Richard E. Honrath

Evidence of NOx production within or upon ice particles in the Greenland snowpack

NOx and NOy were determined in the interstitial air of surface snow and in ambient air at Summit, Greenland. NOx levels in interstitial air were 3 to >10 times those in ambient air, and were generally greater than ambient NOy levels. [NOy] in interstitial air varied diurnally in a manner consistent with photochemical generation within the snowpack. These observations imply that photochemical reactions occurring within or upon the ice crystals of surface snow produced NOx from a N-reservoir compound within the snow. Average [NOx]:[HNO3] and [NOx]:[NOy] ratios in ambient air above the snow were elevated relative to other remote sites, indicating that NOx release within the snowpack may have altered NOx levels in the overlying atmospheric boundary layer. We suggest that the observed release of NOx may have been initiated by photolysis of nitrate, present in relative abundance in surface snow at Summit. Such a process may affect levels of nitrate and other compounds in surface snow, the overlying atmosphere, and glacial ice, and its potential role in cirrus cloud chemistry should be investigated.

International Consortium for Atmospheric Research on Transport and Transformation (ICARTT): North America to Europe—Overview of the 2004 summer field study

In the summer of 2004 several separate field programs intensively studied the photochemical, heterogeneous chemical and radiative environment of the troposphere over North America, the North Atlantic Ocean, and western Europe. Previous studies have indicated that the transport of continental emissions, particularly from North America, influences the concentrations of trace species in the troposphere over the North Atlantic and Europe. An international team of scientists, representing over 100 laboratories, collaborated under the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) umbrella to coordinate the separate field programs in order to maximize the resulting advances in our understanding of regional air quality, the transport, chemical transformation and removal of aerosols, ozone, and their precursors during intercontinental transport, and the radiation balance of the troposphere. Participants utilized nine aircraft, one research vessel, several ground-based sites in North America and the Azores, a network of aerosol-ozone lidars in Europe, satellites, balloon borne sondes, and routine commercial aircraft measurements. In this special section, the results from a major fraction of those platforms are presented. This overview is aimed at providing operational and logistical information for those platforms, summarizing the principal findings and conclusions that have been drawn from the results, and directing readers to specific papers for further details.

Snowpack photochemical production of HONO : a major source of OH in the Arctic boundary layer in springtime

Both snow manipulation experiments and ambient measurements during the Polar Sunrise Experiment 2000 at Alert (Alert2000) indicate intensive photochemical production of nitrous acid (HONO) in the snowpack. This process constitutes a major HONO source for the overlying atmospheric boundary layer in the Arctic during the springtime, and sustained concentrations of HONO high enough that upon photolysis they became the dominant hydroxyl radical (OH) source. This implies a much greater role for OH radicals in Arctic polar sunrise chemistry than previously believed. Although the observations were made in the high Arctic, this finding has a significant implication for the boundary layer atmospheric chemistry in Antarctica during sunlit seasons and in the mid to high latitudes of the Northern Hemisphere during the winter and spring seasons when approximately 50% of the land mass may be covered by snow.

Vertical fluxes of NOx, HONO, and HNO3 above the snowpack at Summit, Greenland

Abstract Vertical gradients of NO x , HONO, and HNO 3 were measured in the lower 1– 2 m above the snowpack at Summit, Greenland, during summer 2000. These measurements are used with simultaneous measurements of atmospheric turbulence using eddy covariance systems to determine the vertical fluxes of NO x , HONO, and HNO 3 . Upward fluxes of NO x and HONO were observed; these emissions were highly correlated with diurnally varying sunlight intensity, consistent with the expectation that they are the result of nitrate photolysis within the snowpack. The HNO 3 flux was smaller in magnitude and more variable than those of HONO and NO x . It was usually downward, but emission was occasionally observed during mid-day. The 24-h average NO x emission (2.52×10 12 molecules m −2 s −1 ) and HONO emission (4.64×10 11 molecules m −2 s −1 ) rates were not balanced by the average HNO 3 deposition rate (7.16×10 11 molecules m −2 s −1 ) , indicating that NO x export may slowly remove nitrogen from the system composed of the atmospheric boundary layer plus the top few cm of the surface snowpack, potentially affecting the amount of nitrate ultimately stored in glacial ice. These measurements imply that snowpack (NO x +HONO) emissions may alter NO x and (through HONO photolysis) OH levels in remote, snow-covered regions, but are small relative to other NO x sources on the global scale.

Significant enhancements of nitrogen oxides, black carbon, and ozone in the North Atlantic lower free troposphere resulting from North American boreal wildfires

enhancements of CO, BC, NOy and NOx, with levels up to 250 ppbv, 665 ng m 3 , 1100 pptv and 135 pptv, respectively. Enhancement ratios relative to CO were variable in the plumes sampled, most likely because of variations in wildfire emissions and removalprocessesduringtransport.AnalysesofDBC/DCO,DNOy/DCOandDNOx/DCO ratios indicate that NOy and BC were on average efficiently exported in these plumes and suggest that decomposition of PAN to NOx was a significant source of NOx. High levels of NOx suggest continuing formation of O3 in these well-aged plumes. O3 levels were also significantly enhanced in the plumes, reaching up to 75 ppbv. Analysis of DO3/DCO ratios showed distinct behaviors of O3 in the plumes, which varied from significant to lower O3 production. We identify several potential reasons for the complex effects of boreal wildfire emissions on O3 and conclude that this behavior needs to be explored further in the future. These observations demonstrate that boreal wildfire emissions significantly contributed to the NOx and O3 budgets in the central North Atlantic lower free troposphere during summer 2004 and imply large-scale impacts on direct radiative forcing of the atmosphere and on tropospheric NOx and O3.

Photochemical production of gas phase NO x from ice crystal NO3

Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NOx (NO+NO2). We report here the results of a series of experiments designed to test the hypothesis that the observed NOx production is the result of nitrate photolysis. Snow produced from deionized water with and without the addition of nitrate was exposed to natural sunlight in an outdoor flow chamber. While NOx release from snow produced without added NO−3 was minimal, the addition of 100 µM NO−3resulted in the release of >500 pptv NOx in a 9 standard liter per minute (sLpm) flow of synthetic air exposed to the snow for 10–20 s; the rate of release was highly correlated with solar radiation. Further addition of radical trap reagents resulted in greatly increased NOx production (to >8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NOx release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism for NOx release from NO−3photolysis consistent with these observations is presented. These results support previous suggestions that surface NOx release may have a significant impact on boundary layer photochemistry in snow-covered regions and that nitrate photolysis on cirrus cloud particles may result in the release of gas phase NOx. A potential for pH-dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.

Release of NOx from sunlight‐irradiated midlatitude snow

Photochemical production and release of gas-phase NOx (NO + NO2) from the natural snowpack at a remote site in northern Michigan were investigated during the Snow Nitrogen and Oxidants in Winter study in January 1999. Snow was collected in an open 34 L chamber, which was then sealed with a transparent Teflon cover and used as an outdoor flow and reaction chamber. Significant increases in NOx mixing ratio were observed in synthetic and ambient air pulled through the sunlit chamber. [NOx] enhancements were correlated to ultraviolet sunlight intensity, reaching ∼300 pptv under partially overcast midday, midwinter conditions. These findings are consistent with NOx production from photolysis of snowpack NO3−; the observed NOx release implies production of significant amounts of OH within the snow. Snowpack NO3− photolysis may therefore significantly alter boundary layer levels of both NOx and oxidized compounds over wide regions of the atmosphere.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Impacts of snowpack emissions on deduced levels of OH and peroxy radicals at Summit, Greenland

Abstract Levels of OH and peroxy radicals in the atmospheric boundary layer at Summit, Greenland, a location surrounded by snow from which HOx radical precursors are known to be emitted, were deduced using steady-state analyses applied to (OH+HO2+CH3O2), (OH+HO2), and OH–HO2 cycling. The results indicate that HOx levels at Summit are significantly increased over those that would result from O3 photolysis alone, as a result of elevated concentrations of HONO, HCHO, H2O2, and other compounds. Estimated midday levels of (HO2+CH3O2) reached 30– 40 pptv during two summer seasons. Calculated OH concentrations averaged between 05:00 and 20:00 (or 21:00) exceeded 4×106 molecules cm−3, comparable to (or higher than) levels expected in the tropical marine boundary layer. These findings imply rapid photochemical cycling within the boundary layer at Summit, as well as in the upper pore spaces of the surface snowpack. The photolysis rate constants and OH levels calculated here imply that gas-phase photochemistry plays a significant role in the budgets of NOx, HCHO, H2O2, HONO, and O3, compounds that are also directly affected by processes within the snowpack.

Ozone production from the 2004 North American boreal fires

We examine the ozone production from boreal forest fires based on a case study of wildfires in Alaska and Canada in summer 2004. The model simulations were performed with the chemistry transport model, MOZART-4, and were evaluated by comparison with a comprehensive set of aircraft measurements. In the analysis we use measurements and model simulations of carbon monoxide (CO) and ozone (O3) at the PICO-NARE station located in the Azores within the pathway of North American outflow. The modeled mixing ratios were used to test the robustness of the enhancement ratio ΔO3/ΔCO (defined as the excess O3 mixing ratio normalized by the increase in CO) and the feasibility for using this ratio in estimating the O3 production from the wildfires. Modeled and observed enhancement ratios are about 0.25 ppbv/ppbv which is in the range of values found in the literature and results in a global net O3 production of 12.9 ± 2 Tg O3 during summer 2004. This matches the net O3 production calculated in the model for a region extending from Alaska to the east Atlantic (9–11 Tg O3) indicating that observations at PICO-NARE representing photochemically well aged plumes provide a good measure of the O3 production of North American boreal fires. However, net chemical loss of fire-related O3 dominates in regions far downwind from the fires (e.g., Europe and Asia) resulting in a global net O3 production of 6 Tg O3 during the same time period. On average, the fires increased the O3 burden (surface −300 mbar) over Alaska and Canada during summer 2004 by about 7–9% and over Europe by about 2–3%.