NO2 Mixing Ratios Research Articles

Nighttime nitrate radical chemistry at Appledore Island, Maine during the 2004 International Consortium for Atmospheric Research on Transport and Transformation

Trace gases including nitrogen dioxide (NO2), nitrate radical (NO3), ozone (O3), and a suite of volatile organic compounds (VOCs) were measured within the New England coastal marine boundary layer on Appledore Island (AI), Maine, USA as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign. These measurements, together with local meteorological records and published kinetic data were used to investigate nighttime NO3 chemistry at AI during the period of 8–28 July 2004. Among the VOCs, isoprene, monoterpenes and dimethylsulfide (DMS) were the dominant NO3 reactants; on average, DMS accounted for 51 ± 34% of the total reactivity. For three case studies, NO3 mixing ratios were calculated from measured parameters with resultant uncertainties of ≤30%. Discrepancies with measured NO3 appeared to result primarily from input parameter variability and exclusion of heterogeneous dinitrogen pentoxide (N2O5) chemistry. We indirectly determined that nighttime NO3 and NOx (=NO + NO2) removal via N2O5 chemistry (gas‐phase + heterogeneous) was on average 51–54% and 63–66% of the total respectively. Our analysis suggested that the minimum average NO3 and NOx removal via heterogeneous N2O5 chemistry was ∼10% of the total. Reducing gas‐phase N2O5 reactivity in accord with Brown et al. (2006a) increased the importance of heterogeneous N2O5 chemistry substantially. It is plausible that the latter pathway was often comparable to gas‐phase removal of NO3 and NOx. Overall, 24 h‐averaged NOx removal was ∼11 ppbv, with nighttime chemical pathways contributing ∼50%.

Methodical study of nitrous oxide eddy covariance measurements using quantum cascade laser spectrometery over a Swiss forest

Abstract. Nitrous oxide fluxes were measured at the Lägeren CarboEurope IP flux site over the multi-species mixed forest dominated by European beech and Norway spruce. Measurements were carried out during a four-week period in October–November 2005 during leaf senescence. Fluxes were measured with a standard ultrasonic anemometer in combination with a quantum cascade laser absorption spectrometer that measured N2O, CO2, and H2O mixing ratios simultaneously at 5 Hz time resolution. To distinguish insignificant fluxes from significant ones it is proposed to use a new approach based on the significance of the correlation coefficient between vertical wind speed and mixing ratio fluctuations. This procedure eliminated roughly 56% of our half-hourly fluxes. Based on the remaining, quality checked N2O fluxes we quantified the mean efflux at 0.8±0.4 μmol m−2 h−1 (mean ± standard error). Most of the contribution to the N2O flux occurred during a 6.5-h period starting 4.5 h before each precipitation event. No relation with precipitation amount could be found. Visibility data representing fog density and duration at the site indicate that wetting of the canopy may have as strong an effect on N2O effluxes as does below-ground microbial activity. It is speculated that above-ground N2O production from the senescing leaves at high moisture (fog, drizzle, onset of precipitation event) may be responsible for part of the measured flux.

Middle atmospheric O3, CO, N2O, HNO3, and temperature profiles during the warm Arctic winter 2001–2002

Ground‐based measurements of stratospheric constituents were carried out from Thule Air Base, Greenland (76.5°N, 68.7°W), during the winters of 2001–2002 and 2002–2003, involving operation of a millimeter‐wave spectrometer (GBMS) and a lidar system. This work focuses on the GBMS retrievals of stratospheric O3, CO, N2O, and HNO3, and on lidar stratospheric temperature data obtained during the first of the two winter campaigns, from mid‐January to early March 2002. For the Arctic lower stratosphere, the winter 2001–2002 is one of the warmest winters on record. During a large fraction of the winter, the vortex was weakened by the influence of the Aleutian high, with low ozone concentrations and high temperatures observed by GBMS and lidar above ∼27 km during the second half of February and in early March. At 900 K (∼32 km altitude), the low ozone concentrations observed by GBMS in the Aleutian high are shown to be well correlated to low solar exposure. Throughout the winter, PSCs were rarely observed by POAM III, and the last detection was recorded on 17 January. During the lidar and GBMS observing period that followed, stratospheric temperatures remained above the threshold for PSCs formation throughout the vortex. Nonetheless, using correlations between GBMS O3 and N2O mixing ratios, in early February a large ozone deficiency owing to local ozone loss is noted inside the vortex. GBMS O3‐N2O correlations suggest that isentropic transport brought a O3 deficit also to regions near the vortex edge, where transport most likely mimicked local ozone loss.

Lightning-produced NO<sub>x</sub> over Brazil during TROCCINOX: airborne measurements in tropical and subtropical thunderstorms and the importance of mesoscale convective systems

Abstract. During the TROCCINOX field experiments in February–March 2004 and February 2005, airborne in situ measurements of NO, NOy, CO, and O3 mixing ratios and the J(NO2) photolysis rate were carried out in the anvil outflow of thunderstorms over southern Brazil. Both tropical and subtropical thunderstorms were investigated, depending on the location of the South Atlantic convergence zone. Tropical air masses were discriminated from subtropical ones according to the higher equivalent potential temperature (Θe) in the lower and mid troposphere, the higher CO mixing ratio in the mid troposphere, and the lower wind velocity in the upper troposphere within the Bolivian High (north of the subtropical jet stream). During thunderstorm anvil penetrations, typically at 20–40 km horizontal scales, NOx mixing ratios were distinctly enhanced and the absolute mixing ratios varied between 0.2–1.6 nmol mol−1 on average. This enhancement was mainly attributed to NOx production by lightning and partly due to upward transport from the NOx-richer boundary layer. In addition, CO mixing ratios were occasionally enhanced, indicating upward transport from the boundary layer. For the first time, the composition of the anvil outflow from a large, long-lived mesoscale convective system (MCS) advected from northern Argentina and Uruguay was investigated in more detail. Over a horizontal scale of about 400 km, NOx, CO and O3 absolute mixing ratios were significantly enhanced in these air masses in the range of 0.6–1.1, 110–140 and 60–70 nmol mol−1, respectively. Analyses from trace gas correlations and a Lagrangian particle dispersion model indicate that polluted air masses, probably from the Buenos Aires urban area and from biomass burning regions, were uplifted by the MCS. Ozone was distinctly enhanced in the aged MCS outflow, due to photochemical production and entrainment of O3-rich air masses from the upper troposphere – lower stratosphere region. The aged MCS outflow was transported to the north, ascended and circulated, driven by the Bolivian High over the Amazon basin. In the observed case, the O3-rich MCS outflow remained over the continent and did not contribute to the South Atlantic ozone maximum.

The NOAA nitrous oxide standard scale for atmospheric observations

A new nitrous oxide (N2O) calibration scale has been developed for atmospheric observations. The NOAA‐2006 N2O scale is based on gravimetrically prepared compressed gas standards. This scale supercedes the NOAA‐2000 scale, which was accepted by the community of experts within the World Meteorological Organization Global Atmosphere Watch program (GAW) as the GAW reference standard. The new scale is defined by thirteen “daughter” standards with dry air mole fractions ranging from 261–371 parts‐per‐billion (nmol mol−1, ppb). These were derived from four part‐per‐million (mmol mol−1, ppm) level “parents”. Standards were evaluated using gas chromatography with electron capture detection. The daughter standards are internally consistent with a standard deviation of residuals of 0.33 ppb, and there is essentially no detectable difference among “cousins” (standards prepared from different parents). The NOAA‐2006 scale is 0.19 ppb lower than the NOAA‐2000 scale at 320 ppb. The global mean N2O mixing ratio (dry air mole fraction), calculated from in situ observations at five monitoring sites was 318.46 ppb in 2004 on the new scale. The NOAA‐2006 scale compares well with other scales based on comparisons of compressed gas standards. The NOAA‐2006 scale is, on average, 0.23% higher than that defined by NIST Standard Reference Materials 2608 and 2609, and an average of 0.01% lower than the Scripps Institution of Oceanography SIO‐98 scale over the range 298–319 ppb.

Influence of nitrate radical on the oxidation of dimethyl sulfide in a polluted marine environment

Simultaneous in situ measurements of dimethyl sulfide (DMS) and nitrate radical (NO3) from the NOAA research vessel Ronald H. Brown off the New England Coast during the summer of 2002 show a clear anticorrelation between these compounds. The anticorrelation indicates a strong interaction between anthropogenic NOx emissions and natural sulfur emissions from the ocean surface. The anticorrelation was invariant with the distance from the continent over the range sampled during this study. Diurnal averages showed maxima in DMS concentrations during dawn and dusk and a lower minimum of ≈30 pptv at night compared to the daytime minimum of ≈40 pptv, indicating a stronger oxidation by NO3 at night than by OH at daytime under polluted conditions. This conclusion is also supported by agreement between a box model calculation and the measurements. Including DMS oxidation by halogens (Cl and BrO) into the model improved the agreement significantly. Calculation of the relative DMS loss due to either NO3 or OH oxidation indicated that between 65 and 90% of the DMS oxidation was due to NO3, depending on NO3 mixing ratios. The area over which DMS oxidation by NO3 is at least as strong as by OH can extend as far as 3000 km from coastal anthropogenic NOx sources. These increased DMS oxidation rates by NO3 may have an impact on global non‐sea‐salt sulfate aerosol production, depending on the DMS chemical oxidation mechanism.

Two-years of NO<sub>3</sub> radical observations in the boundary layer over the Eastern Mediterranean

Abstract. This is the first study that investigates the seasonal variability of nitrate (NO3) radicals in the marine boundary layer over the East Mediterranean Sea. An extensive data set of NO3 radical observations on the north coast of Crete for more than two years (June 2001–September 2003) is presented here. NO3 radicals follow a distinct seasonal dependency with the highest seasonally average mixing ratios in summer (5.6±1.2 pptv) and the lowest in winter (1.2±1.2 pptv). Episodes with high NO3 mixing ratios have been encountered mainly in polluted air masses originating from mainland Greece, Central and East Europe, and Turkey. Ancillary measurements of ozone, nitrogen dioxide (NO2) and meteorological parameters have been conducted and used to reveal possible relationship with the observed NO3 variability. The acquired NO2 nighttime observations provide the up-to-date most complete overview of NO2 temporal variability in the area. The data show correlations of the NO3 nighttime mixing ratios with temperature (positive), relative humidity (negative) and to a lesser extend with O3 (positive). As inferred from these observations, on average the major sink of NO3 radicals in the area is the heterogeneous reaction of dinitrogen pentoxide (N2O5) on aqueous particles whereas the homogeneous gas phase reactions of NO3 are most important during spring and summer. These observations support a significant contribution of NO3 nighttime chemistry to the oxidizing capacity of the troposphere.

Observation of daytime N2O5 in the marine boundary layer during New England Air Quality Study–Intercontinental Transport and Chemical Transformation 2004

The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are key reactive nocturnal nitrogen oxides in the troposphere. The daytime impact of NO3 and N2O5, however, is restricted by photochemical recycling of NO3 to NO2 and O3. In this paper, we report daytime measurements of N2O5 on board the NOAA research vessel Ronald H. Brown in the Gulf of Maine during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS‐ITCT) campaign in the summer of 2004. Daytime N2O5 mixing ratios of up to 4 pptv were observed, consistent with predictions from a steady state analysis. Predicted and observed NO3 mixing ratios were below the instrumental detection limit of ∼1 pptv; the average calculated concentration was 0.09 pptv. Important impacts of daytime NO3 and N2O5 in the marine boundary layer included increased rates of VOC oxidation (in particular dimethyl sulfide) and enhanced NOx to HNO3 conversion, both of which scaled with the available NOx. Smaller effects of daytime NO3 and N2O5 included chemical destruction of O3 and a shift of the NO2:NO ratio. Because the rates of heterogeneous conversion of N2O5 and NO3 to HNO3 scale with the surface area available for uptake, the importance of daytime fog is discussed.

The Vertical Distribution and Trends of the Trace Gases in Stratosphere over China

AbstractIn order to provide some useful data for studies on radiative and chemical processes in the stratosphere, the vertical distribution of trace gases (NO, NO2, HCl, HF, CH4, H2O and O3) and their variation trends in the stratosphere over China have been analyzed by using the HALOE data from 1992 to 2005. Besides the mixing ratio of the traces averaged over China, the values averaged over Qinghai‐Xizang Plateau are also given.The analyses show that the mixing ratios for various trace gases distribute vertically in different patterns in stratosphere, the distribution and seasonal variation of the traces near tropopause are much different from those in stratosphere, The analyses also show that the seasonal variation, the quasi‐biennial oscillation (QBO) signals and the long‐term trends are clear for all the traces, and the trends of the traces gases in various levels of stratosphere are different. In middle stratosphere, the mixing ratios of NO, NO2, HCl and H2O are all increased before 1998 and then decreased after 1998, while it is contrary for mixing ratio of O3 which decreased evidently before 1998 and the decrease trend is not significant after 1998. That means the decrease of those traces lead to the ozone depletion retarded in middle stratosphere in recent years. While in the lower stratosphere, the ozone depletion is still significant.

Probing stratospheric transport and chemistry with new balloon and aircraft observations of the meridional and vertical N<sub>2</sub>O isotope distribution

Abstract. A comprehensive set of stratospheric balloon and aircraft samples was analyzed for the position-dependent isotopic composition of nitrous oxide (N2O). Results for a total of 220 samples from between 1987 and 2003 are presented, nearly tripling the number of mass-spectrometric N2O isotope measurements in the stratosphere published to date. Cryogenic balloon samples were obtained at polar (Kiruna/Sweden, 68° N), mid-latitude (southern France, 44° N) and tropical sites (Hyderabad/India, 18° N). Aircraft samples were collected with a newly-developed whole air sampler on board of the high-altitude aircraft M55 Geophysica during the EUPLEX 2003 campaign. For mixing ratios above 200 nmol mol−1, relative isotope enrichments (δ values) and mixing ratios display a compact relationship, which is nearly independent of latitude and season and which can be explained equally well by Rayleigh fractionation or mixing. However, for mixing ratios below 200 nmol mol−1 this compact relationship gives way to meridional, seasonal and interannual variations. A comparison to a previously published mid-latitude balloon profile even shows large zonal variations, justifying the use of three-dimensional (3-D) models for further data interpretation. In general, the magnitude of the apparent fractionation constants (i.e., apparent isotope effects) increases continuously with altitude and decreases from the equator to the North Pole. Only the latter observation can be understood qualitatively by the interplay between the time-scales of N2O photochemistry and transport in a Rayleigh fractionation framework. Deviations from Rayleigh fractionation behavior also occur where polar vortex air mixes with nearly N2O-free upper stratospheric/mesospheric air (e.g., during the boreal winters of 2003 and possibly 1992). Aircraft observations in the polar vortex at mixing ratios below 200 nmol mol−1 deviate from isotope variations expected for both Rayleigh fractionation and two-end-member mixing, but could be explained by continuous weak mixing between intravortex and extravortex air (Plumb et al., 2000). However, it appears that none of the simple approaches described here can capture all features of the stratospheric N2O isotope distribution, again justifying the use of 3-D models. Finally, correlations between 18O/16O and average 15N/14N isotope ratios or between the position-dependent 15N/14N isotope ratios show that photo-oxidation makes a large contribution to the total N2O sink in the lower stratosphere (possibly up to 100% for N2O mixing ratios above 300 nmol mol−1). Towards higher altitudes, the temperature dependence of these isotope correlations becomes visible in the stratospheric observations.

Long‐term changes of methane and hydrogen in the stratosphere in the period 1978–2003 and their impact on the abundance of stratospheric water vapor

The long‐term changes of the stratospheric mixing ratio of CH4 over the period of 1978–2003 are derived from balloon‐borne data of H2, CH4 and N2O. The data were obtained by collecting whole air samples and subsequent gas chromatographic analyses. To eliminate the short‐term variability attributed to dynamical processes, the N2O mixing ratio is used as a proxy for altitude. A correlation analysis for the individual years is applied and the CH4 mixing ratios are interpolated to four different levels of N2O, corresponding to altitudes of approximately 17, 23, 26 and 30 km at midlatitudes. For the investigated period of 1978 to 2003 we find increases at the four levels of 207 ± 32 ppb, 159 ± 21 ppb, 140 ± 34 ppb and 111 ± 60 ppb, respectively. The CH4 trend has slowed down in recent years and is best fitted by a second‐order polynomial. The increase of CH4 can account for only 25–34% of the increase in stratospheric H2O of 1%/yr over the last decades as derived from previous studies. The simultaneously measured time series of stratospheric H2 mixing ratios shows that the contribution of stratospheric H2 to the H2O trend in the period 1988–2003 is minor.

Measurements of N2O5 near Fairbanks, Alaska

We report measurements of N2O5, CO, and O3 at a subarctic site near Fairbanks, Alaska (65°N), during two periods of approximately 2 weeks each, one in December 2002 and the other in February 2004. N2O5 mixing ratios range from below the instrument detection limit of 2 parts per trillion by volume (pptv) up to approximately 250 pptv. Ozone and carbon monoxide measurements help distinguish several different air mass types: “clean” (near background CO and ozone), “titrated” (ozone titrated to near zero and elevated CO), and “mixed” (intermediate ozone and some CO elevation). Observed N2O5 mixing ratios are near zero for air masses that are either clean or titrated. Times with measurable N2O5 are almost always associated with mixed air masses. Measurements of CO and the CO:NOx emissions ratio in Fairbanks are used to estimate NO2 mixing ratios and calculate steady state lifetimes for N2O5. The lifetime estimates vary from subminute to 4 hours, with a median for the February study of 10 min and a median for the December study of 4 min. These lifetimes are quite short compared to the lifetime of NO2 with respect to reaction with ozone and formation of NO3, suggesting that the rate limiting step of NOx oxidation in the nighttime atmosphere is the formation of NO3, not the losses of NO3 and N2O5. These data show sinks of N2O5 (presumably mostly heterogeneous hydrolysis) are an efficient mechanism of NOx removal near the surface in the dark, high‐latitude winter.

Seasonal and diurnal characteristics of water soluble inorganic compounds in the gas and aerosol phase in the Zurich area

Abstract. Gas and aerosol samples were taken using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to ion chromatography (IC) in the city of Zurich, Switzerland from August to September 2002 and in March 2003. Major water soluble inorganic ions; nitrate, sulfate, and nitrite were analyzed online with a time resolution of two hours for the gas and aerosol phase. The fraction of water soluble inorganic anions in PM10 varied from 15% in August to about 38% in March. Seasonal and diurnal variations of nitrate in the gas and aerosol phase were observed with more than 50% of the total nitrate in the gas phase during August and more than 80% of nitrate in the aerosol phase during March exceeding the concentration of sulfate by a factor of 2. Aerosol sulfate, on the other hand, did not show significant variability with season. However, in the gas phase, the SO2 concentration was 6.5 times higher in winter than in summer. Nitrous acid (HONO) also showed a diurnal variation in both the gas and aerosol phase with the lowest concentration (0.2–0.6 µg/m3) in the afternoon. The primary pollutants, NO, CO and SO2 mixing ratios were often at their highest between 04:00–10:00 local time due to the build up of fresh vehicle emission under a nocturnal inversion.

A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν / δ ν = 76 , 000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50–112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO 2 lines. The derived NO mixing ratio is 5.5 ± 1.5 ppb below 60 km and its flux is ( 6 ± 2 ) × 10 7 cm −2 s −1 . Predissociation of NO at the (0–0) 191 nm and (1–0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of 0.19 ± 0.06 erg cm −2 s −1 . For a flash energy on Venus similar to that on the Earth (∼ 10 9 J ), the global flashing rate is ∼90 s −1 and ∼6 km −2 y −1 which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO 2 lines, and the line at 1900.076 cm −1 is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO ( v = 1 ) in the lowest 20 km on Mars. However, quenching of NO ( v = 1 ) by CO 2 is very effective below 40 km. Excitation of NO ( v = 1 ) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.

Aircraft observations of daytime NO 3 and N 2O 5 and their implications for tropospheric chemistry

The nitrate radical (NO 3) and dinitrogen pentoxide (N 2O 5) are normally considered only in the context of nighttime atmospheric chemistry. Although their importance during the day is often small, it is not always negligible. Here, we present daylight observations of both compounds from the NOAA P-3 aircraft taken during the New England Air Quality Study in the summer of 2004. Observed mixing ratios agreed with predictions from a simple, rapidly established daytime steady state, although observed and calculated mixing ratios of NO 3 were near the instrumental detection limit. The observations have several implications for tropospheric chemistry during the daytime, including the loss of ozone through photolysis of NO 3 to NO + O 2, oxidation of biogenic volatile organic compounds (VOC), and conversion of NO x (=NO + NO 2) to HNO 3 via N 2O 5 hydrolysis. The magnitude of each process is considered in comparison to its photochemically driven analog. For example, NO 3 oxidation of α-pinene in the presence of NO x contributes 10–40% of the total daytime oxidation rate. Hydrolysis of N 2O 5 increases the daytime conversion rate of NO x to HNO 3 by several percent, with a maximum of 13%, relative to OH + NO 2. The implications of these observations for daytime NO 3 and N 2O 5 reactions in a variety of locations and seasons are discussed.

High‐precision determination of the changing isotopic composition of atmospheric N2O from 1990 to 2002

High‐precision nitrous oxide (N2O) concentration and isotope ratio measurements have been carried out on archived air samples from the Antarctic station Neumayer covering the period 1990–2002. The results show that the increase in the N2O mixing ratio over this period is accompanied by a significant decrease in the heavy isotope content. The temporal isotope trends amount to (−0.040 ± 0.003)‰/yr for δ15N (the average of both nitrogen positions) and (−0.021 ± 0.003)‰/yr for δ18O. The individual trends for the terminal (position 1) and central (position 2) nitrogen atoms within the N2O molecule are (−0.064 ± 0.016)‰/yr for 1δ15N and (−0.014 ± 0.016)‰/yr for 2δ15N. The average 15N and 18O trends compare well with recent results from measurements on air extracted from polar firn and ice, confirming earlier estimates that isotopically depleted N2O, mainly from soil emissions, is responsible for a large fraction of the observed N2O increase in the atmosphere. The position‐dependent 15N determinations show a strong difference between the two positions. This is in disagreement with the firn and ice core data, which imply similar fractionations at both positions.

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.

First simultaneous global measurements of nighttime stratospheric NO2 and NO3 observed by Global Ozone Monitoring by Occultation of Stars (GOMOS)/Envisat in 2003

The Global Ozone Monitoring by Occultation of Stars (GOMOS) stellar occultation instrument on board the Envisat European satellite provides global coverage of ozone and other stratospheric species with good vertical resolution and a self‐calibrating method. In this paper we present the first simultaneous global distribution of stratospheric NO2 and NO3 from 1 year of nighttime GOMOS data in 2003. Most previous NO2 satellite observations have been made using the solar occultation technique. They are difficult to interpret due to the fast photochemical evolution of NO2 at sunrise and sunset. There are no published observations of NO3 from space because this constituent is rapidly photodissociated during daytime and is not observable by solar occultation. It is shown that the NO2 mixing ratio reaches a maximum around 40 km with values between 14 and 16 ppbv at low and middle latitudes. The global distribution of NO2 observed by GOMOS is very similar to the NO + NO2 Halogen Occultation Experiment climatology deduced from sunset measurements from 1999 to 2004. At high latitude a high mixing ratio is observed in the north vortex in November 2003 after a strong solar proton event and in the south vortex in July 2003. The NO3 mixing ratio peaks at 40–45 km. NO3 follows a semiannual variation at low latitudes with maxima at equinoxes and an annual variation at middle and high latitudes with a maximum in summer. In the upper stratosphere the mixing ratio of NO3 is strongly correlated with temperature due to the thermal dependence of its formation rate.

Rapid meridional transport of tropical airmasses to the Arctic during the major stratospheric warming in January 2003

Abstract. We present observations of unusually high values of ozone and N2O in the middle stratosphere that were observed by the airborne submillimeter radiometer ASUR in the Arctic. The observations took place in the meteorological situation of a major stratospheric warming that occurred in mid-January 2003 and was dominated by a wave 2 event. On 23 January 2003 the observed N2O and O3 mixing ratios around 69° N in the middle stratosphere reached maximum values of ~190 ppb and ~10 ppm, respectively. The similarities of these N2O profiles in a potential temperature range between 800 and 1200 K with N2O observations around 20° N on 1 March 2003 by the same instrument suggest that the observed Arctic airmasses were transported from the tropics quasi-isentropically. This is confirmed by 5-day back trajectory calculations which indicate that the airmasses between about 800 and 1000 K had been located around 20° N 3–5 days prior to the measurement in the Arctic. Calculations with a linearized ozone chemistry model along calculated as well as idealized trajectories, initialized with the low-latitude ASUR ozone measurements, give reasonable agreement with the Arctic ozone measurement by ASUR. PV distributions suggest that these airmasses did not stay confined in the Arctic region which makes it unlikely that this dynamical situation lead to the formation of dynamically caused pockets of low ozone.

Atmospheric Chemistry Experiment (ACE) Arctic stratospheric measurements of NOxduring February and March 2004: Impact of intense solar flares

[1] Solar occultation measurements recorded in the Arctic by the Atmospheric Chemistry Experiment (ACE) Fourier transform spectrometer show elevated volume mixing ratios of NO and NO2 in the upper stratosphere from mid-February to late March 2004. Mixing ratios of NO exceeded 1.3 ppmv (10−6 parts per million by volume) during mid-February and are higher than any values previously reported in the Arctic during late winter and early spring. The elevated NOx (NO + NO2) mixing ratios likely resulted from particle emissions by intense solar flares that occurred the previous October and November followed by NOx production and downward transport to the upper stratospheric vortex where the elevated levels persisted due to the long lifetime of NOx during darkness in polar winter and early spring. The descent and the NOx decrease rates in the upper stratospheric vortex are estimated from an analysis of the time series. The measured descent rate is compared with previous observations and model predictions.