Sources of ambient PM2.5 exposure in 96 global cities.

To improve air quality, knowledge of the sources and locations of air pollutant emissions is critical. However, for many global cities, no previous estimates exist of how much exposure to fine particulate matter (PM2.5), the largest environmental cause of mortality, is caused by emissions within the city vs. outside its boundaries. We use the Intervention Model for Air Pollution (InMAP) global-through-urban reduced complexity air quality model with a high-resolution, global inventory of pollutant emissions to quantify the contribution of emissions by source type and location for 96 global cities. Among these cities, we find that the fraction of PM2.5 exposure caused by within-city emissions varies widely (µ = 37%; σ = 22%) and is not well-explained by surrounding population density. The list of most-important sources also varies by city. Compared to a more mechanistically detailed model, InMAP predicts urban measured concentrations with lower bias and error but also lower correlation. Predictive accuracy in urban areas is not particularly high with either model, suggesting an opportunity for improving global urban air emission inventories. We expect the results herein can be useful as a screening tool for policy options and, in the absence of available resources for further analysis, to inform policy action to improve public health.

Opportunities and Challenges for Filling the Air Quality Data Gap in Low- and Middle-Income Countries.

Given the millions of people suffering from air pollution, filling the air quality monitoring gap in low- and middle-income countries has been recognized as a global challenge. To meet this challenge and make it work will require private enterprise, multiple levels of government, international organizations, academia and civil society to work together toward the common goal of characterizing, understanding better, and then reducing, the air pollution that causes sickness and preventable death for millions of people each year in lowand middle-income countries around the world. This article offers concrete next steps on how to make progress toward increasing air quality monitoring using a combination of emerging technologies, adaptation to country-specific conditions, and building capacity towards the development of lasting institutions.

CO2 emissions from C40 cities: citywide emission inventories and comparisons with global gridded emission datasets.

Under the leadership of the C40 Cities Climate Leadership Group (C40), approximately 1100 global cities have signed to reach net-zero emissions by 2050. Accurate greenhouse gas emission calculations at the city-scale have become critical. This study forms a bridge between the two emission calculation methods: (a) the city-scale accounting used by C40 cities-the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories (GPC) and (b) the global-scale gridded datasets used by the research community-the Emission Database for Global Atmospheric Research (EDGAR) and Open-Source Data Inventory for Anthropogenic CO2 (ODIAC). For the emission magnitudes of 78 C40 cities, we find good correlations between the GPC and EDGAR (R 2 = 0.80) and the GPC and ODIAC (R 2 = 0.72). Regionally, African cities show the largest variability in the three emission estimates. For the emission trends, the standard deviation of the differences is ±4.7% yr-1 for EDGAR vs. GPC and is ±3.9% yr-1 for ODIAC vs. GPC: a factor of ∼2 larger than the trends that many C40 cities pledged (net-zero by 2050 from 2010, or -2.5% yr-1). To examine the source of discrepancies in the emission datasets, we assess the impact of spatial resolutions of EDGAR (0.1°) and ODIAC (1 km) on estimating varying-sized cities' emissions. Our analysis shows that the coarser resolution of EDGAR can artificially decrease emissions by 13% for cities smaller than 1000 km2. We find that data quality of emission factors (EFs) used in GPC inventories vary regionally: the highest quality for European and North American and the lowest for African and Latin American cities. Our study indicates that the following items should be prioritized to reduce the discrepancies between the two emission calculation methods: (a) implementing local-specific/up-to-date EFs in GPC inventories, (b) keeping the global power plant database current, and (c) incorporating satellite-derived CO2 datasets (i.e. NASA OCO-3).

Sources of Black Carbon Deposition to the Himalayan Glaciers in Current and Future Climates.

WRF-Chem and a modified version of the ECLIPSE 5a emission inventory were used to investigate the sources impacting black carbon (BC) deposition to the Himalaya, Karakoram, and Hindu Kush (HKHK) region. This work extends previous studies by simulating deposition to the HKHK region not only under current conditions, but also in the 2040-2050 period under two realistic emission scenarios and in three different phases of the El Niño-Southern Oscillation (ENSO). Under current conditions, sources from outside our South Asian modelling domain have a similar impact on total BC deposition to the HKHK region (35-87%, varying with month) as South Asian anthropogenic sources (13-62%). Industry (primarily brick kilns) and residential solid fuel burning combined account for 45-66% of the in-domain anthropogenic BC deposition to the HKHK region. Under a no further control emission scenario for 2040-2050, the relative contributions to BC deposition in the HKHK region are more skewed toward in-domain anthropogenic sources (45-65%) relative to sources outside the domain (26-52%). The in-domain anthropogenic BC deposition has significant contributions from industry (32-42%), solid fuel burning (17-28%), and diesel fuel burning (17-27%). Under a scenario in which emissions in South Asia are mitigated, the relative cotribution from South Asian anthropogenic sources is significantly reduced to 11-34%. The changes due to phase of ENSO do not seem to follow consistent patterns with ENSO. Future work will use the high-resolution deposition maps developed here to determine the impact of different sources of BC on glacier melt and water availability in the region.