Mass size distribution of carbon in atmospheric humic-like substances and water soluble organic carbon for an urban environment

Aerosol samples were collected with a micro-orifice uniform deposit impactor in an aerodynamic diameter (AD) range of 0.5–10 μm in central Budapest, Hungary for 12 days. Aqueous aerosol extracts and atmospheric humic-like substances (HULIS) were obtained from the combined aerosol samples for each impactor stage. Water-soluble organic carbon (WSOC) and carbon in HULIS (HULIS-C) were measured in the samples with a total organic carbon analyzer. The analytical data were inverted into semi-smooth mass size distributions, and modal parameters were derived. The size distributions for both WSOC and HULIS-C consisted of three peaks: a coarse mode and two accumulation submodes. The geometric mean AD for the coarse mode was 6.4 μm for both WSOC and HULIS-C. Contribution of the coarse mode to the total concentration of WSOC and HULIS-C were substantial and similar for both components, i.e., approximately 20%. The splitting of the accumulation mode into condensation and droplet submodes was explained by water processing of aerosol particles in the air. The geometric mean ADs of the condensation submode for the WSOC and HULIS-C were 0.37 and 0.31 μm, respectively, and the droplet submode appeared at 1.72 and 1.22 μm, respectively. The condensation submode was larger than the droplet submode by similar ratios of 1.7–1.8 for both WSOC and HULIS-C. The relative concentrations of the two submodes were likely influenced by local meteorology, in particular by relative humidity, pollutant gases, and water uptake properties and aging of fine particles.