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Atmospheric black carbon (BC) warms Earth's climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ∼2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (E abs ) and mixing state are reported for two California regions. The observed E abs is small—6% on average at 532 nm—and increases weakly with photochemical aging. The E abs is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial E abs for BC are possible.

Jacobson argues that our statement that \"many climate models may overestimate warming by BC\" has not been demonstrated. Jacobson challenges our results on the basis that we have misinterpreted some model results, omitted optical focusing under high relative humidity conditions and by involatile components, and because our measurements consist of only two locations over short atmospheric time periods. We address each of these arguments, acknowledging important issues and clarifying some misconceptions, and stand by our observations. We acknowledge that Jacobson identified one detail in our experimental technique that places an additional constraint on the interpretation of our observations and reduces somewhat the potential consequences of the stated implications.

To the Editor: Nancy Lyon, the author of \"The Last Days of a Blue Collar Resort\" (Travel Section, Sept. 16), displays utter contempt for a quaint and charming community while at the same time showing a total lack of understanding for the ambience of a highly eclectic enclave.

Atmospheric concentrations of C$\\sb2$-C$\\sb6$ hydrocarbons were measured at two remote sites in Canada: Fraserdale, ON (50$\\sp\\circ$N) and Alert, NWT (82$\\sp\\circ$N). Air samples were collected in stainless steel canisters and hydrocarbon concentrations determined using gas chromatography and flame ionization detection. Separation was performed on a 50-m Al$\\sb2$O$\\sb3$/KCl PLOT column using above ambient temperature programming. Air samples were collected at Fraserdale at least once weekly from April, 1990 to January, 1993 under the auspices of the Northern Wetlands Study. At Alert, air samples were collected once a day from January 22, to April 19, 1992 during the 1992 Polar Sunrise Experiment. Alkane, acetylene, and benzene concentrations at Fraserdale displayed a distinct seasonal cycle, with a winter maximum and a summer minimum. Individual hydrocarbon profiles differed with respect to phase and amplitude. The acetylene concentration profile was negatively correlated with ambient temperature. Seasonal changes in concentration were consistent with seasonal changes in atmospheric lifetime against HO initiated oxidation. However, deviation of the seasonal trends from pseudo first order kinetic behaviour were apparent. The deviations were qualitatively accounted for by mixing effects based on a Gaussian distribution of air mass ages. A seasonal cycle in the biogenic hydrocarbon isoprene was also observed. Analysis of the hydrocarbon trends at Alert provided evidence of HO, Cl- and possibly Br-atom oxidation of hydrocarbons. The marked decreases in background concentration of C$\\sb2$-C$\\sb6$ hydrocarbons, from late March to mid-April, were well correlated with HO rate constants, yielding an average HO concentration of 1.1 $\\times$ 10$\\sp5$ molecules cm$\\sp{-3}$. Dramatic depletions of C$\\sb2$-C$\\sb5$ alkane and acetylene concentrations in ozone depleted air at Alert, and on an ice floe 150 km north of Alert were observed. The concentration changes of C$\\sb2$-C$\\sb5$ alkanes and benzene in ozone depleted air were well correlated with Cl-atom rate constants, providing the first evidence of Cl-atom oxidation of hydrocarbons in the troposphere. The large decrease in acetylene concentration was attributed to additional reaction with Br-atom. The Br-atom concentration, estimated from the depletion of acetylene, can account for the observed ozone depletion, and supports the hypothesis of Br-atom destruction of ozone in the Arctic boundary layer during polar spring.

Background Cycling is associated with numerous health benefits. Evidence suggests that new cycling infrastructure leads to increases in cycling, though studies of network-level changes are lacking. The objective of this study was to determine the longitudinal effect of cycling infrastructure on cycling engagement among adults living in Montréal, Canada. Methods Using data from the National Population Health Survey (1994–2011), this study included adults who resided in the Montréal Census Metropolitan Area for a minimum of two survey cycles ( N  = 779). Outcomes included self-reported any cycling (transportation or recreation) and time in recreational cycling (minutes/week). Archival maps describing temporal changes in the cycling network for five-year intervals (1991–2011) were classified using the Canadian Bikeway Comfort and Safety Classification System (Can-BICS). Three cycling exposures were calculated from the centroid of each dissemination area: (1) distance to the nearest cycling path categorized by Can-BICS comfort-level (low, medium or high), (2) presence of cycle paths of each comfort level within distance thresholds (low = 321 m, medium = 623 m, high = 1790 m), and (3) density of cycle paths within a 1000 m buffer. Mixed effects logistic regression models estimated associations between cycling infrastructure and any cycling. Linear mixed effects models estimated associations between cycling infrastructure and time spent in recreational cycling. Results Over the study period, low- and medium-comfort cycle paths were more prevalent than high-comfort paths and cycling for recreation was more common than cycling for transportation. Exposure to high-comfort paths within an acceptable distance (< 1790 m) was associated with higher odds of any cycling (aOR = 1.28, 95% CI: 1.00–1.63). Cumulative exposure to medium-comfort paths within an acceptable distance (< 623 m) was associated with greater time spent in recreational cycling (β = 0.09, 95% CI: 0.03–0.16). Gender-stratified analyses suggested that cumulative exposures to low- and medium-comfort infrastructure within distance thresholds was associated with time spent in recreational cycling (low: β = 0.06, 95% CI: 0.00–0.12, medium: β = 0.13, 95% CI: 0.04–0.22,) among women. No significant effects were observed for distance to the nearest cycling infrastructure for either outcome. Density was not examined in models due to low variation with most buffers having no cycling infrastructure. Conclusions This research provides evidence that cycle paths, especially of higher comfort and safety, can promote cycling. Future work is needed to explore cumulative exposures to cycling infrastructure, taking into consideration connectivity of networks, integrated public transport, and accessibility to work.

Anthropogenic secondary organic aerosol (ASOA), formed from anthropogenic emissions of organic compounds, constitutes a substantial fraction of the mass of submicron aerosol in populated areas around the world and contributes to poor air quality and premature mortality. However, the precursor sources of ASOA are poorly understood, and there are large uncertainties in the health benefits that might accrue from reducing anthropogenic organic emissions. We show that the production of ASOA in 11 urban areas on three continents is strongly correlated with the reactivity of specific anthropogenic volatile organic compounds. The differences in ASOA production across different cities can be explained by differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds, indicating the importance of controlling these ASOA precursors. With an improved model representation of ASOA driven by the observations, we attribute 340 000 PM2.5-related premature deaths per year to ASOA, which is over an order of magnitude higher than prior studies. A sensitivity case with a more recently proposed model for attributing mortality to PM2.5 (the Global Exposure Mortality Model) results in up to 900 000 deaths. A limitation of this study is the extrapolation from cities with detailed studies and regions where detailed emission inventories are available to other regions where uncertainties in emissions are larger. In addition to further development of institutional air quality management infrastructure, comprehensive air quality campaigns in the countries in South and Central America, Africa, South Asia, and the Middle East are needed for further progress in this area.