A14C-01 INVITED 16:00h
Trace Gases and Ice: What Conditions Lead to Substantial Uptake?
The non-reactive interactions of ice and atmospheric trace gases have been studied in sufficient detail in the past few years that the general conditions necessary for efficient uptake from the gas phase are becoming apparent. In particular, this talk will summarize the findings made for the interactions of semi-volatile organics, which strongly indicate that the volatility of the organic is the prime factor determining uptake. It will also focus on the uptake of acids, where it has been shown that strong acids, such as nitric acid and hydrogen chloride, are more efficiently adsorbed than weak acids, such as sulfur dioxide and acetic acid. In this context, new experimental results for the uptake of nitric acid at low partial pressures will be presented that are in considerably better agreement with field measurements in cirrus clouds than are measurements conducted at non-atmospheric partial pressures, and for the uptake of gases under conditions where the ice is growing.
A14C-02 INVITED 16:20h
Simulating the Redistribution of Formaldehyde in Deep Convection Using the Weather Research Forecast Model Coupled With Aqueous Chemistry
Processing of chemical species by deep convection affects climate, air quality, and acid deposition. The transport of species to the upper troposphere (UT) is an important way of venting the planetary boundary layer and for contributing to the production of ozone in the UT. Wet deposition of dissolved species is the primary pathway for removing pollutants from the atmosphere, yet this process also contributes to acid rain. Cloud chemistry, the combination of aqueous-phase chemistry and the modification of gas-phase chemistry due to the separation of reactants when cloud drops are present, and cloud microphysics can play an important role in determining the fate of species that participate in ozone chemistry. The fate of formaldehyde (CH$_2$O) is a combination of transport to the upper troposphere where CH$_2$O contributes to ozone formation and of cloud scavenging with subsequent rain out or chemical reaction. By coupling a simple chemical reaction mechanism with the Weather and Research Forecast (WRF) model to simulate the 10 July 1996 STERAO storm which was observed in northeastern Colorado, we examine the relative importance of different chemical and physical cloud processes on formaldehyde concentrations. The specific processes studied include aqueous chemistry and retention of CH$_2$O in frozen hydrometeors. The sensitivity of the modeled processes to the microphysics parameterization will be discussed.
A14C-03 INVITED 16:40h
Pollution Processing by California Radiation Fogs
The San Joaquin Valley (SJV) of California has drawn attention lately for being home to several of the most polluted cities in the U.S. Elevated regional levels of airborne fine particulate matter occur especially in wintertime, when high pressure leads to air stagnation and formation of a subsidence version above the valley floor. Clear skies accompanying these winter stagnation episodes give rise to strong radiative cooling at night and frequent formation of persistent, dense fogs. The chemistry of SJV fogs and their interactions with airborne pollutants have been studied repeatedly over the past two decades. The focus was originally on interactions with sulfate and nitrate, shifting in recent years toward an understanding of fog processing of organic material. A review of key observations and findings from several recent studies of SJV fogs will be presented. SJV fogs contribute both to formation of new particle mass and to particle removal. Particle formation is known to occur via aqueous phase oxidation of sulfur dioxide and by reactions between dissolved sulfur dioxide and formaldehyde to form hydroxymethanesulfonate. Additional production of low volatility solutes from fog processing of volatile organic compounds is also likely, although poorly understood. Fog cleansing of the atmosphere occurs via drop scavenging (via nucleation and other mechanisms) of airborne particles, followed by drop deposition to the surface. Both new particle production and removal are influenced by variations in fog drop composition across the drop size spectrum. The net effect of most SJV fog episodes appears to be to cleanse the atmosphere of fine particulate matter, thereby limiting pollutant buildup during winter stagnation episodes. The effectiveness of the fogs as atmospheric cleansers depends on fog duration, depth and drop size.
A14C-04 17:00h
COUPLING AEROSOL AND CLOUD MICROPHYSICS WITH MULTIPHASE CHEMISTRY
A multiphase chemistry model coupled with a quasi-spectral microphysical model, including aerosol particle activation has been applied to measurements from the European CIME campaign to quantify the formation of the strong acids nitrate and sulfate, and to evaluate the role of microphysical processes in redistributing reactive species among the different phases (gas, aerosols particles versus cloud and/or rain). Significant formation of nitrate and sulfate are found to be due to the reaction of pernitric acid with the sulfite ion. Moreover, pernitric acid, because of its equilibrium in the gas phase and its high solubility, is always available both in cloud water and in rainwater via mass transfer from the gas phase. The sulfite ion comes from the mass transfer from the gas phase of sulfur dioxide in cloud water. When rain formation begins, it is efficiently transferred to the rainwater by collision/coalescence processes. This leads to an enhancement in strong acid production when microphysics is activated in the model. Modeled results have been compared with experimental data, in an effort to retrieve a behavior law related to the partitioning between the gas and aqueous phases of the cloud. In particular, when collision/coalescence processes are considered, an improvement in retrieving the partitioning of soluble species and especially nitrate is observed. Also, model results provide an indication for the relative contributions of gas versus particle phases to sulfate and nitrate concentrations in clouds.
A14C-05 INVITED 17:15h
In-cloud and Clear-sky Supersaturations with Respect to Ice at Low Temperatures in the Atmosphere
Recent in situ measurements in the tropical upper troposphere have documented (1) substantial (about 30%) ice supersaturations within cirrus at temperatures below about 200 K even in the presence of large ice surface areas, and (2) extremely high ice supersaturations (about 100%, near water saturation) at temperatures below about 190 K without any apparent ice nucleation. Both of these observations are unexpected and possibly indicate important cloud chemistry interactions. I will present the available evidence for supersaturations at low temperatures and discuss possible explanations involving nitric acid and organic layers on ice crystal and aerosol surfaces. I will also assess the plausibility of using remote-sensing measurements to gather statistical information about low-temperature supersaturation.
A14C-06 17:30h
Observation of Chlorine Activation near the Midlatitude Tropopause
It has been proposed that heterogeneous chlorine activation on cirrus cloud particles near the tropopause could provide a chemical explanation for ozone trends in the midlatitude tropopause region. During the 1998 WAM (WB-57 Aerosol Mission) campaign, an in situ ClO instrument was deployed on the NASA WB-57 aircraft in the midlatitudes. On the 11 April 1998 flight, clear examples of enhancements to reactive chlorine in sunlit, wet, particle laden air near the tropopause were observed over eastern Wyoming (approximately 42\deg N, 105\deg E) at 11-12 km. The air being sampled appeared to contain evaporating cirrus, and the observed chlorine enhancements (up to 20% activation) were strongly correlated with both particle surface area and total water. Ozone values in this enhanced ClO region ranged from 80-300 ppbv, consistent with both tropospheric and lowermost stratospheric air. These observations suggest that near tropopause reactive chlorine enhancements likely occur in regions of recent stratospheric-tropospheric exchange providing water and increased particle surface area to otherwise relatively dry stratospheric air. Due to greater insolation, ozone loss rates in this region may be higher than those previously reported for similar active chlorine abundances at similar altitudes in the Arctic.
A14C-07 17:45h
Crystallization of Cubic Ice in Liquid Water and Aqueous Solutions
It is often assumed that the only natural phase of ice that forms on Earth is that of hexagonal ice (ice Ih). However, the rare observation of haloes around the sun at 28$^{o}$ and cloud particles of cubic habit indicates that the metastable crystalline form of ice, cubic ice (ice Ic), may form in Earth's atmosphere. The conditions used to produce ice Ic in previous studies are most likely never experienced on Earth outside the laboratory, since they usually involve extremely fast cooling rates. Here we show, using X-ray diffraction, that cubic ice forms when micrometer-sized pure water and aqueous solution droplets freeze homogeneously using a modest cooling rate ( 10 K min$^{-1}$). In fact, ice Ic is the exclusive product when NaCl, (NH$_{4}$)$_{2}$SO$_{4}$, (NH$_{4}$)$_{3}$H(SO$_{4}$)$_{2}$ and HNO$_{3}$ solution droplets freeze at temperatures below 190 K, and pure water droplets freeze to nearly 50% ice Ic at 235 K. These results offers significant insights into the crystallization of ice, a process that occurs widely in nature and strongly suggest that cubic ice will form in the atmosphere. Recently, Murphy (GRL, Art. No. 2230, 2003) has shown that the formation of cubic ice in cirrus clouds may alter the particle size distribution sufficiently to increase dehydration in the tropopause region.