K. Drdla

In‐situ observations of mid‐latitude forest fire plumes deep in the stratosphere

We observed a plume of air highly enriched in carbon monoxide and particles in the stratosphere at altitudes up to 15.8 km. It can be unambiguously attributed to North American forest fires. This plume demonstrates an extratropical direct transport path from the planetary boundary layer several kilometers deep into the stratosphere, which is not fully captured by large-scale atmospheric transport models. This process indicates that the stratospheric ozone layer could be sensitive to changes in forest burning associated with climatic warming.

Carbonaceous aerosol (soot) measured in the lower stratosphere during POLARIS and its role in stratospheric photochemistry

This paper describes recent measurements of carbonaceous aerosol made by wire impactors during the Photochemistry Ozone Loss in the Arctic Region in Summer (POLARIS) campaign and assesses their role in stratospheric photochemistry. Ninety-five percent of the carbonaceous aerosol collected during this campaign was in the form of black carbon aerosol (BCA), or soot. A new method of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results are compared to previously used methods. The new method results in an increase in the measured BCA number density of 4 times, surface area density of ∼15 times, and an increase in mass loading of 6.15 times over one previously used approach. Average values of number, surface area, and mass densities are 0.06 no./cm3,0.03 μm2/ cm3, and 0.64 ng/m3, respectively. BCA number densities are ∼1% of total aerosol number density, and BCA surface area density is ∼10% of the measured sulfuric acid aerosol surface area. Including heterogeneous reactions on BCA in a photochemical model can affect photochemistry leading to renoxification and increased ozone depletion. However, these predicted effects are not supported by the POLARIS observations, in particular, the NOx/NOy ratios. The laboratory data is not conclusive enough to determine to what extent the heterogeneous reaction is catalytic or carbon consuming. Including catalytic reactions on BCA does not statistically improve the agreement between model and measurement in any of the several scenarios considered. Furthermore, if the reactions cause even partial carbon oxidation, the BCA would be consumed at a rate inconsistent with POLARIS observations. These inconsistencies lead us to conclude that the presence of BCA in the stratosphere did not affect stratospheric photochemistry during POLARIS.

Arctic "ozone hole" in a cold volcanic stratosphere.

Optical depth records indicate that volcanic aerosols from major eruptions often produce clouds that have greater surface area than typical Arctic polar stratospheric clouds (PSCs). A trajectory cloud–chemistry model is used to study how volcanic aerosols could affect springtime Arctic ozone loss processes, such as chlorine activation and denitrification, in a cold winter within the current range of natural variability. Several studies indicate that severe denitrification can increase Arctic ozone loss by up to 30%. We show large PSC particles that cause denitrification in a nonvolcanic stratosphere cannot efficiently form in a volcanic environment. However, volcanic aerosols, when present at low altitudes, where Arctic PSCs cannot form, can extend the vertical range of chemical ozone loss in the lower stratosphere. Chemical processing on volcanic aerosols over a 10-km altitude range could increase the current levels of springtime column ozone loss by up to 70% independent of denitrification. Climate models predict that the lower stratosphere is cooling as a result of greenhouse gas built-up in the troposphere. The magnitude of column ozone loss calculated here for the 1999–2000 Arctic winter, in an assumed volcanic state, is similar to that projected for a colder future nonvolcanic stratosphere in the 2010 decade.

Heterogeneous chemistry on Antarctic polar stratospheric clouds: A microphysical estimate of the extent of chemical processing

A detailed model of polar stratospheric clouds (PSCs), which includes nucleation, condensational growth, and sedimentation processes, has been applied to the study of heterogeneous chemical reactions. For the first time, the extent of chemical processing during a polar winter has been estimated for an idealized air parcel in the Antarctic vortex by calculating in detail the rates of heterogeneous reactions on PSC particles. The resulting active chlorine and NOx concentrations at first sunrise are analyzed with respect to their influence upon the Antarctic ozone hole, using a photochemical model. It is found that the species present at sunrise are primarily influenced by the relative values of the heterogeneous reaction rate constants (and thus the “sticking coefficients”) and the initial gas concentrations. However, the extent of chlorine activation is also influenced by whether N2O5 is removed by reaction with HCl or H2O. The reaction of N2O5 with HCl, which occurs rapidly on type 1 PSCs, activates the chlorine contained in the reservoir species HCl. Hence the presence and surface area of type 1 PSCs early in the winter are crucial in determining ozone depletion.

Nitric acid concentrations near the tropical tropopause: Implications for the properties of tropical nitric acid trihydrate clouds

[i] In situ measurements of NO y , NO x , and temperature confirm that nitric acid trihydrate (NAT) particles could form at the tropical tropopause. The HNO 3 mixing ratio near the tropical tropopause is typically no larger than about 0.2-0.3 ppbv, and the corresponding equilibrium mass of nitric acid trihydrate (NAT) is no larger than 0.3 μg m -3 . Considerably larger NAT condensed masses are required to explain the HALOE extinctions; however, localized regions of enhanced HNO 3 produced by oxidation of lightning-generated NO might exist. NAT layers would only be identified as clouds by SAGE II if the particle diameters are in the optimum range of about 0.6 to 2 μm and the condensed NAT mass is larger than about 0.2 μg m 3 . The SAGE II extinction ratio measurements (0.5 μm/1.0 μm) cannot distinguish NAT clouds from mixtures of optically thin ice clouds and background aerosols.

Microphysics and chemistry of sulphate aerosols at warm stratospheric temperatures

Observations of high NO x /NO y ratios (overall 40% larger than modelled values) during the Polar Ozone Loss in the Arctic Region in Summer campaign have led us to re-examine the heterogeneous chemistry of stratospheric aerosol particles during the polar summer period, using the Integrated MicroPhysics and Aerosol Chemistry on Trajectories model. The warm summer temperatures (up to 235 K) imply very concentrated sulphuric acid solutions (80 wt %). On the one hand, these solutions are more likely to freeze, into sulphuric acid monohydrate (SAM), reducing the efficiency of the N 2 O 5 hydrolysis reaction. Including this freezing process increases NO x /NO y ratios but does not improve model/measurement agreement: in polar spring, SAM formation causes the NO x /NO y ratio to be overpredicted whereas freezing has a much smaller effect on nitrogen chemistry during the continuous solar exposure of polar summer. On the other hand, if sulphate aerosols remain liquid, the high acidity may promote acid-catalysed reactions. The most important reaction is CH 2 O+HNO 3 , which effectively increases NO x /NO y ratios across a wide range of conditions, improving agreement with measurements. Furthermore, the production of HONO can either enhance gas-phase OH concentrations or promote secondary liquid reactions, including HONO+HNO 3 and HONO+HCl. Primary uncertainties include the uptake coefficient of CH 2 O relevant to reaction with HNO 3 , the amount of HONO available for secondary reaction, and the relative rates of HONO reaction with HNO 3 and HCI. The fate of the formic acid product, whose presence in the stratosphere may be an indicator for the CH 2 O reaction, and the impact on the stratospheric hydrogen budget are also discussed.

Tracking Polar Stratospheric Cloud Development with POAM II and a Microphysical Model

The formation and development of polar stratospheric clouds are examined using a comprehensive microphysical model applied along air parcel trajectories. During Antarctic winter specific parcels are sampled over a time span of several days using extinction data from the POAM II satellite instrument. The observed evolution of polar stratospheric cloud spectral opacity is compared to predictions from model simulations. Hence, aerosol growth and decay in response to changing environmental conditions can be observed and interpreted. In some instances good agreement between observations and predictions has been obtained over periods of many days. The analysis provides evidence for the role of temperature history in cloud formation and properties, in agreement with previous studies. On occasion, the apparent behavior of a PSC along a trajectory is inconsistent with current theoretical concepts.

Microphysical modeling of southern polar dehydration during the 1998 winter and comparison with POAM III observations

[1] Stratospheric dehydration and high aerosol extinctions are examined for the 1998 Antarctic winter using the Integrated Microphysics and Aerosol Chemistry on Trajectories (IMPACT) model and data obtained by the Polar Ozone and Aerosol Measurement (POAM) III instrument. The model is applied to individual air parcels which are advected along 3-D trajectories using the United Kingdom Meteorological Office (UKMO) global wind and temperature fields. Model results are compared to water vapor and aerosol extinction measurements obtained with the POAM instrument. Results suggest that the water vapor mixing ratio at the end of the season is predicted with reasonable accuracy. However, dehydration occurs more rapidly in the simulation than is indicated by the POAM data. In addition to dehydration results, the frequency of high aerosol extinction measurements is examined for all model runs and compared to POAM data. The aerosol extinction comparisons are consistent with the assumption that heterogeneous nitric acid trihydrate (NAT) freezing occurs in approximately 1% of all particles. Various model parameters influencing ice cloud microphysics are altered to examine their effects on both the water vapor mixing ratio and high aerosol extinction events. While a reduction in the ice accommodation coefficient and an increase in the ice nucleation barrier both improve the agreement in the water vapor mixing ratio, the agreement in aerosol extinction is worsened. Extinction comparisons suggest that the model results are consistent with either high or low NAT-ice lattice compatibility factors, although intermediate values agree poorly with POAM data. The extent of dehydration is highly dependent on temperature; therefore, an uncertainty as small as ±1 K in the UKMO temperature fields may significantly change the model results.