A major challenge in understanding radiative forcing of aerosols is accurately monitoring the light absorbing components and clarifying the main reasons of their spatial and temporal variations. In this study, the optical properties of light-absorbing carbon (LAC) in aerosols were measured over one year and the impacts from various emission sources and other influencing factors were analyzed at three sites (suburban (NJU), urban (PAES) and industrial (NUIST) in Nanjing, a typical polluted city in eastern China. With an improved method that combines online and offline techniques, we revised the multiple scattering correction factors and significantly reduced the uncertainty in measurement of absorption coefficients of black carbon (BC). The result reveals the necessity of developing the regional dependent factor for estimation of BC absorption. Relatively large mass absorption efficiency (MAE) of BC was found in summer and industrial region (NUIST), and the mixing state and coating relevant with secondary aerosol formation were the main reasons for such seasonal and site dependent variations. Distinct seasonal variations existed in the MAE of brown carbon (BrC) at NJU. In the winter, BrC from primary emissions such as diesel vehicles had a stronger absorption ability than that from secondary aerosol formation. The lowest MAE values of BrC appeared in summer, reflecting the formation of the non-absorbing biogenic secondary organic aerosol, and the effect of photobleaching. At the urban site PAES, BrC was expected to be mainly from gasoline vehicles and transport of biomass burning emissions, and had a stronger light absorbing ability than the other two sites. The results of simultaneous observations at NJU and PAES indicated that the formation of fresh secondary organic aerosol enhanced the optical absorption of BC but reduced that of BrC. The impacts of various influencing factors on LAC provided effective ways to alleviate their regional radiative forcing at the city scale.

Using data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) program, Bahadur et al. (2011) report that average fine particulate light absorbing carbon (LAC) concentrations in California decreased by about 50% from 0.46 mu g m(-3) in 1989 to 0.24 mu g m(-3) in 2008. They attribute most of the LAC decline in California to reductions in the state's diesel emissions. These findings are encouraging, but in this comment we call attention to a significant methodological issue that can arise in any long-term trends analysis using IMPROVE data. In the Bahadur et al. analysis, LAC data from eighteen remote monitoring sites were aggregated with data from three urban sites that only operated for 1-8 years. The large absolute decrease of 0.22 mu g m(-3) they reported in the statewide California average was largely driven by one urban site, South Lake Tahoe (Tahoe), which was dropped from the network in mid-1997. LAC concentrations at Tahoe were an order of magnitude higher than those at nearby Bliss State Park indicative of large local source contributions. The exclusion of the three locally influenced urban sites substantially reduces the magnitude of the decreasing LAC trends shown in Bahadur et al., though this does not necessarily invalidate the paper's conclusion that LAC is broadly decreasing and diesel emission controls are likely to be responsible for part of this decrease. Control of emissions from wood-burning stoves may also have contributed to decreases in LAC and other particulate compounds; like diesel emission controls, this too is an important regulatory success. Published by Elsevier Ltd.