Wide-angle acousto-optic devices based on isotropic light scattering in biaxial crystals

Using the example of a rhombic crystal of alpha-iodic acid, a detailed study of new variants of acousto-optic isotropic light scattering in oblique sections of biaxial crystals, whose refractive surface contains a concave portion, is carried out. For these cases, the frequency dependences of the Bragg angles and the ranges of acousto-optic interaction are calculated. The possibility of implementing a unique, simultaneously broadband and wide-angle variant of light scattering is established. It is shown that the two-dimensional transfer function of such acousto-optic geometry has a complex X-shaped structure that has no analogues among uniaxial crystals. The possibility of applying this diffraction regime in light beam spatial filtering devices is studied.

Measurement report: Long-term measurements of aerosol precursor concentrations in the Finnish sub-Arctic boreal forest

Aerosol particles form in the atmosphere by clustering of certain atmospheric vapors. After growing to larger particles by condensation of low volatile gases, they can affect the Earth’s climate directly by scattering light and indirectly by acting as cloud condensation nuclei. Observations of low-volatility aerosol precursor gases have been reported around the world but longer-term measurement series and any Arctic data sets showing seasonal variation are close to non-existent. In here, we present ~7 months of aerosol precursor gas measurements performed with the nitrate based chemical ionization mass spectrometer (CI-APi-TOF). We deployed our measurements ~150 km North of the Arctic Circle at the continental Finnish sub-Arctic field station, SMEAR I, located in Värriö strict nature reserve. We report concentration measurements of the most common new particle formation related compounds; sulfuric acid (SA), methane sulfonic acid (MSA), iodic acid (IA) and a total concentration of highly oxygenated organic compounds (HOMs). At this remote measurement site, SA is originated both from anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle with concentrations peaking in the summer. Of the measured compounds, iodic acid concentrations are the most stable throughout the measurement period, except in April, when the concentration of IA is significantly higher than during the rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and autumn, and on new particle formation event or non-event days. HOMs are abundant during the summer months and low in winter months. Due to the low winter concentrations and their high correlation with ambient air temperature, we suggest that most of HOMs are products of biogenic emissions, most probably monoterpene oxidation products. New particle formation events at SMEAR I happen under relatively low temperatures with a fast temperature rise in the morning followed by decreasing relative humidity during the day. The ozone concentrations are on average ~10 ppbv higher on NPF days than non-event days. During NPF days, we have on average higher SA concentration peaking at noon, higher MSA concentrations in the afternoon and slightly higher IA concentration than during non-event days. All together, these are the first long term measurements of aerosol forming vapors from the SMEAR I in the sub-arctic region, and the results help us to understand atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.

Molecular-level evidence for marine aerosol nucleation of iodic acid and methanesulfonic acid

Both iodic acid (HIO3, IA) and methanesulfonic acid (CH3S(O)2OH, MSA) have been identified by field studies as important precursors of new particle formation (NPF) in marine areas. However, the mechanism of NPF in which IA and MSA are jointly involved is still unclear. Hence, we investigated the IA-MSA nucleation system under different atmospheric conditions and uncovered the corresponding nucleating mechanism at a molecular level for the first time using quantum chemical approach and Atmospheric Cluster Dynamics Code (ACDC). The findings showed that MSA can stabilize IA clusters via both hydrogen and halogen bonds. Moreover, the joint nucleation rate of IA-MSA system is significantly higher than that of IA self-nucleation, particularly in relatively cold marine regions with sparse IA and rich MSA. For the IA-MSA nucleation mechanism, in addition to self-nucleation of IA, the IA-MSA-involved clusters can also directly participate in the nucleation process, and their contribution is particularly prominent in the polar regions with rich MSA and sparse IA. The IA-MSA nucleation mechanism revealed in this work may help to elucidate some missing sources of marine NPF.

Iodide CIMS and <i>m</i>∕<i>z</i> 62: the detection of HNO₃ as NO₃⁻ in the presence of PAN, peroxyacetic acid and ozone

Chemical ionisation mass spectrometry (CIMS) using I− (the iodide anion), hereafter I-CIMS, as a primary reactant ion has previously been used to measure NO3 and N2O5 both in laboratory and field experiments. We show that reports of large daytime mixing ratios of NO3 and N2O5 (both usually present in detectable amounts only at night) are likely to be heavily biased by the ubiquitous presence of HNO3 in the troposphere and lower stratosphere. We demonstrate in a series of laboratory experiments that the CIMS detection of HNO3 at m/z 62 using I− ions is efficient in the presence of peroxy acetyl nitric anhydride (PAN) or peroxyacetic acid (PAA) and especially O3. We have characterised the dependence of the sensitivity to HNO3 detection on the presence of acetate anions (CH3CO2-, m/z 59, from either PAN or PAA). The loss of CH3CO2- via conversion to NO3- in the presence of HNO3 may represent a significant bias in I-CIMS measurements of PAN and PAA in which continuous calibration (e.g. via addition of isotopically labelled PAN) is not carried out. The greatest sensitivity to HNO3 at m/z 62 is achieved in the presence of ambient levels of O3 whereby the thermodynamically disfavoured, direct reaction of I− with HNO3 to form NO3- is bypassed by the formation of IOx-, which reacts with HNO3 to form, for example, iodic acid and NO3-. The ozone and humidity dependence of the detection of HNO3 at m/z 62 was characterised in laboratory experiments and applied to daytime, airborne measurements in which good agreement with measurements of the I−(HNO3) cluster ion (specific for HNO3 detection) was obtained. At high ozone mixing ratios, we show that the concentration of I− ions in our ion–molecule reactor (IMR) is significantly depleted. This is not reflected by changes in the measured I− signal at m/z 127 as the IOx- formed does not survive passage through the instrument but is likely detected after fragmentation to I−. This may result in a bias in measurements of trace gases using I-CIMS in stratospheric air masses unless a calibration gas is continuously added or the impact of O3 on sensitivity is characterised.

Iodide-CIMS and <i>m/z</i> 62: The detection of HNO₃ as NO₃⁻ in the presence of PAN, peracetic acid and O₃

Chemical Ionisation Mass Spectrometry (CIMS) using I− (the iodide anion) as primary chemi-ion has previously been used to measure NO3 and N2O5 both in laboratory and field experiments. We show that reports of the large daytime mixing ratios of NO3 and N2O5 (usually only present in detectable amounts at night-time) are likely to be heavily biased by the ubiquitous presence of HNO3 in the troposphere and lower stratosphere. We demonstrate in a series of laboratory experiments that the CIMS detection of HNO3 at m/z 62 using I− ions is efficient in the presence of PAN or peracetic acid (PAA) and especially O3. We have characterised the dependence of the sensitivity to HNO3 detection on the presence of acetate anions (CH3CO2−, m/z 59, from either PAN or PAA). The loss of CH3CO2− via conversion to NO3− in the presence of HNO3 may represent a significant bias in I-CIMS measurements of PAN and CH3C(O)OOH. The largest sensitivity to HNO3 at m/z 62 is achieved in the presence of ambient levels of O3 whereby the thermodynamically disfavoured, direct reaction of I− with HNO3 to form NO3− is bypassed by the formation of IOX− which react with HNO3 to form e.g. iodic acid and NO3−. The ozone and humidity dependence of the detection of HNO3 at m/z 62 was characterised in laboratory experiments and applied to daytime, airborne measurements in which very good agreement with measurements of the I−(HNO3) cluster-ion (specific for HNO3 detection) was obtained.

Ion-induced iodic acid nucleation

&lt;p&gt;Aside from capable of influencing atmospheric oxidation capacity, iodine species are known to contribute to particle formation processes. Iodine particle formation was commonly believed to be important in coastal regions only, e.g. Mace Head, but emerging evidence shows that it also plays an important role in Arctic regions.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Although the nucleation mechanisms have been proposed to involve mainly iodine oxides, recent field observations suggest that HIO&lt;sub&gt;3&lt;/sub&gt; plays a key role in the cluster formation processes. Despite these advances, experiments with atmospherically relevant vapor concentrations are lacking and the time evolution of charged cluster formation processes has never been detected at the molecular level to validate the mechanisms observed in the field.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In this study, we carried out iodine particle formation experiments in the CLOUD chamber at CERN. The precursor vapor (I&lt;sub&gt;2&lt;/sub&gt;) and oxidation products were carefully controlled at concentrations relevant to those in marine boundary layer conditions. Natural galactic cosmic rays were used to produce ions in the chamber which further initiated ion-induced nucleation processes. An atmospheric pressure interface time-of-flight mass spectrometer was used to trace the time evolution of charged iodine clusters which revealed HIO&lt;sub&gt;3&lt;/sub&gt; as the major contributor.&lt;/p&gt;

Composition and concentrations of aerosol precursor gases in the sub-Arctic boreal forest

&lt;p&gt;One way to form aerosol particles is the condensation of oxidized atmospheric trace gases, such as sulfuric acid (SA) into small molecular clusters. After growing to larger particles by condensation of low volatile gases, they can affect the planets climate directly by scattering light and indirectly by acting as cloud condensation nuclei. Observations of low-volatility aerosol precursor gases have been reported around the world but long-term measurement series and Arctic data sets showing seasonal variation are close to non-existent. In here, we present ~7 months of aerosol precursor gas measurements performed with the nitrate based chemical ionization mass spectrometer (CI-APi-TOF). We deployed our measurements ~250 km above the Arctic Circle at the Finnish sub-Arctic field station, SMEAR I in V&amp;#228;rri&amp;#246;. We report concentration measurements of the most common new particle formation related compounds; sulfuric acid, methanesulfonic acid (MSA), iodic acid (IA) and highly oxygenated organic compounds, HOMs. At this remote measurement site, surrounded by a strict nature preserve, that gets occasional pollution from a Russian city of Murmansk, SA is originated both from anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation, while MSA as an oxidation product of purely biogenic sources is showing a more distinct seasonal cycle. Iodic acid concentrations are the most stable throughout the measurement period, showing almost identical peak concentrations for spring, summer and autumn. HOMs are abundant during the summer months and due to their high correlation with ambient air temperature, we suggest that most of HOMs are products of monoterpene oxidation. New particle formation events at SMEAR I happen under relatively low temperatures, low relative humidity, high ozone concentration, high SA concentration in the morning and high MSA concentrations in the afternoon. The role of HOMs in aerosol formation will be discussed. All together, these are the first long term measurements of aerosol forming precursor from the sub-arctic region helping us to understand atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Observations of New Particle Formation events during summertime in Helsinki

&lt;p&gt;Aerosols can originate from different sources and undergo various formation pathways. New Particle formation (NPF) events occur when precursor vapors nucleate and vapors with low volatility condense on the critical nuclei enabling them to grow to cloud condensation nuclei (CCN) relevant sizes. As CCN, these aerosols affect the occurrence of clouds and their lifetime on local, regional and global level. &amp;#160;Many studies have investigated new particle formation events from various sites ranging from urban areas, boreal forests to pristine locations; however, there is still a dearth of studies investigating coastal new particle formation, which is a complex phenomenon due to the dynamic and ever-changing atmospheric conditions at the coast.&amp;#160; A comprehensive study of particle number distributions and aerosol forming precursor vapors was carried out in a coastal capital city of Finland, Helsinki, during the summer of 2019. The experimental setup comprising of a nitrate-based chemical ionization atmospheric pressure interface time of flight mass spectrometer (CI-APi-TOF), a neutral cluster-air ion spectrometer (NAIS) and a particle size magnifier (PSM) were housed in and around the SMEAR III station in Kumpula Science campus. SMEAR III is a unique site situated in a semi-urban yet coastal location. The period of experiment coincided with the cyanobacterial bloom in the coastal areas of Finland and in the Baltic Sea region. Our study recorded several regional NPF and aerosol burst events during this period. High concentrations of sulfuric acid was found to be associated with the regional NPF events whereas increasing iodic acid concentrations was mostly associated with the initiation of burst events. The sources of sulfuric acid and iodic acid has been carefully evaluated in this study.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;