Surface ozone and its precursors at Summit, Greenland: Comparison between observations and model simulations

Recent studies have shown significant challenges for atmospheric models to simulate tropospheric ozone (O3/and its precursors in the Arctic. In this study, ground-based data were combined with a global 3-D chemical transport model (GEOS-Chem) to examine the abundance and seasonal variations of O3 and its precursors at Summit, Greenland (72.34° N, 38.29° W; 3212 ma.s.l.). Model simulations for atmospheric nitrogen oxides (NOx/, peroxyacetyl nitrate (PAN), ethane (C2H6/, propane (C3H8/, carbon monoxide (CO), and O3 for the period July 2008-June 2010 were compared with observations. The model performed well in simulating certain species (such as CO and C3H8/, but some significant discrepancies were identified for other species and further investigated. The model generally underestimated NOx and PAN (by ∼50 and 30 %, respectively) for March-June. Likely contributing factors to the low bias include missing NOx and PAN emissions from snowpack chemistry in the model. At the same time, the model overestimated NOx mixing ratios by more than a factor of 2 in wintertime, with episodic NOx mixing ratios up to 15 times higher than the typical NOx levels at Summit. Further investigation showed that these simulated episodic NOx spikes were always associated with transport events from Europe, but the exact cause remained unclear. The model systematically overestimated C2H6 mixing ratios by approximately 20% relative to observations. This discrepancy can be resolved by decreasing anthropogenic C2H6 emissions over Asia and the US by ∼20 %, from 5.4 to 4.4 Tg year-1. GEOS-Chem was able to reproduce the seasonal variability of O3 and its spring maximum. However, compared with observations, it underestimated surface O3 by approximately 13% (6.5 ppbv) from April to July. This low bias appeared to be driven by several factors including missing snowpack emissions of NOx and nitrous acid in the model, the weak simulated stratosphereto-troposphere exchange flux of O3 over the summit, and the coarse model resolution

Late summer changes in burning conditions in the boreal regions and their implications for NO x and CO emissions from boreal fires

Copyright © 2008 American Geophysical Union. All Rights Reserved.Building emission inventories for the fires in boreal regions remains a challenging task with significant uncertainties in the methods used. In this work, we assess the impact of seasonal trends in fuel consumption and flaming/smoldering ratios on emissions of species dominated by flaming combustion (e.g., NO x ) and species dominated by smoldering combustion (e.g., CO). This is accomplished using measurements of CO and NO y at the free tropospheric Pico Mountain observatory in the central North Atlantic during the active boreal fire seasons of 2004 and 2005. ΔNO y /ΔCO enhancement ratios in aged fire plumes had higher values in June-July (7.3 × 10−3 mol mol−1) relative to the values in August-September (2.8 × 10−3 mol mol−1), indicating that NO x /CO emission ratios declined significantly as the fire season progressed. This is consistent with our understanding that an increased amount of fuel is consumed via smoldering combustion during late summer, as deeper burning of the drying organic soil layer occurs. A major growth in fuel consumption per unit area is also expected, due to deeper burning. Emissions of CO and NO x from North American boreal fires were estimated using the Boreal Wildland Fire Emissions Model, and their long-range transport to the sampling site was modeled using FLEXPART. These simulations were generally consistent with the observations, but the modeled seasonal decline in the ΔNO y /ΔCO enhancement ratio was less than observed. Comparisons using alternative fire emission injection height scenarios suggest that plumes with the highest CO levels at the observatory were lofted well above the boundary layer, likely as a result of intense crown fires

NO \u3c inf\u3e x and NO \u3c inf\u3e y over the northwestern North Atlantic: Measurements and measurement accuracy

Measurements of NOx (NO+NO2) and NOy (total reactive nitrogen oxides) during February-April 1996 at the northern tip of Newfoundland are used to determine levels in the local marine boundary layer (MBL) and assess the adequacy of current understanding of the processes controlling NOx levels over the northern North Atlantic, as expressed through previously reported simulations using the Geophysical Fluid Dynamics Laboratory (GFDL) Global Chemical Transport Model (GCTM). Median mixing ratios of NOx and NOy in the local MBL were 24 parts per trillion by volume (pptv) and 200 pptv, respectively. These levels are 35-64% above background levels measured in the remote MBL in summer and fall during the Chemical Instrumentation Test and Evaluation (CITE 2), North Atlantic Regional Experiment (NARE-93), and Pacific Exploratory Mission-West (PEM-West) A measurement campaigns and are similar to or somewhat higher than anthropogenically influenced levels observed in winter-spring during the PEM-West B campaign. The magnitude of median NOx and NOy levels in the local MBL is not due to events with high reactive nitrogen oxides levels. Instead, these relatively high median levels are likely the result of dispersion of anthropogenic emissions over a large region. A detailed comparison with results from the GFDL GCTM indicates that measured March and April average NOx levels are significantly lower than simulated levels over the north central North Atlantic. The frequency and magnitude of modeled and observed elevated-NOx events were similar, indicating that the conditions responsible for relatively direct long-range transport events were similar. This indicates that interannual variability probably did not cause the discrepancy in monthly average NOx values. However, simulated elevated NOx events are much longer than are observed. This difference appears to be at least partially responsible for the higher average NOx values simulated by the model. These results indicate that model-based estimates of this region\u27s contributions to the global ozone budget may be too high. Accuracy of the NOx measurements is estimated to be 6%, while conservative analysis of conversion efficiencies indicates a negative bias of ≲18% in the determination of gas-phase NOy compounds. Copyright 1999 by the American Geophysical Union

Seasonal variability of atmospheric nitrogen oxides and non-methane hydrocarbons at the GEOSummit station, Greenland

Measurements of atmospheric nitrogen oxides NOx (NOx = NO + NO2), peroxyacetyl nitrate (PAN), NOy, and non-methane hydrocarbons (NMHC) were taken at the Greenland Environmental Observatory at Summit (GEOSummit) station, Greenland (72.34° N, 38.29° W; 3212 m a.s.l.), from July 2008 to July 2010. The data set represents the first year-round concurrent record of these compounds sampled at a high latitude Arctic site. Here, the study focused on the seasonal variability of these important ozone (O3) precursors in the Arctic troposphere and the impact from transported anthropogenic and biomass burning emissions. Our analysis shows that PAN is the dominant NOy species in all seasons at Summit, varying from 42 to 76 %; however, we find that odd NOy species (odd NOy = NOy − PAN − NOx) contribute a large amount to the total NOy speciation. We hypothesize that the source of this odd NOy is most likely alkyl nitrates and nitric acid (HNO3) from transported pollution, and photochemically produced species such as nitrous acid (HONO). FLEXPART retroplume analyses and black carbon (BC) tracers for anthropogenic and biomass burning (BB) emissions were used to identify periods when the site was impacted by polluted air masses. Europe contributed the largest source of anthropogenic emissions during the winter months (November–March) with 56 % of the total anthropogenic BC tracer originating from Europe in 2008–2009 and 69 % in 2009–2010. The polluted plumes resulted in mean enhancements above background levels up to 334, 295, 88, and 1119 pmol mol−1 for NOy, PAN, NOx, and ethane, respectively, over the two winters. Enhancements in O3 precursors during the second winter were typically higher, which may be attributed to the increase in European polluted air masses transported to Summit in 2009–2010 compared to 2008–2009. O3 levels were highly variable within the sampled anthropogenic plumes with mean ΔO3 levels ranging from −6.7 to 7.6 nmol mol−1 during the winter periods. North America was the primary source of biomass burning emissions during the summer; however, only 13 BB events were observed as the number of air masses transported to Summit, with significant BB emissions, was low in general during the measurement period. The BB plumes were typically very aged, with median transport times to the site from the source region of 14 days. The analyses of O3 and precursor levels during the BB events indicate that some of the plumes sampled impacted the atmospheric chemistry at Summit, with enhancements observed in all measured species

Analysis and application of Sheppard\u27s airflow model to predict mechanical orographic lifting and the occurrence of mountain clouds

Mechanically driven orographic lifting is important for air pollution dispersion and weather prediction, but the small dimensions of mountain peaks often prevent numerical weather models from producing detailed forecasts. Mechanical lifting in stratified flow over mountains and associated thermodynamic processes were quantified and evaluated using Sheppard\u27s model to estimate the dividing-streamline height zt. The model was based on numerical weather model profile data and was evaluated using ground-based measurements on a tall, axisymmetric mountaintop for which the nondimensional mountain height hND = hN/U∞ is frequently between 1 and 10 (here h is mountain height, N is Brunt-Väisälä frequency, and U∞ is upstream horizontal wind speed). Sheppard\u27s formula was successful in predicting water vapor saturation at the mountaintop, with a false-prediction rate of 14.5%. Wind speed was found to be strongly related to the likelihood of forecast errors, and wind direction, season, and stratification did not play significant roles. The potential temperature (water vapor mixing ratio) at zt in the sounding was found to be slightly smaller (larger) than at the mountaintop, on average, indicating less lifting than predicted and/or turbulent mixing with higher-altitude air during parcel ascent. Detailed analysis revealed that this difference is a result of less lifting than predicted for small U∞(Nh), whereas Sheppard\u27s model predicts the relative increase in uplift with increasing U∞(Nh) correctly for U∞/(Nh) \u3e 0.2. © 2006 American Meteorological Society

Transport of ozone precursors from the Arctic troposphere to the North Atlantic region

The importance of Arctic outflow events to the budgets of nitrogen oxides and hydrocarbons in the North Atlantic region is estimated using a climatology of isentropic airflow trajectories, in combination with current understanding of the levels of these compounds in the Arctic troposphere. We first review available measurements of nonmethane hydrocarbons (NMHCs), total reactive oxidized nitrogen (NOy), and major NOy species in the Arctic troposphere to develop best estimate average vertical profiles during January-May outflow events. Measurements of these compounds in the winter-spring Arctic are generally consistent. Average levels during March are ≥500 parts per 1012 by volume (NOy) and ∼20 parts per billion carbon (NMHC). Current evidence for a significant vertical gradient above the boundary layer is weak, although additional measurements are needed. Secondly, the flow patterns and frequency of Arctic outflow events which reach the North Atlantic region south of 50°-55°N are investigated using an 11-year climatology of isentropic forward trajectories originating at 70°N in the months of January-May. The dominant route of trajectories reaching the temperate North Atlantic originates north of Canada at 2-6 km altitude and continues southward along a semipermanent trough located near the East Coast of North America. Trajectories reaching the temperate North Atlantic originated in this region on ∼70% of the days analyzed. Significant subsidence occurs during the southward flow, resulting in warming conducive to photochemical processing of the Arctic pollutants. Based on these analyses, the southward fluxes of NOy, and NMHCs out of the Arctic in events which reach the North Atlantic south of 50°N total 7.3 GgN/month NOy and 250 GgC/month NMHC during March. These values are biased low as they include only those trajectories originating below 6 km and exclude trajectories which pass over the United States or southeastern Canada. The calculated NOy flux during May is lower but may be underestimated due to uncertainty in conditions in the Arctic free troposphere in that month. The May flux of NMHCs is larger than that in March as a result of a more frequent occurrence of outflow events. These fluxes impact air parcels which are not affected by direct transport from source regions and appear to be seasonally significant relative to other sources of ozone precursors to the North Atlantic troposphere. If a significant fraction of the peroxyacetyl nitrate and alkyl nitrates which comprise most of the advected NOy decomposes over the North Atlantic, the transport of anthropogenic pollutants through the Arctic may play a significant role in the ozone budget of the North Atlantic troposphere

Surface exchange and transport processes governing atmospheric PCB levels over lake superior

Measurements of gas phase PCB concentrations obtained at a site on the shore of Lake Superior are analyzed to determine the importance of regional air/surface exchange as a determinant of PCB concentration over the lake. During periods of onshore flow, results obtained in congener-specific regression analyses of log concentration against reciprocal temperature are strongly dependent upon time of year. During months when the lake is colder than the overlying air (resulting in a stable atmosphere over the lake), regressions for onshore flow explain 48-63% of the observed variance in log C, and the regression slope and intercept are consistent with Henry\u27s law equilibrium between the lower atmosphere over the lake and aqueous concentrations. Results consistent with Henry\u27s law are obtained only when water temperature is assumed to be similar to air temperature measured at the site during flow off the take; poor regression results (r2 = 0.00-0.06) are obtained when lake water temperature measured by data buoys is used. Based on consideration of the spatial variability of lake skin temperature during onshore flow periods, it is inferred that, under stable conditions, air/water equilibrium is reached rapidly and that atmospheric concentrations at the measurement site are primarily affected by air/water exchange within 10-30 km offshore. These results are consistent with consideration of potential limitations of regression analyses of log C versus reciprocal temperature. Furthermore, they indicate that the net exchange of PCBs during the stable season over large lakes (April-August or September for Lake Superior) may be significantly less than previously estimated. Regression analyses based on samples obtained during onshore flow in the unstable season and during over- land flow exhibit increased scatter and are inconsistent with the hypothesis of upwind equilibrium controlled by either Henry\u27s law- or vapor pressure- mediated exchange at local temperatures. This result is consistent with rate limitations to atmosphere/surface exchange over the lake and land surface and with the effect of atmospheric mixing processes, implications of these results to the interpretation of concentration measurements at lakeside sites and the use of such measurements in calculations of the air/water exchange flux of PCBs over large lakes are discussed

Measurements of nitrogen oxides and a simple model of NO \u3c inf\u3e y fate in the remote North Atlantic marine atmosphere

NO and NOy (total reactive oxidized nitrogen) were measured at a site in the Azores (27.322°W, 38.732°N, 1 km altitude) over a 3 week period in August and September 1993, during the first summer intensive of the North Atlantic Regional Experiment (NARE). These measurements were performed to determine background reactive nitrogen oxides levels and to assess the impact that long-range transport of reactive nitrogen oxides associated with human activities have on these levels. Median NOy mixing ratios during background marine boundary layer (MBL) periods ranged from 59 to 93 parts per trillion by volume (pptv), with an overall median of 73 pptv. Analysis of back trajectories, low and uncorrelated CO and O3 levels, and low levels of MBL NOy indicate that the central North Atlantic region was not influenced by direct transport of anthropogenic emissions during the period of this study. Changes in NOy levels during two MBL periods with adjacent in and out-of-cloud events indicated that up to 45-47% of MBL NOy was scavenged by clouds. However, mean NOy levels during all in-cloud periods (~70 pptv) and all out-of-cloud periods (~80 pptv) were not significantly different, apparently because of variabiltiy in MBL NOy levels. In addition to the MBL periods, there were two periods when the site was within the free troposphere (FT), as indicated by vertical soundings and weather conditions at the site. FT NOy mixing ratios were ≥280 pptv and ≥400 pptv during these two periods. The median clear-sky FT NO level during the hour centered on solar noon was 16 pptv. A mass balance model considering FT/MBL exchange and MBL removal processes is used to find the NOy MBL effective first-order loss lifetime (~1.2 days) and the NOy MBL e-folding response time due to both effective first-order loss processes and subsidence-induced ventilation of the MBL (~0.9 days). The apparent rapid loss of MBL NOy implies that it will respond rapidly to changes in the overlying FT, but that correlations of NOy with trace gases with slower MBL removal, such as O3 and CO, will be degraded within the subsidence-influenced MBL

NO \u3c inf\u3e x during background and ozone depletion periods at Alert: Fluxes above the snow surface

Measurements of nitric oxide (NO) and nitrogen dioxide (NO2) at Alert, Nunavut, Canada showed median background mixing ratios of 0.2 and 1.3 pmol mol-1, respectively, during darkness in late winter 2000, and 2.8 and 10.8 pmol mol-1 during spring in 24-hour light. Both NO and NO2 showed clear diurnal cycles with noontime maxima during spring. In darkness, no NOx exchange between the snow surface and the overlying atmosphere was detected. During the period of 24-hour sunlight, the snow surface constituted a source of NOx, whose noon-time flux reached approximately 40 nmol m-2 h-1. Measured NO x fluxes were roughly equal to HONO fluxes reported during the Alert campaign. The fluxes were correlated to ultraviolet light intensity, but anticorrelated to wind speeds. During 2 days of high wind speeds under O 3 depletion conditions, the fluxes were not significantly different from zero. However, under low wind speeds during the O3 depletion event, the snowpack continued to emit a detectable NOx flux. The observed release of NOx and HONO during the sunlit period was small relative to the observed decrease in the snowpack surface-layer nitrate inventory. Finally, as part of this study, the nitrous acid (HONO) interference in the Xe-lamp-based photolytic NO2 measurements was determined; it amounted to 24% of the HONO mixing ratio. Copyright 2002 by the American Geophysical Union