Uptake of nitric acid, ammonia, and organics in orographic clouds: mass spectrometric analyses of droplet residual and interstitial aerosol particles

Abstract

Abstract. Concurrent in situ analyses of interstitial aerosol and cloud droplet residues have been conducted at the Schmücke mountain site during the Hill Cap Cloud Thuringia campaign in central Germany in September and October 2010. Cloud droplets were sampled from warm clouds (temperatures between −3 and +16 • C) by a counterflow virtual impactor and the submicron-sized residues were analyzed by a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS), while the interstitial aerosol composition was measured by an high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). During cloud-free periods, the submicron out-of-cloud aerosol was analyzed using both instruments, allowing for intercomparison between the two instruments. Further instrumentation included black carbon measurements and optical particle counters for the aerosol particles as well as optical sizing instrumentation for the cloud droplets. The results show that, under cloud conditions, on average 85 % of the submicron aerosol mass partitioned into the cloud liquid phase. Scavenging efficiencies of nitrate, ammonium, sulfate, and organics ranged between 60 and 100 %, with nitrate having, in general, the highest values. For black carbon, the scavenging efficiency was markedly lower (about 24 %). The nitrate and ammonium mass fractions were found to be markedly enhanced in cloud residues, indicating uptake of gaseous nitric acid and ammonia into the aqueous phase. This effect was found to be temperature dependent: at lower temperatures, the nitrate and ammonium mass fractions in the residues were higher. Also, the oxidation state of the organic matter in cloud residues was found to be temperature dependent: the O : C ratio was lower at higher temperatures. A possible explanation for this observation is a more effective uptake and/or higher concentrations of low-oxidized water-soluble volatile organic compounds, possibly of biogenic origin, at higher temperatures. Organic nitrates were observed in cloud residuals as well as in the out-of-cloud aerosol, but no indication of a preferred partitioning of organic nitrates into the aqueous phase or into the gas phase was detected. Assuming the uptake of nitric acid and ammonia in cloud droplets will be reversible, it will lead to a redistribution of nitrate and ammonium among the aerosol particles, leading to more uniform, internally mixed particles after several cloud passages.

In situ chemical composition measurement of individual cloud residue particles at a mountain site, southern China

“…S8) possibly reflect the uptake of gaseous HNO 3 during cloud processes (Hayden et al, 2008;Roth et al, 2016). A recent study also confirmed that the update of gaseous HNO 3 is an important contributor for the increased nitrate level in the cloud residuals (Schneider et al, 2017). Interestingly, we observed a decrease in nitrate intensity in all cloud residues except for dust type (Fig.…”

“…These studies were mainly conducted at sites in North America including Wyoming (Pratt et al, 2010a), Ohio (Hayden et al, 2008), Oklahoma (Berg et al, 2009), Florida Twohy et al, 2005) and California (Coggon et al, 2014); in Europe including Schmücke (Roth et al, 2016;Schneider et al, 2017), Jungfraujoch (Kamphus et al, 2010), Åresku-tan (Drewnick et al, 2007) and Scandinavia (Targino et al, 2006); in the Arctic (Zelenyuk et al, 2010); in Central America (Cziczo et al, 2013); in western Africa (Matsuki et al, 2010); and in marine areas (Twohy and Anderson, 2008;Twohy et al, 2009;Shingler et al, 2012).…”

“…Similarly, the heterogeneous chemistry of HNO 3 in the dust particles also contributes the preferential enrichment of nitrate (Tang et al, 2016). Moreover, after activation, uptake of gas-phase HNO 3 would increase nitrate level in the cloud residues (Schneider et al, 2017). The nitrate in the cloud residues was thought to be in the form of ammonium nitrate by estimating the ratio of m/z 30 to m/z 46 in AMS data (Drewnick et al, 2007;Hayden et al, 2008).…”

Aircraft-based observations of isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) in the tropical upper troposphere over the Amazon region

“…Several studies show that qualitative measurements and quantification of organic nitrates are of major interest (e.g., Fry et al, 2009;Bruns et al, 2010;Farmer et al, 2010;Ayres et al, 2015;Kiendler-Scharr et al, 2016;Liu et al, 2017;Schneider et al, 2017). Organic nitrates are decomposed during the evaporation and/or ionisation processes in the C-ToF-AMS and, therefore,…”

“…Many studies reported on the difficulties to measure organic 10 nitrates quantitatively, but suggested also a possibility to estimate the amount or at least the presence of organic nitrates based on the measured ion ratios of NO + to NO + 2 using aerosol mass spectrometry (e.g., Bruns et al, 2010;Farmer et al, 2010;Rollins et al, 2010;Fry et al, 2013;Ayres et al, 2015;Kiendler-Scharr et al, 2016;Ng et al, 2017;Schneider et al, 2017).…”