Possible contribution of N₂O₅ scavenging to HNO₃ observed in winter stratiform cloud

Abstract

During a winter field study in early 1984, the concentrations of NO3−, SO4=, and H+, measured in cloud water samples collected in the bases of deep stratiform clouds and in the aerosol and gas phases below cloud, suggested a substantial enhancement of NO3− in the cloud water samples for 8 of 12 days. For these 8 days, variations in cloud water NO3−/SO4= were associated with an excess H+/SO4= in the cloud water, indicating HNO3. For the conditions of study, which included subfreezing temperatures and weak solar fluxes to the low‐level cloud, the possibility exists that much of the NO3− in the cloud water was from the formation and subsequent scavenging of N2O5 by cloud droplets. A chemical model set in a Lagrangian framework is used to describe the daytime evolution of odd nitrogen species, including dry deposition of HNO3, under conditions approximating the average conditions for the 8 days. Computations with no cloud scavenging of N2O5 predict an average cloud water NO3− mixing ratio well below the geometric mean determined for the measurements. For the conditions most typical of the observations, the production rate of HNO3 via the cloud scavenging of N2O5 was computed to be approximately equal to that for the gas‐phase production of HNO3.

Classification of clouds sampled at the puy de Dôme (France) based on 10 yr of monitoring of their physicochemical properties

“…At the pdD sampling site, since the origin of the air masses is variable with high nitrate fractions for polluted air masses, this conclusion cannot be applied. Several values of this ratio reported elsewhere for cloud water samples were below 1 (Baltensperger et al, 1998;Bridges et al, 2002) but the observation of elevated NO − 3 relative to SO 2− 4 in cloud water has been also reported in other studies Hayden et al, 2008;Hill et al, 2007;Leaitch et al, 1986;Lee et al, 2012). This could be related to high NO x emission and the predominance of nitrate over sulfate in the acidification of aerosols.…”

“…The potential sources of nitrate, sulfate and ammonium in cloud water are multiple and include particles (ammonium sulfate and ammonium nitrate particles) and gases (from SO 2 , HNO 3 , NH 3 , N 2 O 5 ) (Leaitch et al, 1988;Sellegri et al, 2003). The concentrations of ammonium, nitrate and sulfate are significantly correlated within each category of air mass (R varies between 0.64 to 0.94, n = 6 to 70).…”

Cloud processing of nitrate

“…In the concentrated volume of the cloud droplets, HNO 3 can react with NH 3 , also absorbed from the gas phase, or with alkaline compounds that may enter the droplets via nucleation scavenging. Also, clouds change the actinic radiation both in and around them that can promote or inhibit the formation of N 2 O 5 [ Leaitch et al , 1988]. In turn, nitrate particles are effective cloud condensation nuclei, and therefore can contribute to the indirect effects of aerosols on climate [ Intergovernmental Panel on Climate Change , 2007].…”

“…The data presented include only measurements taken within daytime hours. The presence of N 2 O 5 will be less likely than at nighttime; however, inside clouds the actinic radiation can be significantly reduced, so potential contributions from gas‐phase N 2 O 5 cannot be ruled out [ Leaitch et al , 1988].…”

Heterogeneous Atmospheric Chemistry, Ambient Measurements, and Model Calculations of N2O5: A Review

“…Leaitch et al (1988) studied the dissolution of N 2 O 5 in cloud droplets by using a Lagrangian chemical model with winter conditions in northeastern region of the United States and compared the resulting NO 3 − content in cloud water with measured values. They showed that the mean production rate of HNO 3 via cloud scavenging is also much more efficient than homogeneous N 2 O 5 hydrolysis, which was observed by Colvile et al (1994) as well.…”