The long-term trend for aerosol optical properties and climate impact sensitivity in terms of radiative forcing efficiency were analyzed at a suburban station in Athens, Southeast Mediterranean, using an extensive dataset from 2008 to 2022. The study examined scattering (nsc) and absorption (nap) coefficients, scattering & Aring;ngstrom exponent (SAE), absorption & Aring;ngstrom exponent (AAE), single scattering albedo (SSA), asymmetry parameter (g), and radiative forcing efficiency (RFE). Seasonal variability was linked to meteorological conditions and human activities. Single Scattering Albedo (SSA) was lowest (0.86), and Radiative Forcing Efficiency (RFE) was highest (-61 W/m2) in winter, confirming enhanced contributions from traffic and biomass burning. Lower SAE values (1.5) in spring indicate a greater presence of coarse particles due to frequent Saharan dust events (SDEs). Daily patterns of nap and SSA reflect local emissions, with pronounced traffic-related peaks. Aerosol classification revealed that Black Carbon (BC) dominates the suburban aerosol (51 %), with mixed BrC-BC (16 %) peaking in winter and dust-pollution mixtures (13 %) increasing in spring. The presence of large particles mixed with BC (11 %) was more frequent in spring, further highlighting seasonal variability. Trend analysis showed statistically significant (ss) decreases in nsc (-0.611) and SSA (-0.003), alongside increases in nap (+0.027) and RFE (+0.270) at a 95 % confidence level, suggesting a shift toward more absorbing aerosols. The findings provide new insights and reveal a new aerosol regime, where a reduction in anthropogenic emissions is affecting the scattering rather than the absorbing aerosol component, while the impact from forest fires as a climate feedback mechanism has a significant effect in the Eastern Mediterranean. It is important for future studies and climate modelling to account for the regionally observed changes of the state of mixing of ambient aerosol leading to a shift in radiative forcing efficiency through the reduction in SSA. This is evident in the long term for the east Mediterranean region and must be accounted for in radiative forcing estimates and future climate projections.

Black carbon (BC) is one of the major aerosol components with relatively high implications on climatic patterns through its radiative forcing (RF). South Asia has recently experienced an increased concentration of pollution; however, relatively fewer studies have been carried out on long-term assessment of BC and its implications. The present study analyzed the long-term concentration of BC in selected urban locations over South Asia using the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2). The study employed statistical analysis, including linear regression techniques, to assess the long-term concentration of BC. The results show that a rapid increase of BC is observed over most urban locations of South Asia with the predominance in winter and hence requires strict regional control measures to reduce the excess concentration of BC in the atmosphere. High concentration of BC in winter is attributed to anthropogenic activities and changes in meteorological conditions that enhance the accumulation of pollutants in the atmosphere. The relationship of BC with cloud top temperature and cloud effective radius demonstrates the direct and indirect effect of BC on cloud properties in this region. The RF results reveal that aerosol optical depth has positive aerosol RF in the atmosphere and negative RF at the top of the atmosphere (TOA) as well as at the bottom of the atmosphere (BOA). Negative RF at the TOA indicates less forcing efficiency due to fewer BC aerosols. On the other hand, averaging aerosol RF within the atmosphere reveals positive forcing, which suggests the efficiency force exerted by BC aerosols after absorbing solar radiation.

Estimates of the effective radiative forcing from aerosol-radiation interaction (ERFari) of anthropogenic Black Carbon (BC) have been disputable and require better constraints. Here we find a substantial decline in atmospheric absorption of -5.79Wm(-2)decade(-1) over eastern central China (ECC) responding to recent anthropogenic BC emission reductions. By combining the observational finding with advances from Coupled Model Intercomparison Project phase6 (CMIP6), we identify an emergent constraint on the ERFari of anthropogenic BC. We show that across CMIP6 models the simulated trends correlate well with simulated annual mean shortwave atmospheric absorption by anthropogenic BC over China. Making use of this emergent relationship allows us to constrain the aerosol absorption optical depth of anthropogenic BC and further provide a constrained range of 2.4-3.0 Wm(-2) for its top-of-atmosphere ERFari over China, higher than existing estimates. Our work supports a strong warming effect of BC over China, and highlights the need to improve BC simulations over source regions.

The Pan-Third Pole (PTP), stretching from Eastern Asia to Middle-central Europe, has experienced unprecedented accelerated warming and even retreat of glaciers. Absorbing aerosols reduce snow and ice albedo and radiative forcing, consequently enhancing a great melting of snow cover and ice sheet in the PTP. Employing the 10-year (2007-2016) space-based active and passive measurements, this study investigated the distribution, optical properties and decadal trends for dominating aerosols at a seasonal scale in the PTP divided into six subregions. Results showed that the sub-regions of PTP were mainly dominated by dust, polluted dust and elevated smoke. The Taklimakan Desert (TD) and the Iranian Plateau (IP) were dominated by mineral dust, accounting for 96% and 86% of the total aerosol extinction while the Central Europe (CE), Indo-China (IC) and Anatolia Plateau (AP) were dominated by the mixture of the dominating aerosol types. The mean aerosol extinction coefficient (MAEC) showed an obvious variability depending on the sub-regions and a tendency of decreasing with an increase in the topographic height. The strongest extinction layer (>0.1 km(-1)) mainly occurred below 4 km and the weak extinction layers (>0.001 km(-1)) were mainly distributed between 5 km and 8 km, indicating pronounced vertical transport in the region. The decadal trends of columnar aerosol optical depth (AOD) showed a relation with the contributions of the dominating aerosol types. For example, significant upward or downward trends of total aerosol loading in the IC region were driven by elevated smoke while the AOD trends of total aerosol loading for the CE, the AP and the IP were driven by the dominating aerosol types. The Tibetan Plateau (TP), the cleanest region in the PTP, has been regularly exposed to polluted air masses with significant amounts of absorbing aerosols. Therefore, understanding the dominating aerosol types, properties and decadal trends in the PTP region will contribute considerably to assessing their effects on radiative forcing, climate change, and even snowmelt and glacier retreat.

Due to U.S. air pollution regulations, aerosol and precursor emissions have decreased during recent decades, while changes in emissions in other regions of the world also influence U.S. aerosol trends through long-range transport. We examine here the relative roles of these domestic and foreign emission changes on aerosol concentrations and direct radiative forcing at the top of the atmosphere over the continental United States. Long-term (1980-2014) trends and aerosol source apportionment are quantified in this study using a global aerosol-climate model equipped with an explicit aerosol source tagging technique. Due to U.S. emission control policies, the annual mean near-surface concentration of particles, consisting of sulfate, black carbon, and primary organic aerosol, decreases by about -1.1 (+/- 0.1)/-1.4 (+/- 0.1) mu g/m(3) in western United States and -3.3 (+/- 0.2)/-2.9 (+/- 0.2) mu g/m(3) in eastern United States during 2010-2014, as compared to those in 1980-1984. Meanwhile, decreases in U.S. emissions lead to a warming of +0.48 (+/- 0.03)/+0.46 (+/- 0.03)W/m(2) in western United States and +1.41 (+/- 0.07)/+1.32 (+/- 0.09)W/m(2) in eastern United States through changes in aerosol direct radiative forcing. Increases in emissions from East Asia generally have a modest impact on U.S. air quality but mitigated the warming effect induced by reductions in U.S. emissions by 25% in western United States and 7% in eastern United States. As U.S. domestic aerosol and precursor emissions continue to decrease, foreign emissions may become increasingly important to radiative forcing over the United States.

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.