ALBERT

Description: One of the objectives of the Deep Convective Clouds and Chemistry (DC3) field experiment was to determine the scavenging of soluble trace gases by thunderstorms. We present an analysis of scavenging of hydrogen peroxide (H 2 O 2 ) and methyl hydrogen peroxide (CH 3 OOH) from six DC3 cases that occurred in Oklahoma and northeast Colorado. Estimates of H 2 O 2 scavenging efficiencies are comparable to previous studies ranging from 79-97% with relative uncertainties of 5-25%. CH 3 OOH scavenging efficiencies ranged from 12-84% with relative uncertainties of 18-558%. The wide range of CH 3 OOH scavenging efficiencies is surprising, as previous studies suggested that CH 3 OOH scavenging efficiencies would be 〈10%. Cloud chemistry model simulations of one DC3 storm produced CH 3 OOH scavenging efficiencies of 26-61% depending on the ice retention factor of CH 3 OOH during cloud drop freezing, suggesting ice physics impacts CH 3 OOH scavenging. The highest CH 3 OOH scavenging efficiencies occurred in two severe thunderstorms, but there is no obvious correlation between the CH 3 OOH scavenging efficiency and the storm thermodynamic environment. We found a moderate correlation between the estimated entrainment rates and CH 3 OOH scavenging efficiencies. Changes in gas-phase chemistry due to lightning production of nitric oxide and aqueous-phase chemistry have little effect on CH 3 OOH scavenging efficiencies. To determine why CH 3 OOH can be substantially removed from storms, future studies should examine effects of entrainment rate, retention of CH 3 OOH in frozen cloud particles during drop freezing, and lightning-NO x production.

Description: We investigate the contribution of oceanic methyl iodide (CH3I) to the stratospheric iodine budget. Based on CH3I measurements from three tropical ship campaigns and the Lagrangian transport model FLEXPART, we provide a detailed analysis of CH3I transport from the ocean surface to the cold point in the upper tropical tropopause layer (TTL). While average oceanic emissions differ by less than 50% from campaign to campaign, the measurements show much stronger variations within each campaign. A positive correlation between the oceanic CH3I emissions and the efficiency of CH3I troposphere–stratosphere transport has been identified for some cruise sections. The mechanism of strong horizontal surface winds triggering large emissions on the one hand and being associated with tropical convective systems, such as developing typhoons, on the other hand, could explain the identified correlations. As a result of the simultaneous occurrence of large CH3I emissions and strong vertical uplift, localized maximum mixing ratios of 0.6 ppt CH3I at the cold point have been determined for observed peak emissions during the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere)-Sonne research vessel campaign in the coastal western Pacific. The other two campaigns give considerably smaller maxima of 0.1 ppt CH3I in the open western Pacific and 0.03 ppt in the coastal eastern Atlantic. In order to assess the representativeness of the large local mixing ratios, we use climatological emission scenarios to derive global upper air estimates of CH3I abundances. The model results are compared with available upper air measurements, including data from the recent ATTREX and HIPPO2 aircraft campaigns. In the eastern Pacific region, the location of the available measurement campaigns in the upper TTL, the comparisons give a good agreement, indicating that around 0.01 to 0.02 ppt of CH3I enter the stratosphere. However, other tropical regions that are subject to stronger convective activity show larger CH3I entrainment, e.g., 0.08 ppt in the western Pacific. Overall our model results give a tropical contribution of 0.04 ppt CH3I to the stratospheric iodine budget. The strong variations in the geographical distribution of CH3I entrainment suggest that currently available upper air measurements are not representative of global estimates and further campaigns will be necessary in order to better understand the CH3I contribution to stratospheric iodine.