Measurements in marine stratocumulus over the northeast Pacific help scientists unravel the mysteries of this important cloud regime.T he stratocumulus-topped boundary layer (hereafter the STBL), which prevails in the subtropics in regions where the underlying ocean is much colder than the overlying atmosphere, is thought to be an important component of the climate system. Perhaps most striking is its impact on the radiative balance at the top of the atmosphere. The seasonally averaged net cloud radiative forcing from the STBL has been estimated to be as large as 70 W nr 2 (Stephens and Greenwald 1991), more than an order of magnitude larger than the radiative forcing associated with a doubling of atmospheric C0 2 . This means that even rather subtle sensitivities of the STBL to changes in the properties of the atmospheric aero-

Abstract. Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for •e•earch on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOz levels and high isoprene emissions. Detailed HOz budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.

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Abstract. Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state.These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere.Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso-or synoptic scale conditions.Ground-based measurements combined with tetheredballoon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period.Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.

Single-point failures of natural gas infrastructure can hamper methane emission control strategies designed to mitigate climate change. The 23 October 2015 blowout of a well connected to the Aliso Canyon underground storage facility in California resulted in a massive release of natural gas. Analysis of methane and ethane data from dozens of plume transects, collected during 13 research-aircraft flights between 7 November 2015 and 13 February 2016, shows atmospheric leak rates of up to 60 metric tons of methane and 4.5 metric tons of ethane per hour. At its peak, this blowout effectively doubled the methane emission rate of the entire Los Angeles basin and, in total, released 97,100 metric tons of methane to the atmosphere.

Abstract. Direct quantification of fossil fuel CO 2 (CO 2 ff) in atmospheric samples can be used to examine several carbon cycle and air quality questions. We collected in situ CO 2 , CO, and CH 4 measurements and flask samples in the boundary layer and free troposphere over Sacramento, California, USA, during two aircraft flights over and downwind of this urban area during spring of 2009. The flask samples were analyzed for 14 CO 2 and CO 2 to determine the recently added CO 2 ff mole fraction. A suite of greenhouse and other trace gases, including hydrocarbons and halocarbons, were measured in the same samples. Strong correlations were observed between CO 2 ff and numerous trace gases associated with urban emissions. From these correlations we estimate emission ratios between CO 2 ff and these species, and compare these with bottom-up inventory-derived estimates. Recent county level inventory estimates for carbon monoxide (CO) and benzene from the California Air Resources Board CEPAM database are in good agreement with our measured emission ratios, whereas older emissions inventories appear to overestimate emissions of these gases by a factor of two. For most other trace species, there are Correspondence to: J. C. Turnbull (jocelyn.turnbull@noaa.gov) substantial differences (200-500%) between our measured emission ratios and those derived from available emission inventories. For the first flight, we combine in situ CO measurements with the measured CO:CO 2 ff emission ratio of 14 ± 2 ppbCO/ppmCO 2 to derive an estimate of CO 2 ff mole fraction throughout this flight, and also estimate the biospheric CO 2 mixing ratio (CO 2 bio) from the difference of total and fossil CO 2 . The resulting CO 2 bio varies dramatically from up to 8 ± 2 ppm in the urban plume to −6 ± 1 ppm in the surrounding boundary layer air. Finally, we use the in situ estimates of CO 2 ff mole fraction to infer total fossil fuel CO 2 emissions from the Sacramento region, using a mass balance approach. The resulting emissions are uncertain to within a factor of two due to uncertainties in wind speed and boundary layer height. Nevertheless, this first attempt to estimate urban-scale CO 2 ff from atmospheric radiocarbon measurements shows that CO 2 ff can be used to verify and improve emission inventories for many poorly known anthropogenic species, separate biospheric CO 2 , and indicates the potential to constrain CO 2 ff emissions if transport uncertainties are reduced.

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