Analysis of trace oxygenated hydrocarbons in the environment

A technique has been developed which can measure low-molecular-weight (C₂–C₅) oxygenated hydrocarbons down to concentrations of less than 10 parts per trillion (10^-12) in the atmosphere. The method uses cryogenic trapping of trace gases from the air, and two-dimensional gas chromatography (2DGC) with flame ionization and photo-ionization detectors to analyze the samples. The method has been used to make extensive measurements in the field, and it is capable of measuring all of the C₂–C₅ carbonyl compounds in clean tropospheric air. The 2DGC analytical system also makes it possible to prepare accurate, reproducible standards of the low-molecular-weight oxygenated hydrocarbons at trace levels.     

Long-Term Measurements of Light Hydrocarbons (C2–C5) at Schauinsland (Black Forest)

Long-term measurements of light hydrocarbons(C₂–C₅ HCs) were performed in the courseof the EUROTRAC sub-project TOR (Tropospheric Ozone Research) in thesouthern part of the Black Forest in southwest Germany. The measurementscover a time period of five years (January 1989–January 1994) and theair samples were analyzed onsite by an automated GC-system. Pronouncedannual cycles with maxima in late winter and minima in late summer wereobserved in the case of the slowly reacting hydrocarbons ethane, propane andacetylene, reflecting the fact that the seasonal variation of these speciesis photochemically driven. For the shorter lived compounds the seasonalvariations are considerably weaker, connected with a stronger scatter of theindividual measurements, which is caused by different distances to theirmain sources for different wind vectors as well as by varying sourcestrengths. From a detailed characterization of the hydrocarbon patterns theinfluence of two different sources could be distinguished. An extrapolationto photochemical age of zero and completion of our data with those from aspeciated VOC inventory yields an estimated [VOC]/NO_x sourceratio for Schauinsland of 5 [ppbC/ppb]. Comparable[VOC]/NO_x ratios are observed in automobile exhaust gasesunder low speed conditions, which points to the important role of trafficunder conditions, when freshly polluted air masses from a near-by city areadvected to the site. From an investigation of the photochemical age of theadvected air masses it becomes clear that there must exist biogenic sourcesof light olefins in the vicinity of the observatory during the vegetationperiod. For propene and the butenes we are able to estimate a lower limit oftheir contributions in terms of reactivity to the total reactivity( [HC](i) k_OH(i),i=C₂-C₅) of the measured hydrocarbons. Forlowest pollution levels in summer (acetylene <300 ppt, about 40%of the summer values) this source is found to be responsible for15–20% of the total C₂–C₅reactivity observed at Schauinsland. On the average, this source accountsfor 5–10% of the total C₂–C₅reactivity.     

European VOC Emission Estimates Evaluated by Measurements and Model Calculations

Observations and model calculations of the concentration of hydrocarbonsat five Scandinavian rural sites during March–June 1993are reported.Decreasing concentrations from March to June are observedat all sites. The highest concentrations of hydrocarbons were found in air massescoming in from the southwest to southeast, indicating that long rangetransport fromcontinental Europe and the U.K. is important in pollution episodes. An episode of elevated concentrations of hydrocarbons observed at three of the sites in the middle of Marchis described and discussed in relation to the synoptic situation and thepresenceof other chemical compounds (NO₂, PAN, total nitrate andozone).A Lagrangian numerical model is used to calculate the concentrations of theindividual hydrocarbons at the fivesites and comparison with observations is made.The calculated concentrations for nonmethane hydrocarbons with quite longchemicallifetimes agree well with the observations.For the sum of observed and calculated hydrocarbons the correlationcoefficientsare in the range of 0.65–0.88 for the five sitesand the ratio between calculated and measured concentrations was0.72–0.97, indicating thatthe European VOC emission inventory is quite well estimated.     

Meteorology and haze structure during AGASP-II,Part 1: Alaskan Arctic flights, 2–10 April 1986

The second Arctic Gas and Aerosol Sampling Program (AGASP-II) was conducted across the Alaskan and Canadian Arctic in April 1986, to study the in situ aerosol, and the chemical and optical properties of Arctic haze. The NOAA WP-3D aircraft, with special instrumentation added, made six flights during AGASP-II. Measurements of wind, pressure, temperature, ozone, water vapor, condensation nuclei (CN) concentration, and aerosol scattering extinction (b_sp) were used to determine the location of significant haze layers. The measurements made on the first three flights, over the Arctic Ocean north of Barrow and over the Beaufort Sea north of Barter Island, Alaska are discussed in detail in this report of the first phase of AGASP II. In the Alaskan Arctic the WP-3D detected a large and persistent region of haze between 960 and 750 mb, in a thermally stable layer, on 2, 8, and 9 April 1986. At its most dense, the haze contained CN concentrations >10,000 cm^–3 and b_sp of 80×10^–6 m^–1 suggesting active SO₂ to H₂SO₄ gas-to-particle conversion. Calculations based upon observed SO₂ concentrations and ambient relative humidities suggest that 10⁴–10⁵ small H₂SO₄ droplets could have been produced in the haze layers. High concentrations of sub-micron H₂SO₄ droplets were collected in haze. Ozone concentrations were 5–10 ppb higher in the haze layers than in the surrounding troposphere. Outside the regions of haze, CN concentrations ranged from 100 to 400 cm^–3 and b_sp values were about (20–40)×10^–6 m^–1. Air mass trajectories were computed to depict the air flow upwind of regions in which haze was observed. In two cases the back trajectories and ground measurements suggested the source to be in central Europe.     

C2-C7 Hydrocarbon Concentrations in Arctic Snowpack Interstitial Air: Potential Presence of Active Br within the Snowpack

Samples of interstitial air from within the snow pack on an ice floe on the Arctic Ocean were collected during the April 1994 Polar Sunrise Experiment. The concentrations of C₂-C₇ hydrocarbons are reported for the first time in the snow pack interstitial air. Alkane concentrations tended to be higher than concentrations in free air samples above the snow but very similar to winter measurements at various locations in the Arctic archipelago. However, ethyne concentrations in both interstitial and free air were highly correlated with ozone mixing ratios, consistent with previous demonstrations of the effects of Br atom chemistry. The analysis of total bromine within the snow pack indicate an enrichment in total Br at the interface layer between snow and free troposphere. The mixing ratios of some brominated compounds, such as CHBr₃ and CHBr₂Cl, are found to be higher in this top layer of snow relative to the boundary layer. Results were inconclusive due to the limited number of samples, but suggest the possible presence of active bromine in the snow pack and also that some differences exist between chemical reactions occurring in interstitial air compared to air in the boundary layer.     

Reactive Nitrogen Compounds at Spitsbergen in the Norwegian Arctic

Simultaneousindependent measurements of NO_y and NO_x(NO_x= NO + NO₂) by high-sensitivitychemiluminescence systems and of PAN (peroxyacetylnitrate) and PPN (peroxypropionyl nitrate) by GC-ECDwere made at Spitsbergen in the Norwegian Arcticduring the first half year of 1994. The average mixingratio of the sum of PAN and PPN (denoted PANs)increased from around 150 pptv in early winter to amaximum of around 500 pptv in late March, whereasepisodic peak values reached 800 pptv. This occurredsimultaneously with a maximum in ozone which increasedto 45–50 ppbv in March–April. The average NO_xmixing ratio was 27 pptv and did not show any cyclethrough the period. The NO_y mixing ratio showeda maximum in late March, while the difference betweenNO_y and PAN decreased during spring. This is anindication of the dominance of PAN in the NO_ybudget in the Arctic, but possible changes in theefficiency of the NO_y converter could alsocontribute to this. Although most PAN in theArctic is believed to be due to long range transport,the observations indicate local loss and formationrates of up to 1–2 pptv h^-1 in April–May.Measurements of carbonyl compounds suggest thatacetaldehyde was the dominant, local precursor ofPAN.Now at 1.     

Arctic hazes in summer over Greenland and the North American Arctic. I: Incidence and origins

Airborne observations during August 1985 over Greenland and the North American Arctic revealed that dense, discrete haze layers were common above 850 mb. No such hazes were found near the surface in areas remote from local sources of particles. The haze layers aloft were characterized by large light-scattering coefficients due to dry particles (maximum value 1.24 × 10^–4m^–1) and relatively high total particle concentrations (maximum value 3100 cm^–3). Sulfate was the dominant ionic component of the aerosol (0.06 – 1.9 g m^–3); carbon soot was also present. Evidence for relatively fresh aerosols, accompanied by NO₂ and O₃ depletion, was found near, but not within, the haze layers. The hazes probably derived from anthropogenic sources and/or biomass burning at midlatitudes.It is hypothesized that the scavenging of particles by stratus clouds plays an important role in reducing the frequency and intensity of hazes at the surface in the Arctic in summer. Since the detection of haze layers aloft through measurements of column-integrated parameters from the surface (e.g., by lidar) cannot be carried out reliably when clouds are present, such measurements have likely underestimated the occurrence of haze layers in the Arctic, particularly in summer.     

The partitioning of nitrogen oxides in the lower Arctic troposphere during spring 1988

Surface observations of several nitrogen oxides in the Canadian high Arctic during the period March-April 1988 are reported. These include data on NO₂, the inorganic nitrates HNO₃ and particulate nitrate, and the organic nitrates PAN and C₃–C₇ alkyl-nitrates. It is found that the organic nitrates make up 70–80% of the sum of the measured nitrogen oxides. Based on concurrently measured sulphur oxides, the period of observation was divided into two halves with the first half representing less polluted, more aged air than the second. The preponderance of the organic nitrates was less in the first period than the second. In contrast, there was little difference in the inorganic nitrates and NO₂ concentrations. The dominant inorganic nitrate shifted from particulate nitrate in the first period towards gaseous HNO₃ in the second. No correlation between the nitrates (inorganic or organic) and O₃ was observed; although some indication of a positive correlation between NO₂ and O₃ has been reported earlier (Bottenheimet al., 1990). Possible explanations for these observations are proposed. A survey of other potential nitrogen oxides that may be present in the Arctic air but not measured in these experiments suggests that the nitrogen oxides not measured here constitute a minor fraction of the total reactive nitrogen (NO _y ).Paper submitted to the 7th International Symposium of the Commission for Atmospheric Chemistry and Global Pollution on the Chemistry of the Global Atmosphere held in Chamrousse, France, from 5 to 11 September 1990.     

Total column densities of tropospheric and stratospheric trace gases in the undisturbed Arctic summer atmosphere

Ground-based FTIR measurements have been performed in the Arctic summer in July 1993 and June 1994 at 79° N to study the zenith column densities of several trace gases in the undisturbed Arctic summer atmosphere. Zenith column densities of H₂O, N₂O, HNO₃, NO₂, NO, ClONO₂, ClO, HCl, HF, COF₂, OCS, SF₆, HCN, CH₄, C₂H₆, C₂H₂, CO, O₃, CFC-12, CFC-22, and CO₂ were retrieved by line-by-line calculations. The results are compared with winter and springtime observations measured at the same site, with column densities obtained in the Antarctic summer atmosphere, and with measurements at midlatitudes. For HCl the spectra give lower total zenith columns than expected, but the ratio HF/HCl agrees well with midlatitude literature data. Measurements of ClONO₂ give low total columns in agreement with observations at midlatitudes. In the undisturbed atmosphere HCl was found to be in excess of ClONO₂. The total columns of HNO₃, N₂O and the sum of NO and NO₂ agree with summer observations in Antarctica. Results for the tropospheric trace gas C₂H₆ are higher by 250% when compared with Antarctic observations. Contrary to N₂O and CH₄ the seasonal cycle of C₂H₆ and C₂H₂ give much higher total columns in winter/spring compared to the summer observations. This is assigned to transport of polluted airmasses from mid-latitudes into the Arctic.     

The evolution of Pinatubo aerosols in the Arctic stratosphere during 1994–2000

Balloon-borne aerosol measurements were performed with an optical particle counter between 1994 and 2000 at Ny-Ålesund (79°N), Svarbard. Throughout the observation period, continuous decay was found in the concentrations of particles with 0.4–0.6 μm in radius in the Arctic stratosphere, suggesting that Pinatubo aerosols remained even at the end of the 1990s. The decay rate was clearly higher for larger particle sizes, and higher at higher altitude (e-folding time of 970–526 days), suggesting a gravitational sedimentation effect. For smaller particles with R<0.4 μm, slight increases in concentration with time were found, which agreed with the measurements at mid-latitude. The sulfate mass mixing ratio in the Arctic stratosphere before 1998 showed values higher than those at middle latitude, while values were almost the same in both regions after 1998. A possible explanation of the latitudinal difference is a time lag (of 0.5–1 year) in the arrival of Pinatubo aerosols in the Arctic.     

Is the Arctic Surface Layer a Source and Sink of NOx in Winter/Spring?

Measurements of NO_x (NO +NO₂) and the sum of reactive nitrogenconstituents, NO_y, were made near the surface atAlert (82.5°N), Canada during March and April1998. In early March when solar insolation was absentor very low, NO_x mixing ratios were frequentlynear zero. After polar sunrise when the sun was abovethe horizon for much or all of the day a diurnalvariation in NO_x and NO_y was observed withamplitudes as large as 30–40 pptv. The source ofactive nitrogen is attributed to release from the snowsurface by a process that is apparently sensitized bysunlight. If the source from the snowpack is a largescale feature of the Arctic then the diurnal trendsalso require a competing process for removal to thesurface. From the diurnal change in the NO/NO₂ratio, mid-April mixing ratios for the sum of peroxyand halogen oxide radicals of 10 pptv werederived for periods when ozone mixing ratios were inthe normal range of 30–50 ppbv. Mid-day ozoneproduction and loss rates with the active nitrogensource were estimated to be 1–2 ppbv/day and in nearbalance. NO_y mixing ratios which averaged only295±66 pptv do not support a large accumulation inthe high Arctic surface layer in the winter and springof 1998. The small abundance of NO_y relative tothe elevated mixing ratios of other long-livedanthropogenic constituents requires that reactivenitrogen be removed to the surface during transport toor during residence within the high Arctic.     

Nonmethane hydrocarbons in an oceanic atmosphere

C₂–C₆ Nonmethane hydrocarbons (NMHC) and radioactive continental tracers were measured during two oceanographic cruises, in June 1982 in the Mediterranean and Red Sea, and in November 1982 across the North Atlantic and South Pacific oceans. Typical concentrations in marine atmosphere are between 0.05 and 0.2 ppbv. Owing to their similar lifetimes, propane and radon-222 are found to be well correlated. This relationship establishes that propane is mainly produced over lands and enables us to estimate its continental source strength at about 60×10⁶ tons of carbon per year.Also at Université de Picardie     

The tropospheric distribution of formaldehyde during TROPOZ II

A series of 149 measurements of the HCHO mixing ratio were made between 0 and 10 km altitude and 70° N to 60° S latitude during TROPOZ II. The data show a vertical decrease of the HCHO mixing ratio with altitude at all latitudes and a broad latitudinal maximum in the HCHO mixing ratio between 30° N and 30° S at all altitudes. The measured mixing ratios of HCHO are considerably higher than those expected from CH₄ oxidation alone, but agree broadly with the average latitude by altitude distribution of HCHO derived by a 2D model including emissions of C₁–C₇ hydrocarbons. A number of the regional scale deviations of the measured HCHO distribution from the average modelled one can be explained in terms of the local wind field.     

Simultaneous measurements of peroxy and nitrate radicals at Schauinsland

We present simultaneous field measurements of NO₃ and peroxy radicals made at night in a forested area (Schauinsland, Black Forest, 48° N, 8° N, 1150 ASL), together with measurements of CO, O₃, NO _x , NO _y , and hydrocarbons, as well as meteorological parameters. NO₂, NO₃, HO₂, and (RO₂) radicals are detected with matrix isolation/electron spin resonance (MIESR). NO₃ and HO₂ were found to be present in the range of 0–10 ppt, whilst organic peroxy radicals reached concentrations of 40 ppt. NO₃, RO₂, and HO₂ exhibited strong variations, in contrast to the almost constant values of the longer lived trace gases. The data suggest anticorrelation between NO₃ and RO₂ radical concentrations at night.The measured trace gas set allows the calculation of NO₃ and peroxy radical concentrations, using a chemical box model. From these simulations, it is concluded that the observed anthropogenic hydrocarbons are not sufficient to explain the observed RO₂ concentrations. The chemical budget of both NO₃ and RO₂ radicals can be understood if emissions of monoterpenes are included. The measured HO₂ can only be explained by the model, when NO concentrations at night of around 5 ppt are assumed to be present. The presence of HO₂ radicals implies the presence of hydroxyl radicals at night in concentrations of up to 10⁵ cm^–3.     

Characteristics of Arctic synoptic activity, 1952–1989

Summary Synoptic activity for the Arctic is examined for the period 1952–1989 using the National Meteorological Center sea level pressure data set. Winter cyclone activity is most common near Iceland, between Svalbard and Scandinavia, the Norwegian and Kara seas, Baffin Bay and the eastern Canadian Arctic Archipelago; the strongest systems are found in the Iceland and Norwegian seas. Mean cyclone tracks, prepared for 1975–1989, confirm that winter cyclones most frequently enter the Arctic from the Norwegian and Barents seas. Winter anticyclones are most frequent and strongest over Siberia and Alaska/Yukon, with additional frequency maxima of weaker systems found over the central Arctic Ocean and Greenland.During summer, cyclonic activity remains common in the same regions as observed for winter, but increases over Siberia, the Canadian Arctic Archipelago and the Central Aretic, related to cyclogenesis over northern parts of Eurasia and North America. Eurasian cyclones tend to enter the Aretic Ocean from the Laptev Sea eastward to the Chukchi Sea, augmenting the influx of systems from the Norwegian and Barents seas. The Siberian and Alaska/Yukon anticyclone centers disappear, with anticyclone maxima forming over the Kara, Laptev, East Siberian and Beaufort seas, and southeastward across Canada. Summer cyclones and anticyclones exhibit little regional variability in mean central pressure, and are typically 5–10 mb weaker than their winter counterparts.North of 65°N, cyclone and anticyclone activity peaks curing summer, and is at a minimum during winter. Trends in cyclone and anticyclone activity north of 65°N are examined through least squares regression. Since 1952, significant positive trends are found for cyclone numbers during winter, spring and summer, and for anticyclone numbers during spring, summer and autumn.With 11 Figures     

Relations between Light Hydrocarbon, Carbon Monoxide, and Carbon Dioxide Concentrations in the Plume from the Combustion of Plant Material in a Furnace

To systematically explain relations between light hydrocarbons, CO, and CO₂ concentrations/emissions of biomassburning, we measured concentrations/emissions of carbon gases – CO,CO₂, light hydrocarbons (CH₄, C₂H₆,C₂H₄, C₂H₂, C₃H₈, C₃H₆,n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂,i-C₅H₁₂), and THC (total hydrocarbon) – in the burning of dead plant material, mainly Imperata grass, byclosed-chamber experiments and by time-series analyses of gas concentrations in combustion plumes in relatively efficient and inefficient combustion situations. Concentrations of hydrocarbons measured were well correlated to [CO] although [C₂H₂] was exceptionally well correlated to[CO₂]. The phase diagrams (relation between [CO]/ [CO₂] and [hydrocarbon]/ [CO₂]) obtained by the time-seriesexperiments well illustrated the variation in the overall emission rates of the closed-chamber experiments. The higher rates of decrease in hydrocarbon concentration with increasing carbon number in the efficient case compared with the inefficient case probably reflected the rate of oxidation and the amount of radicals. The overall concentrations (or emissions) of C₂H₄ and C₃H₆ were higher thanthose of C₂H₆ and C₃H₈, suggesting a linkage to mechanisms in whichthe predominant path of hydrocarbon oxidation is through the degradation of alkyl radicals, which can be immediately converted into or formed from alkenes. For C₃ and C₄ species, normal-chain species hadhigher emissions than iso-chain species under lower combustion efficiency. This may be attributable to the presence of tertiary C–H bonds in iso-species,which show more reactivity in the abstraction of H than secondary C–H bonds unless the carbon number is large.     

Ground-Based and Airborne Measurements of Nonmethane Hydrocarbons in BERLIOZ: Analysis and Selected Results

During the Berlin Ozone Experiment BERLIOZ in July–August 1998 quasi-continuous measurements ofC₂–C₁₂ nonmethane hydrocarbons (NMHCs) were carried out at 10 sites in and around the city of Berlin using on-line gas-chromatographic systems (GCs) with a temporal resolution of 20–120 minutes. Additional airborne NMHCmeasurements were made using canister sampling on three aircraft and an on-line GC system on a fourth aircraft. The ground based data are analyzed to characterize the different sites and to identify the influence of emissions from Berlin on its surroundings. Benzene mixing ratios at the 4 rural sites were rather low (<0.5 ppbv). Berlin (and the surrounding highway ring) was identified as the main source of anthropogenic NMHCs at Eichstädt and Blossin, whilst other sources were important at the furthermost site Menz. The median toluene/benzene concentration ratio in Berlin was 2.3 ppbv/ppbv, agreeing well with measurements in other German cities. As expected, the ratios at the background sites decreased with increasing distance to Berlin and were usually around one or below. On 20 and 21 July, the three northwesterly sites were situated downwind of Berlin and thus were influenced by its emissions. Considering the distance between the sites and the windspeed, the city plume was observed at reasonable time scales, showing decreasing toluene/benzene ratios of 2.3, 1.6 and 1.3 with increasing distance from Berlin. Isoprene was the only biogenic NMHC measured at BERLIOZ. It was themost abundant compound at the background sites on the hotter days, dominating the local NMHC reactivity with averaged contributions to the total OH loss rate of 51% and 70% at Pabstthum and Blossin, respectively. Emissionratios (relative to CO and to the sum of analysed NMHCs) were derived from airborne measurements. The comparison with an emission inventory suggests traffic-related emissions to be the predominating source of the considered hydrocarbon species. Problems were identified with the emission inventory for propane, ethene and pentanes.     

Boundary-layer ozone depletion as seen in the Norwegian Arctic in spring

Several years of measurements of ozone, hydrocarbons, sulphate and meteorological parameters from Spitsbergen in the Norwegian Arctic are presented. Most of the measurements were taken on the Zeppelin Mountain at an altitude of 474 m a.s.l. The focus is the episodes of ozone depletion in the lower troposphere in spring, which are studied in a climatological way. Episodes of very low ozone concentrations are a common feature on the Zeppelin Mountain in spring. The low ozone episodes were observed from late March to the beginning of June. When the effect of transport direction was subtracted, the frequenty of the low ozone episodes was found to peak in the beginning of May, possibly reflecting the seasonal cycle in the actual depletion process. Analyses based on trajectory calculations show that most of the episodes occurred when the air masses were transported from W-N. Ozone soundings show that the ozone depletion may extend from the surface and up to 3–4 km altitude. The episodes were associated with a cold boundary layer beneath a thermally stable layer, suppressing mixing with the free troposphere. The concentration of several individual hydrocarbons was much lower during episodes of low ozone than for the average conditions. The change in concentration ratio between the hydrocarbons was in qualitative agreement with oxidation of hydrocarbons by Br and Cl atoms rather than by OH radicals.     

Pan over the Arctic; Observations during AGASP-2 in April 1986

During three of the flights with the NOAA P3 Orion over the Arctic icecap in April 1986, the atmospheric concentration of PAN (peroxyacetyl nitrate) was measured. Due to major experimental problems, the uncertainty in the data is large (+/–50%), but, nevertheless, some important trends can be resolved. More than 600 (+/–300) ppt(v) of PAN was present in a moderately dense arctic haze layer, confirming conclusions reached from surface observations at Alert, N.W.T., Canada, that PAN is a major odd nitrogen species in Arctic polluted air masses. In relatively clean air off Barrow, Alaska, PAN levels were well below 100 (+/–50) ppt(v), increasing with altitude, in agreement with theoretical predictions concerning the occurrence of PAN in clean air. PAN mixing ratios in the upper troposphere or lower stratosphere were variable (from ca. 30 (+/–15) ppt(v) on April 13 up to 140 (+/–70) ppt(v) on April 8), suggesting involvement in the tropospheric-stratospheric exchange of odd nitrogen. To place the PAN data in a broader context, measurements of other NO_y compounds as well as integrated SO_x data are also reported.     

Modified HNO3 seasonality in volcanic layers of a polar ice core: Snow-pack effect or photochemical perturbation?

Using the chemical composition of snow and ice of a central Greenland ice core, we have investigated changes in atmospheric HNO₃ chemistry following the large volcanic eruptions of Laki (1783), Tambora (1815) and Katmai (1912). The concentration of several cations and anions, including SO ₄ ^2– and NO ₃ ^– , were measured using ion chromatography. We found that following those eruptions, the ratio of the concentration of NO ₃ ^– deposited during winter to that deposited during summer was significantly higher than during nonvolcanic periods. Although we cannot rule out that this pattern originates from snow pack effects, we propose that increased concentrations of volcanic H₂SO₄ particles in the stratosphere may have favored condensation and removal of HNO₃ from the stratosphere during Arctic winter. In addition, this pattern might have been enhanced by slower formation of HNO₃ during summer, caused by direct consumption of OH through oxidation of volcanic SO₂.