles present in a borealforest was measured at a relative humidity of 88%. Simultaneouslythe gas phase concentration of sulfuric acid, a very hygroscopiccompound, was monitored. The focus was mainly on days with newparticle formation by nucleation. The measured hygroscopic growthfactors (GF) correlated positively with the gaseous phase sulfuricacid concentrations. The smaller the particles, the stronger thecorrelation, with r=0.20 for 50 nm and r=0.50 for 10 nmparticles. The increase in GF due to condensing sulfuric acid isexpected to be larger for particles with initially smaller masses.During new particle formation, the changes in solubility of the newparticles were calculated during their growth to Aitken modesizes. As the modal diameter increased, the solubility of theparticles decreased. This indicated that the initial particlegrowth was due to more hygroscopic compounds, whereas the latergrowth during the evening and night was mainly caused by lesshygroscopic or even hydrophobic compounds. For all the measuredsizes, a diurnal variation in GF was observed both during dayswith and without particle formation. The GF was lowest at aroundmidnight, with a mean value of 1.12–1.24 depending on particlesize and if new particle formation occurred during the day, andincreased to 1.25–1.34 around noon. This can be tentativelyexplained by day- and nighttime gas-phase chemistry; differentvapors will be present depending on the time of day, and throughcondensation these compounds will alter the hygroscopic propertiesof the particles in different ways.

[学科分类] 大气科学