ALBERT

Description: We undertook three-dimensional numerical studies of a marine stratus deck under a strong inversion using an interactive shortwave- and longwave-radiation module. A suite of sensitivity tests were conducted to address the effects of model resolution on entrainment (inversion heights), cloud-radiation interactions, and cloud radiative-forcings by varying model horizontal resolution only, varying vertical resolution only, and varying horizontal- and vertical-resolution simultaneously but with a fixed aspect ratio of 2.5. Our results showed that entrainment (inversion height) is more sensitive to vertical- than to horizontal-resolution. A vertical resolution finer than 40 m can simulate spatial- and temporal-variations in the inversion height well. The inversion height decreases with increasing vertical resolution, but tends to increase with increasing horizontal resolution. Cloud liquid water path doubles after refining both the vertical- and horizontal-resolution by a factor of four. This doubling is associated with a positive feedback between cloud water and cloud top radiative cooling, which amplifies small differences initiated by changes in the model resolution. The magnitude of the cloud radiative-forcing tends to increase with increasing model resolution, mainly attributable to the increase in the cloud liquid water path. Shortwave radiative forcing is dominant, and more sensitive to model resolution than the longwave counterpart.

Description: Beijing is a megacity situated in the rapidly developing Beijing-Tianjin-Hebei region of northern China. In this study, we analyze data on ozone and nitrogen oxide levels obtained at six urban sites in Beijing between the months of July and September. Our goal is to investigate average trends and interpretations over the 2001–2006 period. Average concentrations of NOx (NOx=NO+NO2), O3, and Ox (Ox=O3+NO2) were 49.2±5.9 ppbv, 26.6±2.8 ppbv, and 60.3±1.9 ppbv, respectively. NOx concentrations decreased linearly at a rate of 3.9±0.5 ppbv/yr after 2002, while ozone concentrations increased at a rate of 1.1±0.5 ppbv/yr in a two-year cycle during 2001–2006, and Ox concentrations remained nearly constant. The reduction of NOx emissions and elevated non-methane hydrocarbon (NMHCs) emissions may have contributed to the increased O3 concentrations in Beijing. When the contributions from Beijings urban and surrounding areas were disaggregated via trajectory cluster analysis, daily maximum and average Ox concentrations attributable to Beijing local emissions increased linearly at rates of 1.3±0.6 ppbv/yr and 0.8±0.6 ppbv/yr, while the Ox concentrations attributable to regional areas decreased linearly at rates of 0.6±0.3 ppbv/yr and 0.5±0.3 ppbv/yr, respectively. The decrease in Ox concentrations of surrounding areas was found to counteract increasing Beijing urban Ox production, leading to nearly constant Ox concentrations in the Beijing region over the study period. Our results may be helpful for redefining government strategies to control the photochemical formation of air pollutants in the Beijing region. Our conclusions have relevance for developing megacities worldwide.

Description: We performed MAX-DOAS measurements during the PRiDe-PRD2006 campaign in the Pearl River Delta region 50 km north of Guangzhou, China, for 4 weeks in June 2006. We used an instrument which simultaneously sampled the wavelength range from 292 nm to 443 nm at 7 different elevation angles between 3° and 90°. Here we show that the O4 (O2 dimer) absorption at 360 nm can be used to retrieve the aerosol extinction and the height of the boundary layer. A comparison with simultaneously recorded, ground based nephelometer data shows an excellent agreement.

Description: Total atmospheric OH reactivities (kOH) have been measured as reciprocal OH lifetimes by a newly developed instrument at a rural site in the densely populated Pearl River Delta (PRD) in Southern China in summer 2006. The deployed technique, LP-LIF, uses laser flash photolysis (LP) for artifical OH generation and laser-induced fluorescence (LIF) to measure the time-dependent OH decay in samples of ambient air. The reactivities observed at PRD covered a range from 10 s−1 to 120 s−1, indicating a large load of chemical reactants. On average, kOH exhibited a pronounced diurnal profile with a mean maximum value of 50 s−1 at daybreak and a mean minimum value of 20 s−1 at noon. The reactivity was dominated by anthropogenic pollutants (e.g., CO, NOx, light alkenes and aromatic hydrocarbons) at night, while it was strongly influenced by local, biogenic emissions of isoprene at day. The comparison of reactivities calculated from measured trace gases with measured kOH reveals a missing reactivity of about a factor of 2 at day and night. Box model calculations initialized by measured parameters reproduce the observed OH reactivity well and suggest that the missing reactivity is contributed by unmeasured, secondary chemistry products (mainly aldehydes and ketones) that were photochemically formed by hydrocarbon oxidation. Overall, kOH was dominated by organic compounds, which had a maxium contribution of 85% in the afternoon. The paper demonstrates the usefulness of direct reactivity measurements and emphasizes the need for direct measurements of oxygenated organic compounds in atmospheric chemistry studies.