Diurnal cycle of iodine, bromine, and mercury concentrations in Svalbard surface snow

Sunlit snow is highly photochemically active and plays a key role in the exchange of gas phase species between the cryosphere and the atmosphere. Here, we investigate the behaviour of two selected species in surface snow: mercury (Hg) and iodine (I). Hg can deposit year-round and accumulate in the snowpack. However, photo-induced re-emission of gas phase Hg from the surface has been widely reported. Iodine is active in atmospheric new particle formation, especially in the marine boundary layer, and in the destruction of atmospheric ozone. It can also undergo photochemical re-emission. Although previous studies indicate possible post-depositional processes, little is known about the diurnal behaviour of these two species and their interaction in surface snow. The mechanisms are still poorly constrained, and no field experiments have been performed in different seasons to investigate the magnitude of re-emission processes Three sampling campaigns conducted at an hourly resolution for 3 d each were carried out near Ny-Ålesund (Svalbard) to study the behaviour of mercury and iodine in surface snow under different sunlight and environmental conditions (24 h darkness, 24 h sunlight and day–night cycles). Our results indicate a different behaviour of mercury and iodine in surface snow during the different campaigns. The day–night experiments demonstrate the existence of a diurnal cycle in surface snow for Hg and iodine, indicating that these species are indeed influenced by the daily solar radiation cycle. Differently, bromine did not show any diurnal cycle. The diurnal cycle also disappeared for Hg and iodine during the 24 h sunlight period and during 24 h darkness experiments supporting the idea of the occurrence (absence) of a continuous recycling or exchange at the snow–air interface. These results demonstrate that this surface snow recycling is seasonally dependent, through sunlight. They also highlight the non-negligible role that snowpack emissions have on ambient air concentrations and potentially on iodine-induced atmospheric nucleation processes.

Diurnal cycle of iodine and mercury concentrations in Svalbard surface snow

Sunlit snow is highly photochemically active and plays an important role in the exchange of gas-phase species between the cryosphere to the atmosphere. Here, we investigate the behaviour of two selected species in surface snow: mercury (Hg) and iodine (I). Hg can deposit year-round and accumulate in the snowpack. However, photo-induced re-emission of gas phase Hg from the surface has been widely reported. Iodine is active in atmosphere new particle formation, especially in the marine boundary layer, and in the destruction of atmospheric ozone. It can also undergo photochemical re-emission. Although previous studies indicate possible post-depositional processes, little is known about the diurnal behaviour of these two species and their interaction in surface snow. The mechanisms are still poorly constrained and no field experiments have been performed in different seasons to investigate the magnitude of re-emission processes. Three high temporal resolution (hourly samples) 3 days long sampling campaign were carried out near Ny-Ålesund (Svalbard) to study the behaviour of mercury and iodine in surface snow under different sunlight and environmental conditions (24 h-darkness, 24 h-sunlight and day/night cycles). Our results indicate a clearly different behaviour of Hg and I in surface snow during the different campaign. The day/night experiments demonstrate the existence of a diurnal cycle in surface snow for Hg and iodine, indicating that these species are indeed influenced by the daily solar radiation cycle. Differently bromine did not show any diurnal cycle. The diurnal cycle disappeared also for Hg and iodine during the 24 h-sunlight period and during 24 h-darkness experiments supporting the idea of the occurrence (absence) of a continuous recycling/exchange at the snow-air interface. These results demonstrate that this surface snow recycling is seasonally dependent, through sunlight. They also highlight the non-negligible role that snowpack emissions have on ambient air concentrations and potentially on iodine-induced atmospheric nucleation processes.

Observations of particles at their formation sizes in Beijing, China

New particle formation (NPF) has been observed in many highly polluted environments of South East Asia, including Beijing, where the extent of its contribution to intense haze events is still an open question. Estimated characteristics of NPF events, such as their starting times and formation and growth rates of particles, are more accurate when the detection range of particles extends to smaller sizes. In order to understand the very first steps of particle formation, we used a neutral cluster and air ion spectrometer (NAIS) to investigate particle characteristics at sizes exactly at which atmospheric nucleation and cluster activity occurs. Observations over a continuous 3-month period in Beijing showed 26 NPF events. These events generally coincided with periods with relatively clean air when the wind direction was from the less industrialised north. No NPF events were observed when the daily mean PM2. 5 concentration exceeded 43 µg m−3, which was the upper threshold for particle formation in Beijing. The fraction of particles that are charged in the size range 2–42 nm was normally about 15 %. However, this fraction increased to 20–30 % during haze events and decreased to below 10 % during NPF events. With the NAIS, we very precisely determined the starting times of NPF to a greater accuracy than has been possible in Beijing before and provided a temporal distribution of NPF events with a maximum at about 08:30 LT. Particle formation rates varied between 12 and 38 cm−3 s−1. Particle growth rates were estimated to be in the range of 0.5–9.0 nm h−1. These results are more reliable than previous studies in Beijing as the measurements were conducted for the first time at the exact sizes at which clusters form into particles and provide useful insight into the formation of haze events.

Observations of Particles at their Formation Sizes in Beijing, China

New particle formation (NPF) has been observed in many highly polluted environments of South-East Asia, including Beijing, where the extent of its contribution to intense haze events is still an open question. Estimated characteristics of NPF events, such as their starting times and formation and growth rates of particles, are very different when the measurements are restricted to particles in larger size ranges. In order to understand the very first steps of particle formation, we used a neutral cluster and air ion spectrometer (NAIS) to investigate particle characteristics at sizes exactly where atmospheric nucleation and cluster activity occurs. Observations over a continuous three-month period in Beijing showed 26 NPF events. These events generally coincided with periods with relatively clean air when the wind direction was from the less-industrialized north. No NPF were observed when the daily mean PM2.5 concentration exceeded 43 µg m-3, which was the upper threshold for particle formation in Beijing. The fraction of particles that are charged in the size range 2–42 nm was normally about 15%. However, this fraction increased to 20–30% during haze events and decreased to below 10 % during NPF events. With the NAIS, we determined the starting times of NPF very precisely to a greater accuracy than has been possible in Beijing before and provided a temporal distribution of NPF events with a maximum at about 8.30 am. Particle formation rates varied between 10–36 cm-3 s-1. Particle growth rates were estimated to be in the range 0.5–9.0 nm h-1. These results are more reliable than previous studies in Beijing as the measurements were conducted for the first time at the exact sizes where clusters form into particles and provide useful insight into the formation of haze events.

Chemical ionization of clusters formed from sulfuric acid and dimethylamine or diamines

Chemical ionization (CI) mass spectrometers are used to study atmospheric nucleation by detecting clusters produced by reactions of sulfuric acid and various basic gases. These instruments typically use nitrate to deprotonate and thus chemically ionize the clusters. In this study, we compare cluster concentrations measured using either nitrate or acetate. Clusters were formed in a flow reactor from vapors of sulfuric acid and dimethylamine, ethylene diamine, tetramethylethylene diamine, or butanediamine (also known as putrescine). These comparisons show that nitrate is unable to chemically ionize clusters with high base content. In addition, we vary the ion–molecule reaction time to probe ion processes which include proton-transfer, ion–molecule clustering, and decomposition of ions. Ion decomposition upon deprotonation by acetate/nitrate was observed. More studies are needed to quantify to what extent ion decomposition affects observed cluster content and concentrations, especially those chemically ionized with acetate since it deprotonates more types of clusters than nitrate.Model calculations of the neutral and ion cluster formation pathways are also presented to better identify the cluster types that are not efficiently deprotonated by nitrate. Comparison of model and measured clusters indicate that sulfuric acid dimers with two diamines and sulfuric acid trimers with two or more base molecules are not efficiently chemical ionized by nitrate. We conclude that acetate CI provides better information on cluster abundancies and their base content than nitrate CI.

How to reliably detect molecular clusters and nucleation mode particles with Neutral cluster and Air Ion Spectrometer (NAIS)

To understand the very first steps of atmospheric particle formation and growth processes, information on the size where the atmospheric nucleation and cluster activation occurs, is crucially needed. The current understanding of the concentrations and dynamics of charged and neutral clusters and particles is based on theoretical predictions and experimental observations. This paper gives a standard operation procedure (SOP) for Neutral cluster and Air Ion Spectrometer (NAIS) measurements and data processing. With the NAIS data, we have improved the scientific understanding by (1) direct detection of freshly formed atmospheric clusters and particles, (2) linking experimental observations and theoretical framework to understand the formation and growth mechanisms of aerosol particles, and (3) parameterizing formation and growth mechanisms for atmospheric models. The SOP provides tools to harmonize the world-wide measurements of small clusters and nucleation mode particles and to verify consistent results measured by the NAIS users. The work is based on discussions and interactions between the NAIS users and the NAIS manufacturer.