Field observations and quantifications of atmospheric formaldehyde partitioning in gaseous and particulate phases

Formaldehyde (HCHO) can possibly be taken by atmospheric particles due to its moderate solubility. Although previous model studies have proposed that uptake by particles was a large sink for HCHO, direct observation of HCHO partitioning and estimation of HCHO uptake coefficient (γ) for tropospheric conditions are still limited. In this work, online measurements of gaseous HCHO (HCHO_g) and particulate HCHO (HCHO_p) were carried out simultaneously at an urban site in Beijing in winter and spring. The results indicated that the average concentrations of HCHO_p ranged from 0.15 to 0.4 μg m^-3, accounting for 1.2% to 10% of the total HCHO (i.e., HCHO_g + HCHO_p). The median values of estimated γ based on the measured data were in the range of about 1.09 ∗ 10^-5-2.42 ∗ 10^-4, with lower values during PM_2.5 pollution episodes. Besides, the pH and liquid water content of aerosols that are mainly determined by ambient relative humidity (RH) and inorganic salt composition were identified as the main influencing factors of γ. We propose that the HCHO uptake process was mainly driven by hydrone and hydrogen ions in particles.