Matthew C Peterson

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.

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.

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.

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.

Do emissions from ships have a significant impact on concentrations of nitrogen oxides in the marine boundary layer

The potential impact of ship emissions on concentrations of nitrogen oxides and reactive nitrogen compounds in the marine boundary layer is assessed using a global chemical transport model. The model predicts significant enhancements of these compounds over large regions, especially over the northern midlatitude oceans. This result is consistent with a recently published study, though the impacts predicted here are more widespread and the peak enhancements are not as large. However, comparisons of model results with recent measurements over the central North Atlantic Ocean do not provide support for these model predictions. While one cannot completely overlook the possibility that emissions of nitrogen oxides from ships may be overestimated, our analysis suggests that there may be a gap in our understanding of the chemical evolution of ship plumes as they mix into the background atmosphere in the marine boundary layer. On a related note, it is also possible that the overestimate of the impacts of ships on nitrogen oxides in the marine boundary layer by global models is due to the lack of parameterized representations of plume dynamics and chemistry in these models.

Observations of rapid photochemical destruction of ozone in snowpack interstitial air

Measurements in central Greenland demonstrate that ozone is rapidly destroyed through a photochemical process in the surface snowpack. The rate of destruction is too high to result from reaction with NOx or HOx, but could result from catalytic destruction by bromine if its release from snowpack bromide is highly efficient. The pristine nature of the Greenland snowpack implies that ozone destruction may be widespread in illuminated snowpacks and thus influence the budget of boundary-layer ozone. Similar destruction on tropospheric cloud ice crystals may explain observations of very low ozone levels associated with cirrus clouds.

NO x and NO 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 regions 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.

Monte Carlo modeling and measurements of actinic flux levels in Summit, Greenland snowpack

Abstract Knowledge of actinic flux levels in snowpack is needed to find the influence of snowpack photochemical processes on atmospheric composition. Measurements show that while

Measurements of nitrogen oxides and a simple model of NO 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 variability 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.