Imaging radar polarimetry from wave synthesis

It was shown that it is possible to measure the complete scattering matrix of an object using data acquired on a single aircraft pass, and can combine the signals later in the data processor to generate radar images corresponding to any desired combination of transmit and receive polarization. Various scattering models predict different dependence on polarization state of received power from an object. The imaging polarimeter permits determination of this dependence, which is called the polarization signature, of each point in a radar image. Comparison of the theoretical predictions and observational data yield identification of possible scattering mechanisms for each area of interest. It was found that backscatter from the ocean is highly polarized and well-modeled by Bragg scattering, while scattering from trees in a city park possesses a considerable unpolarized component. Urban regions exhibit the characteristics expected from dihedral corner reflectors and their polarization signatures are quite different from the one-bounce Bragg model

Air-snow exchange of HNO3 and NOy at Summit, Greenland

Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol-associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3− in snow may be reemitted to the atmosphere after deposition. Here we report on extensive measurements of HNO3, including vertical gradients between 1.5 and 7 m above the snow, made during the summers of 1994 and 1995 at Summit. These HNO3 data are compared with NO3− concentrations in surface snow and the first measurements of the concentrations and fluxes of total reactive nitrogen oxides (Ny) on a polar glacier. Our results confirm that HNO3 concentrations are quite low (mean 0.5 nmol m−3) during the summer, while NO3− is the dominant ion in snow. Daytime peaks in HNO3− appear to be due at least partly to emissions from the snow, an assertion supported by gradients indicating a surface source for HNO3− on many days. Observed short-term increases in NO3− inventory in the snow can be too large to be readily attributed to deposition of HNO3− suggesting that deposition of one or more other N oxides must be considered. We found that the apparent fluxes of HNO3 and NOy were in opposite directions during about half the intervals when both were measured, with more cases of HNO3 leaving the snow, against an NOy flux into the snow, than the reverse. The concentrations of NOy are generally about 2 orders of magnitude greater than those of HNO3; hence deposition of only a small, non-HNO3, fraction of this pool could dominate NO3− in snow, if the depositing species converted to NO3−, either in the snowpack or upon melting for analysis

Impact of New Laboratory Studies of N2O5 Hydrolysis on Global Model Budgets of Tropospheric Nitrogen Oxides, Ozone, and OH

We investigate the impact of new laboratory studies of N2O5 hydrolysis in aerosols on global model simulations of tropospheric chemistry. We use data from these new studies to parameterize the reaction probability (γN2O5) in the GEOS-CHEM global model as a function of local aerosol composition, temperature, and relative humidity. We find a much lower global mean γN2O5 (0.02) than commonly assumed in models (0.1). Relative to a model simulation assuming a uniform γN2O5 = 0.1, we find increases in mass-averaged tropospheric NOx, O3, and OH concentrations of 7%, 4%, and 8% respectively. The increases in NOx and O3 concentrations bring the GEOS-CHEM simulation in better agreement with climatological observations.Earth and Planetary Science

Chemical nonlinearities in relating intercontinental ozone pollution to anthropogenic emissions

Model studies typically estimate intercontinental influence on surface ozone by perturbing emissions from a source continent and diagnosing the ozone response in the receptor continent. Since the response to perturbations is non-linear due to chemistry, conclusions drawn from different studies may depend on the magnitude of the applied perturbation. We investigate this issue for intercontinental transport between North America, Europe, and Asia with sensitivity simulations in three global chemical transport models. In each region, we decrease anthropogenic emissions of NOx and nonmethane volatile organic compounds (NMVOCs) by 20% and 100%. We find strong nonlinearity in the response to NOx perturbations outside summer, reflecting transitions in the chemical regime for ozone production. In contrast, we find no significant nonlinearity to NOx perturbations in summer or to NMVOC perturbations year-round. The relative benefit of decreasing NOx vs. NMVOC from current levels to abate intercontinental pollution increases with the magnitude of emission reductions

Effect of Climate Change on Air Quality

Air quality is strongly dependent on weather and is therefore sensitive to climate change. Recent studies have provided estimates of this climate effect through correlations of air quality with meteorological variables, perturbation analyses in chemical transport models (CTMs), and CTM simulations driven by general circulation model (GCM) simulations of 21st-century climate change. We review these different approaches and their results. The future climate is expected to be more stagnant, due to a weaker global circulation and a decreasing frequency of mid-latitude cyclones. The observed correlation between surface ozone and temperature in polluted regions points to a detrimental effect of warming. Coupled GCM–CTM studies find that climate change alone will increase summertime surface ozone in polluted regions by 1–10 ppb over the coming decades, with the largest effects in urban areas and during pollution episodes. This climate penalty means that stronger emission controls will be needed to meet a given air quality standard. Higher water vapor in the future climate is expected to decrease the ozone background, so that pollution and background ozone have opposite sensitivities to climate change. The effect of climate change on particulate matter (PM) is more complicated and uncertain than for ozone. Precipitation frequency and mixing depth are important driving factors but projections for these variables are often unreliable. GCM–CTM studies find that climate change will affect PM concentrations in polluted environments by ±0.1–1 μg m−3 over the coming decades. Wildfires fueled by climate change could become an increasingly important PM source. Major issues that should be addressed in future research include the ability of GCMs to simulate regional air pollution meteorology and its sensitivity to climate change, the response of natural emissions to climate change, and the atmospheric chemistry of isoprene. Research needs to be undertaken on the effect of climate change on mercury, particularly in view of the potential for a large increase in mercury soil emissions driven by increased respiration in boreal ecosystems.Engineering and Applied Science

Instrument to collect fogwater for chemical analysis

An instrument is presented which collects large samples of ambient fogwater by impaction of droplets on a screen. The collection efficiency of the instrument is determined as a function of droplet size, and it is shown that fog droplets in the range 3–100-µm diameter are efficiently collected. No significant evaporation or condensation occurs at any stage of the collection process. Field testing indicates that samples collected are representative of the ambient fogwater. The instrument may easily be automated, and is suitable for use in routine air quality monitoring programs