Chemistry of riming: the retention of organic and inorganic atmospheric trace constituents

Abstract

Abstract. During free fall in clouds, ice hydrometeors such as snowflakes and ice particles grow effectively by riming, i.e., the accretion of supercooled droplets. Volatile atmospheric trace constituents dissolved in the supercooled droplets may remain in ice during freezing or may be released back to the gas phase. This process is quantified by retention coefficients. Once in the ice phase the trace constituents may be vertically redistributed by scavenging and subsequent precipitation or by evaporation of these ice hydrometeors at high altitudes. Retention coefficients of the most dominant carboxylic acids and aldehydes found in cloud water were investigated in the Mainz vertical wind tunnel under dry-growth (surface temperature less than 0 • C) riming conditions which are typically prevailing in the mixed-phase zone of convective clouds (i.e., temperatures from −16 to −7 • C and a liquid water content (LWC) of 0.9±0.2 g m −3 ). The mean retention coefficients of formic and acetic acids are found to be 0.68 ± 0.09 and 0.63 ± 0.19. Oxalic and malonic acids as well as formaldehyde show mean retention coefficients of 0.97 ± 0.06, 0.98 ± 0.08, and 0.97 ± 0.11, respectively. Application of a semi-empirical model on the present and earlier wind tunnel measurements reveals that retention coefficients can be well interpreted by the effective Henry's law constant accounting for solubility and dissociation. A parameterization for the retention coefficients has been derived for substances whose aqueous-phase kinetics are fast compared to mass transport timescales. For other cases, the semi-empirical model in combination with a kinetic approach is suited to determine the retention coefficients. These may be implemented in high-resolution cloud models.

Effects of Scavenging, Entrainment, and Aqueous Chemistry on Peroxides and Formaldehyde in Deep Convective Outflow Over the Central and Southeast United States

“…Using the CH 2 O SE calculated by Fried et al () using the n‐butane ratio method (67 ± 20%), to avoid potential errors from using a horizontally averaged background profile in the altitude‐dependent calculation, we still find r f <0.25 for CH 2 O. These results conflict with the laboratory measurements of r f =0.97 for CH 2 O (Jost et al, ). Nevertheless, for the severe storm and MCS, the analysis in the previous section indicates that the simulations are overestimating precipitation production, and thus, the true r f values may be higher.…”

“…Retention fractions for hydrometeor surfaces with temperatures above freezing (wet growth riming) are likely lower (e.g., r f (i l) < 0.2, where r f (i l) refers to the fraction remaining in the ice core to that in the liquid (Michael & Stuart, )) than for below freezing hydrometeor surfaces (dry growth riming) because in wet growth riming, species are preferentially expelled from the frozen portion of the hydrometeor interior into the quasi‐liquid surface layer. Dry growth riming ice retention fractions have been measured in laboratory and field experiments (e.g., r f =0.05–1.0 for H 2 O 2 , Jost et al, ; Iribarne & Pyshnov, ; Snider et al, ; Snider & Huang, ; Voisin et al, ; von Blohn et al, , ). Ice retention of species that dissociate or react in cloud water can be affected by the reaction kinetics.…”

“…Ice retention of species that dissociate or react in cloud water can be affected by the reaction kinetics. For example, Jost et al (2017) determined an average r f value (the ratio of the concentration in the rime ice versus the liquid phase concentration in the supercooled cloud droplets prior to riming) of 0.97 for CH 2 O, which they attribute to the diol (CH 2 (OH) 2 ) formed by aqueous CH 2 O not being able to dehydrate before the drop freezes. In the WRF-Chem simulations in this study, diol formation is included in the effective Henry's law coefficient for CH 2 O because of the rapid formation of the diol.…”

“…Model simulations combined with aircraft measurements can be utilized to determine r f values that result in simulated gas mixing ratios in convective outflow consistent with observations. While the laboratory studies can report r f for single processes (e.g., dry-growth riming, Jost et al, 2017), the combined model simulations and aircraft measurements approach encompasses several processes that occur within the storm, including scavenging, entrainment, and chemistry. Thus, differences between the theoretical and laboratory studies with the field observations may be reconciled based on multiple processes affecting the trace gases.…”

Wet Scavenging in WRF‐Chem Simulations of Parameterized Convection for a Severe Storm During the DC3 Field Campaign

“…For example, HNO 3 is completely retained (Iribarne & Pyshnov, ; von Blohn et al, ), while the observed retention fraction is 0.62 for SO 2 (Iribarne et al, ), which is less soluble than HNO 3 . Jost et al () measured the retention coefficients of formaldehyde by wind tunnel experiments. The retention coefficients they obtained for CH 2 O were 0.97 ± 0.11 and did not depend significantly on temperature or ventilation.…”