Measurements of atmospheric dimethyl sulfide and carbon disulfide in the western Atlantic boundary layer

Shipboard measurements of atmospheric dimethyl sulfide were made during two transects along the east coast of the United States and at several stations in the Gulf of Maine. Limited measurements of carbon disulfide and hydrogen sulfide are also reported. The mean DMS mixing ratio was 29 pptv (=25, n=84, median 19 pptv) during the Atlantic transects, and 101 pptv (=67, n=77, median 79 pptv) in the Gulf of Maine. Distinct diurnal variations were found in the DMS data from the transects. The meteorology of the study area appears to control day-to-day differences in the magnitude of these diurnal variations, although rapid daytime oxidation is suggested in some cases. Diurnal variations were also evident in near-shore stations in the Gulf of Maine due to nocturnal boundary-layer inversion. Diurnal variation was not evident at other sites in the Gulf due to large scale changes in the atmospheric circulation pattern, which effectively masked any effects due to oxidation processes. Model simulations confirm that the DMS levels and diurnal variation found during the transects are not consistent with atmospheric oxidation processes alone. Atmospheric CS₂ and H₂S mixing ratios were less than 3 pptv during the transects, except for a single period of higher CS₂ mixing ratios (reaching 11 pptv) during advection of continental air. Calculations of the flux of oceanic sulfur to the eastern United States show that the contribution of natural sulfur to the North American sulfur budget is small compared to anthropogenic sources.     

Short-Term Variability of Atmospheric DMS and Its Oxidation Products at Amsterdam Island during Summer Time

A one-month experiment was performed at Amsterdam Island in January 1998, to investigate the factors controlling the short-term variations of atmospheric dimethylsulfide (DMS) and its oxidation products in the mid-latitudes remote marine atmosphere. High mixing ratios of DMS, sulfur dioxide (SO₂) and dimethylsulfoxide (DMSO) have been observed during this experiment, with mean concentrations of 395 parts per trillion by volume (pptv) (standard deviation, = 285, n = 500), 114 pptv ( = 125, n = 12) and 3 pptv ( = 1.2, n = 167), respectively. Wind speed and direction were identified as the major factors controlling atmospheric DMS levels. Changes in air temperature/air masses origin were found to strongly influence the dimethylsulfoxide (DMSO)/DMS and SO₂/DMS molar ratios, in line with recent laboratory data. Methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄ ^2–) mean concentrations in aerosols during this experiment were 12.2± 6.5 pptv (1, n=47) and 59 ± 33 pptv (1, n=47), respectively. Evidence of vertical entrainment was reported following frontal passages, with injection of moisture-poor, ozone-rich air. High MSA/ nss-SO₄ ^2– molar ratios (mean 0.44) were calculated during these events. Finally following frontal passages, few spots in condensation nuclei (CN) concentration were also observed.     

Shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide in the Caribbean and Gulf of Mexico

Simultaneous shipboard measurements of atmospheric dimethylsulfide and hydrogen sulfide were made on three cruises in the Gulf of Mexico and the Caribbean. The cruise tracks include both oligotrophic and coastal waters and the air masses sampled include both remote marine air and air masses heavily influenced by terrestrial or coastal inputs. Using samples from two north-south Caribbean transects which are thought to represent remote subtropical Atlantic air, mean concentrations of DMS and H₂S were found to be 57 pptv (74 ng S m^-3, =29 pptv, n=48) and 8.5 pptv (11 ng S m^-3, =5.3 pptv, n=36), respectively. The ranges of measured concentrations for all samples were 0–800 pptv DMS and 0–260 pptv H₂S. Elevated concentrations were found in coastal regions and over some shallow waters. Statistical analysis reveals slight nighttime maxima in the concentrations of both DMS and H₂S in the remote marine atmosphere. The diurnal nature of the H₂S data is only apparent after correcting the measurements for interference due to carbonyl sulfide. Calculations using the measured ratio of H₂S to DMS in remote marine air suggest that the oxidation of H₂S contributes only about 11% to the excess (non-seasalt) sulfate in the marine boundary layer.     

Measurements of Volatile Organic Compounds (VOC) During POPCORN 1994: Applying a New On-Line GC–MS-Technique

Atmospheric concentrations of ca. 250 C6–C15 hydrocarb on and C4–C12 oxygenated volatile organic compounds (VOC) including alkanes, benzene and alkyl benzenes, monoterpenes and aldehydes were measured in August 1994 during the POPCORN campaign (POPCORN = Photo-Oxidant formation by Plant emitted Compounds and OH Radicals in North-Eastern Germany). About 80 substances together contributed 90% of the atmospheric carbon in this range of molecular weight. During this field campaign VOC-emissions from several crop and tree species and the ambient concentrations of CO, C2–C7 non-methane hydrocarbons (NMHC), C1 and C2 aldehydes, nitrogen oxides, ozone and hydroxyl-radicals (OH) were also measured. These data were used to interpret the VOC measurements presented here. The on-line GC–MS used for the VOC measurements combines adsorptive sampling with thermal desorption and GC–MS analysis in an automated system. Internal standards were used to quantify the measurements. Ozone was destroyed prior to the sample preconcentration through the gas phase reaction with NO. Aromatic compounds like benzene, toluene and xylenes were the most abundant compound class among the measured substances, -pinene and 3-carene, most probably originating from pineforests ca. 1 km away from the measuring site, were the most abundant monoterpenes. The highest mixing ratios of most compounds were measured in nights with strong inversion situations. The toluene mixing ratios then reached 630 pptv; -pinene mixing ratios went up to 430 pptv. The median of all toluene and -pinene measurements during the campaign was 125 pptv or 22 pptv, respectively. These values are on the lower end of ambient measurements reported for continental sites. In most samples also n-pentanal, n-hexananl, n-nonanal and n-undecanal were present. Median mixing ratios were 9, 16, 14 and 8 pptv, respectively. Emission studies indicate that these highly reactive compounds are most probably emitted from maize. It is shown by a simple first order approach that the potential for ozone formation during the POPCORN campaign was roughly equal for anthropogenic and biogenic VOC. From measured concentrations of ozone, OH-radicals, methane, CO, C2–C15 nonmethane hydrocarbons (NMHC) and C5–C11 aldehydes a photochemical production of ozone in the order of 3.5 ppb/h can be estimated. Apart from formaldehyde and acetaldehyde, which are at least partly products of VOC oxidation, the substance group with the largest contribution to the VOC turnover are the monoterpenes. They contribute ca. 30%. However, the mechanism of terpene oxidation is very complex and presently only partly understood. Thus the actual contribution of monoterpenes to ozone formation is very uncertain. Other measured compound classes such as light alkenes, alkanes, aromatics, and C5–C11 aldehydes contribute each between 10% and 15% to ozone formation. The measuring site was not influenced directly from strong biogenic or anthropogenic sources, and the results obtained during the POPCORN campaign can be regarded as a typical picture of a remote rural central European environment.     

Surface NO and NO2 mixing ratios measured between 30° N and 30° S in the Atlantic region

Surface NO and NO₂ mixing ratios were measured aboard the research vessel Polarstern during the mission ANT VII/1 from 24 September to 5 October 1988. The measurements were taken along the meridian at 30° W in the Atlantic region covering latitudes between 30° N and 30° S. The average mixing ratios were about 12 pptv NO/30 pptv NO₂ in the Northern Hemisphere and about 7 pptv NO/22 pptv NO₂ in the Southern. Elevated mixing ratios of 20 pptv NO/70 pptv NO₂ were found at 12° N (probably due to air masses originating from the surface of West Africa) and in the region of the ITCZ between 8° N and 5° N. Because of probable contamination by the ship, the measured mixing ratios mostly represent upper limits.     

DOAS observations of formaldehyde and its impact on the HO<Subscript>x</Subscript> balance in the tropical Atlantic marine boundary layer

Measurements of formaldehyde (HCHO) were made at the Cape Verde Atmospheric Observatory between November 2006 and June 2007 using the Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) technique. Observations show that typical HCHO mixing ratios ranged between 350 and 550 pptv (with typical 2-σ uncertainties of ~110 pptv), with several events of high HCHO, the maximum being 1,885?±?149 pptv. The observations indicate a lack of strong seasonal or diurnal variations, within the uncertainty of the measurements. A box model is employed to test whether the observations can be explained using known hydrocarbon photochemistry; the model replicates well the typical diurnal profile and monthly mean values. The model results indicate that on average 20% of HO₂ production and 10% of OH destruction can be attributed to the mean HCHO levels, suggesting that even at these low average mixing ratios HCHO plays an important role in determining the HO_x (HO₂+OH) balance of the remote marine boundary layer.     

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.     

Hydrogen peroxide measurements in the marine atmosphere

Hydrogen peroxide, one of the key compounds in multiphase atmospheric chemistry, was measured on an Atlantic cruise (ANT VII/1) of the German research vessel Polarstern from 15 September to 9 October 1988, in rain and ambient air by a chemiluminescence technique. For gas phase H₂O₂ cryogenic sampling was employed. The presented results show an increase of gas-phase mixing ratios of about 45 pptv per degree latitude between 50° N and 0°, and a maximum of 3.5 ppbv around the equator. Generally higher mixing ratios were observed in the Southern Hemisphere, with a clear diurnal variation. The H₂O₂ mixing ratio is correlated to the UV radiation intensity and to the temperature difference between air and ocean surface water.     

Partitioning Between Chlorine Reservoir Species Deduced from Observations in the Arctic Winter Stratosphere

Simultaneous observations of several chlorine source gases, as well asHCl and ClO, have been performed in the Arctic stratosphere on 1 and 9February 1994, using balloon-borne instrumentation as a contribution toSESAME (Second European Stratospheric Arctic and Mid latitude Experiment).The observed mixing ratios of HCl and N₂O show a clearanticorrelation. No severe loss of HCl was observed inside the vortex duringour measurement. These measurements showed that during this period at 20 kmand above, HCl was either in excess, or at least as abundant, asClONO₂ and comprised between 50 and 70% of theavailable chlorine, Cl_y. On 1 February, measurements were madeinside the polar vortex. The air mass sampled on this day showed a clearsignature of diabatic descent, and also enhanced levels of ClO with amaximum of 230 pptv at 22.5 km. A 10 day backward trajectory analysis showedthat these air masses had passed a large region of low temperatures a fewhours prior to the measurement. Temperatures along the back trajectory atthe 475 K and 550 K levels (20.1 and 23.7 km respectively) were cold enoughfor heterogeneous chlorine activation to occur, in agreement with theobserved elevated ClO mixing ratios.     

Seasonal variation of atmospheric dimethylsulfide at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric concentrations of dimethylsulfide (DMS) were carried out for two years in a marine site at remote area: the Amsterdam Island (37°50S–77°31E) located in the southern Indian Ocean. DMS concentrations were also measured in seawater. A seasonal variation is observed for both DMS in the atmosphere and in the sea-surface. The monthly averages of DMS concentrations in the surface coastal seawater and in the atmosphere ranged, respectively, from 0.3 to 2.0 nmol l^-1 and from 1.4 to 11.3 nmol m^-3 (34 to 274 pptv), with the highest values in summer. The monthly variation of sea-to-air flux of DMS from the southern Indian Ocean ranges from 0.7 to 4.4 mol m^-2 d^-1. A factor of 2.3 is observed between summer and winter with mean DMS fluxes of 3.0 and 1.3 mol m^-2 d^-1, respectively.     

Grab sampling for the determination of sulfur dioxide and dimethyl sulfide in air by isotope dilution gas chromatography/mass spectrometry

Developments allowing the direct determination of sulfur dioxide and dimethyl sulfide in grab samples by gas chromatography/mass spectrometry with isotopically labeled standards (GC/MS/ILS) are reported. Isotopomers of DMS and SO₂ are used as internal standards. Spiked air samples are dried to a dew point of <–60 °C and trapped cryogenically in loops of Teflon tubing. Sealed samples are transported to the laboratory under liquid nitrogen and later subjected to GC/MS analysis. Holding times of up to one month do not result in significant sample loss. For samples collected in a clean marine environment, concentrations of SO₂ and DMS greater than 5 and 8 pptv, respectively, are significantly different from blanks at the 95% confidence level. Average measurement precision derived from a propagation of errors are 9% for SO₂ and 42% for DMS at concentrations from 5–15 pptv.Improvements are outlined which should provide sensitivity and precision comparable to that of on-site GC/MS. The technique will allow increased flexibility for the determination of trace sulfur species in the field under conditions where deployment of a mass spectrometer is not possible.     

Hydroxyl and Peroxy Radical Chemistry in a Rural Area of Central Pennsylvania: Observations and Model Comparisons

Atmospheric hydroxyl (OH), hydroperoxy (HO₂), total peroxy (HO₂ and organic peroxy radicals, RO₂) mixing ratios and OH reactivity (first order OH loss rate) were measured at a rural site in central Pennsylvania during May and June 2002. OH and HO₂ mixing ratios were measured with laser induced fluorescence (LIF); HO₂ + RO₂ mixing ratios were measured with chemical ionization mass spectrometry (CIMS). The daytime maximum mixing ratios were up to 0.6 parts per trillion by volume (pptv) for OH, 30 pptv for HO₂, and 45 pptv for HO₂ + RO₂. A parameterized RACM (Regional Atmospheric Chemistry Mechanism) box model was used to predict steady state OH, HO₂ and HO₂ + RO₂ concentrations by constraining the model to the measured OH reactivity and previously measured volatile organic compound (VOC) distributions. The averaged model calculations are generally in good agreement with the observations. For OH, the model matched the observations for day and night, with an average observed-to-modeled ratio of 0.80. In previous studies such as PROPHET98, nighttime NO was near 0 pptv and observed nighttime OH was significantly larger than modeled OH. In this study, nighttime observed and modeled OH agree to within measurement and model uncertainties because the main source of the nighttime OH was the reaction HO₂ + NO → OH + NO₂, with the NO being continually emitted from the surrounding fertilized corn field. The observed-to-modeled ratio for HO₂ is 1.0 on average, although daytime HO₂ is underpredicted by a factor of 1.2 and nighttime HO₂ is over-predicted by a factor of ∼2. The average measured and modeled HO₂ + RO₂ agree well during daytime, but the modeled value is about twice the measured value during nighttime. While measured HO₂ + RO₂ values agree with modeled values for NO mixing ratios less than a few parts per billion by volume (ppbv), it increases substantially above the expected value for NO greater than a few ppbv. This observation of the higher-than-expected HO₂ + RO₂ with the CIMS technique confirms the observed increase of HO₂ above expected values at higher NO mixing ratios in HO₂ measurements with the LIF technique. The maximum instantaneous O₃ production rate calculated from HO₂ and RO₂ reactions with NO was as high as 10–15 ppb h⁻¹ at midday; the total daily O₃ production varied from 13 to 113 ppbv d⁻¹ and was 48 ppbv d⁻¹ on average during this campaign.     

Aircraft measurements of oxides of nitrogen along the eastern rim of the Asian continent: Winter observations

Aircraft observations of oxides of nitrogen (NO _y ), measured with a ferrous sulfate converter, over the sea surrounding the Japanese islands (30–43° N, 131–141° E) were carried out in the winter of 1983 and 1984 at altitudes mostly between 3 and 8 km. NO _y defined here is the sum of NO, NO₂, and other unstable oxides of nitrogen that are converted to NO by ferrous sulfate. The main observations were:

1. Over the Pacific Ocean between the latitudes of 30–35° N, the observed NO _y mixing ratio between 3 and 8 km was a fairly constant 200 pptv. The NO mixing ratio increased with altitude from 15 pptv at 3 km to 35 pptv at 7 km.
2. Over the Sea of Japan, tropospheric NO _y mesured between 1 and 6 km started increasing with latitude North of 35° N and reached about 1000 pptv at 40° N.
3. NO _y was measured in an air mass transported from the stratosphere near a tropopause fold region. When the ozone mixing ratio was between 80 and 140 ppbv, the NO _y mixing ratio was about 200 pptv.

     

Measurement of gaseous HNO3 over the Atlantic Ocean

A latitudinal profile (30° W, from 30° N to 30° S) of mixing ratios of nitric acid and particulate nitrate was determined on the Atlantic Ocean during the Polarstern cruise ANT VII/1 from Bremerhaven, Germany, to Rio Grande, Brazil. The detection of HNO₃ was performed simultaneously by laser-photolysis fragment-fluorescence (LPFF) and by nylon filter packs. The detection limit was about 30 pptv for a signal accumulation time of 1 h for LPFF and about 5 pptv for the filters at a collection time of 4 h. In general, the mixing ratios of HNO₃ in the Northern Hemisphere were found to be significantly higher than those in the Southern Hemisphere. The Atlantic background concentrations frequently varied between 80 pptv and the detection limit. Larger deviations from this trend were found for the more northern latitudes and for episodes like crossings of exhaust plumes from ships or from continental pollutions sources.     

High-Latitude Springtime Photochemistry. Part I: NOx, PAN and Ozone Relationships

Springtime measurements of NO_x, ozone, PAN,J(NO₂), and other compounds were made near Ny-Ålesund,Svalbard (78°54N, 11°53E), in 1994 and Poker Flat,Alaska (65°08N, 147°29W), in 1995. At Svalbard medianmixing ratios for PAN and NO_x of 237 and 23.7 pptv,respectively, were observed. The median mixing ratios at Poker Flat for PANand NO_x were 79.5 and 85.9 pptv, respectively. These data areused to estimate thermal PAN decomposition using several differentapproaches. At Svalbard PAN decomposition was very small, while at PokerFlat up to 30 pptv/h PAN decomposed. At both sites the NO_x/PANratio increased with temperature between –10 and 20°C implyingthat PAN decomposition is an important NO_x source. In-situozone production was calculated from the measured NO, NO₂,O₃, J(NO₂), and temperature data, using thesteady state assumption Median ozone production was 605 pptv/h at PokerFlat, and one order of magnitude smaller at Svalbard during the daytime.Only at Poker Flat could a direct influence on the diurnal ozone cycle beobserved from in-situ production. These results imply that PAN decompositionis a major source of NO_x in the high latitude troposphere, andthat this contributes to the observed spring maximum in surface ozone.     

The Impact of the Afternoon Planetary Boundary-Layer Height on the Diurnal Cycle of CO and $$\hbox {CO}_{2}$$ Mixing Ratios at a Low-Altitude Mountaintop

Mountaintop trace-gas mixing ratios are often assumed to represent free atmospheric values, but are affected by valley planetary boundary-layer (PBL) air at certain times. We hypothesize that the afternoon valley–PBL height relative to the ridgetop is important in the diurnal cycle of mountaintop trace-gas mixing ratios. To investigate this, we use, (1) 4-years (1 January 2009–31 December 2012) of CO and \(\hbox {CO}_{2}\) mixing-ratio measurements and supporting meteorological observations from Pinnacles (\(38.61^{\circ }\hbox {N}\), \(78.35^{\circ }\hbox {W}\), 1017 m a.s.l.), which is a monitoring site in the Appalachian Mountains, (2) regional \(\hbox {O}_{3}\) mixing-ratio measurements, and (3) PBL heights determined from a nearby sounding station. Results reveal that the amplitudes of the diurnal cycles of CO and \(\hbox {CO}_{2}\) mixing ratios vary as a function of the daytime maximum valley–PBL height relative to the ridgetop. The mean diurnal cycle for the subset of days when the afternoon valley–PBL height is at least 400 m below the ridgetop shows a daytime CO mixing-ratio increase, implying the transport of PBL air from the valley to the mountaintop. During the daytime, on days when the PBL heights exceed the mountaintop, PBL dilution and entrainment cause CO mixing ratios to decrease. This decrease in CO mixing ratio, especially on days when PBL heights are at least 400 m above the ridgetop, suggests that measurements from these days can be used as with afternoon measurements from flat terrain in applications requiring regionally-representative measurements.     

Atmospheric Measurements of HONO by Tunable Diode Laser Absorption Spectroscopy

Ambient gas phase nitrous acid (HONO) has been measured by Tunable Diode Laser Absorption Spectroscopy with sub-ppbv detection limits. An R-branch line in the 1263.4 cm^–1 ₃ band was found tobe free of interference and suitable for ambient HONO measurements. Nitrous acid was measured during the night and early morning at an urban site in Toronto, Canada, during the summer of 1998. Average mixing ratios, integrated over 30 minutes, ranged from below the minimum detection limit of 300 pptv to 1.9 ppbv, with the highest concentrations observed during the early morning hours. During the night of 19 June 1998, the concentration of HONO increased by as much as 0.5 ppbv/hr. The usual decrease in HONO after sunrise was delayed by a few hours, possibly due to a combination of an increase in the production rate of HONO with rush hour, and attenuation of the early morning light by high NO₂ within the aerosol fog/haze.     

An upper limit on the HCl near-surface mixing ratio over the Atlantic measured using TDLAS

A study of the gas phase mixing ratio of hydrochloric acid was made during the 1988 Polarstern expedition using tunable diode laser absorption spectroscopy (TDLAS). The measurements were made 22 m above sea level at 28° N, 30° W. The signal-to-noise ratio was unity for <50 pptv HCl, but because of wall effects in the sampling system, only an upper limit for the HCl mixing ratio of <250 pptv can be stated.     

Measurements of peroxyacetylnitrate in the marine boundary layer over the Atlantic

The atmospheric concentration of peroxyacetylnitrate (PAN) was measured during a cruise of the R.S. Polarstern from Bremerhaven (Germany) to Rio Grande do Sul (Brazil) in September/ October 1988. The measurements were made in-situ by a combination of electron capture gaschromatography with a cryogenic preconcentration step. The theoretical lower limit of detection (3) was 0.4 ppt. The mixing ratios of PAN varied by more than three orders of magnitude from 2000 ppt in the English Channel to less than 0.4 ppt south of the Azores (38° N). South of 35° N, PAN levels were below the detection limit, except at 30–31° S off the eastern coast of South America. Here, PAN mixing ratios of 10 to 100 ppt were detected in continentally influenced air masses. Detectable levels of PAN were mostly observed in air masses of continental or high northern origin. Changes in the wind directions were usually associated with substantial changes in the PAN mixing ratios.     

Photochemistry in the Arctic Free Troposphere: Ozone Budget and Its Dependence on Nitrogen Oxides and the Production Rate of Free Radicals

Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO_x mixing ratiosup to 300 pptv in February and for NO_x mixing ratios up to 500 pptv in May. These NO_x limits are an order of magnitude higher thanmedian NO_x levels observed, illustrating the strong dependence ofgross ozone production rates on NO_x mixing ratios for the majority of theobservations. The threshold NO_x mixing ratio needed for netpositive ozone production was also calculated to increase from NO_x 10pptv in February to 25 pptv in May, suggesting that the NO_x levels needed to sustain net ozone production are lower in winter than spring. This lower NO_x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO_x neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign.