10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Enhanced Formation of Brown Carbon Particles at Low Relative Humidity
Masao Gen, Dandan Huang, CHAK K. CHAN, City University of Hong Kong
Abstract Number: 54 Working Group: Aerosol Chemistry
Abstract Reactions between dissolved ammonia, NH3(aq), and carbonyls can form light-absorbing species in atmospheric particles. According to previous research, such reactions can be accelerated by actively removing water from the reaction system. Here, we examine the effects of relative humidity (RH) on the reactive uptake of glyoxal (Gly) by aqueous particles of ammonium sulfate (AS), ammonium bisulfate (ABS), sodium sulfate, magnesium sulfate, ammonium nitrate (AN), and sodium nitrate (SN) at RHs of 30, 45, 60, and 75%. In-situ Raman analysis was used to quantify particle-phase Gly via the v(C-H) peak at 2900-3050 cm-1 and a colored product, 2,2’-biimidazole (BI), by its broadband fluorescence as a function of uptake time. The presence of BI was verified by off-line UV-vis and fluorescence characterization, as well as surface-enhanced Raman spectroscopy measurements. Overall, the Gly uptake rate increases with decreasing RH, reflecting the “salting-in” effect, but is suppressed for SN particles at 30% RH, probably due to increased particle viscosity resulting from the formation of Gly oligomers. For NH3(aq)-containing particles, the BI formation rate increases significantly with decreasing RH or aerosol liquid water (ALW); however, the BI formation rate is negligible for crystalline AS particles at 30% RH and ABS particles under all RH conditions due to extremely low NH3(aq). Compared to that at 75% RH, the BI formation rate is enhanced by factors of 2.9 x 10 at 60% RH and 3.3 x 102 at 45% RH for AS particles and 6.5 x 10 at 60% RH, 2.1 x 102 at 45% RH, and 4.6 x 102 at 30% RH for AN particles. These enhancement factors are much larger than those estimated from increased reactant concentrations due to decreases in RH and ALW alone. We postulate that the reduction in ALW at low RH increases the Gly uptake rate via the “salting-in” effect and the BI formation rate by facilitating dehydration reactions.