One of the least understood aspects in atmospheric chemistry is how urban emissions influence the formation of natural organic aerosols, which affect Earth’s energy budget. The Amazon rainforest, during its wet season, is one of the few remaining places on Earth where atmospheric chemistry transitions between preindustrial and urban-influenced conditions. Here, we integrate insights from several laboratory measurements and simulate the formation of secondary organic aerosols (SOA) in the Amazon using a high-resolution chemical transport model. Simulations show that emissions of nitrogen-oxides from Manaus, a city of ~2 million people, greatly enhance production of biogenic SOA by 60–200% on average with peak enhancements of 400%, through the increased oxidation of gas-phase organic carbon emitted by the forests. Simulated enhancements agree with aircraft measurements, and are much larger than those reported over other locations. The implication is that increasing anthropogenic emissions in the future might substantially enhance biogenic SOA in pristine locations like the Amazon.

We analyze summertime photochemistry near the surface in Beijing, China, using a 1-D photochemical model (Regional chEmical and trAnsport Model, REAM-1D) constrained by in situ observations, focusing on the budgets of RO_x (OH + HO₂ + RO₂) radicals and O₃ formation. While the modeling analysis focuses on near-surface photochemical budgets, the implications for the budget of O₃ in the planetary boundary layer are also discussed. In terms of daytime average, the total RO_x primary production rate near the surface in Beijing is 6.6 ppbv per hour (ppbv h⁻¹, among the highest found in urban atmospheres. The largest primary RO_x source in Beijing is photolysis of oxygenated volatile organic compounds (OVOCs), which produces HO₂ and RO₂ at 2.5 ppbv h⁻¹ and 1.7 ppbv h⁻¹, respectively. Photolysis of excess HONO from an unknown heterogeneous source is the predominant primary OH source at 2.2 ppbv h⁻¹, much larger than that of O¹D+H₂O (0.4 ppbv h⁻¹). The largest RO_x sink is via OH + NO₂ reaction (1.6 ppbv h⁻¹), followed by formation of RO₂NO₂ (1.0 ppbv h⁻¹) and RONO₂ (0.7 ppbv h⁻¹). Due to the large aerosol surface area, aerosol uptake of HO₂ appears to be another important radical sink, although the estimate of its magnitude is highly variable depending on the uptake coefficient value used. The daytime average O₃ production and loss rates near the surface are 32 ppbv h⁻¹ and 6.2 ppbv h⁻¹, respectively. Assuming NO₂ to be the source of excess HONO, the NO₂ to HONO transformation leads to considerable O₃ loss and reduction of its lifetime. Our observation-constrained modeling analysis suggests that oxidation of VOCs (especially aromatics) and heterogeneous reactions (e.g. HONO formation and aerosol uptake HO₂) play potentially critical roles in the primary radical budget and O₃ formation in Beijing. One important ramification is that O₃ production is neither NO_x nor VOC limited, but in a transition regime where reduction of either NO_x or VOCs could result in reduction of O₃ production. The transition regime implies more flexibility in the O₃ control strategies than a binary system of either NO_x or VOC limited regime. The co-benefit of concurrent reduction of both NO_x and VOCs in reducing column O₃ production integrated in the planetary boundary layer is significant. Further research on the spatial extent of the transition regime over the polluted eastern China is critically important for controlling regional O₃ pollution.

Isoprene dominates global non-methane volatile organic compound emissions, and impacts tropospheric chemistry by influencing oxidants and aerosols. Isoprene emission rates vary over several orders of magnitude for different plants, and characterizing this immense biological chemodiversity is a challenge for estimating isoprene emission from tropical forests. Here we present the isoprene emission estimates from aircraft eddy covariance measurements over the Amazonian forest. We report isoprene emission rates that are three times higher than satellite top-down estimates and 35% higher than model predictions. The results reveal strong correlations between observed isoprene emission rates and terrain elevations, which are confirmed by similar correlations between satellite-derived isoprene emissions and terrain elevations. We propose that the elevational gradient in the Amazonian forest isoprene emission capacity is determined by plant species distributions and can substantially explain isoprene emission variability in tropical forests, and use a model to demonstrate the resulting impacts on regional air quality.

Using a GC/FID/MS system, we analyzed the mixing ratio of 16 halocarbon species in more than 100 air samples collected in 2004 from the Pearl River Delta (PRD) region of southern China. The results revealed that there are elevated mixing ratios for most of halocarbons, especially for HClC = CCl₂ (trichloroethylene, TCE), CH₂Cl₂ (dichloromethane, DCM), CH₃ Br (bromomethane), HCFC-22, CHCl₃ (trichloromethane), CCl₄ (tetrachloromethane), Cl₂C = CCl₂ (perchloroethylene, PCE), CH₃CCl₃ (methyl chloroform, MCF), and CFC-12. Comparisons were done with the data from TRACE-P and ALE/GAGE/AGAGE experiments, we found that the large variability in mixing ratios (relative standard deviation ranged from 9.31 % to 96.55 %) of the halocarbons suggested substantial local emissions from the PRD region in 2004. Correlations between the mixing ratio of each species and carbon monoxide (CO) was examined, and then the emission of each halocarbon was quantified based on scaling the optimized CO emission inventory with the slope of the regression line fitted to each species relative to CO. The calculated results revealed that mass of CH₂Cl₂ (7.0 Gg), CH₃CCl₃ (6.7 Gg), and Cl₂C = CCl₂ (2.3 Gg) accounted for about 62.9 % of total halocarbon emissions, it suggested a significant contribution from solvent use in the PRD region. Emissions of HCFC-22 (3.5 Gg), an alternative refrigerant to chlorofluorocarbons (CFCs), were about 2.3 times greater than those of CFC-12 (1.6 Gg). CFC-12 and HCFC-22 accounted for 21.5 % of total emissions of halocarbons, so that the refrigerant would be the second largest source of halocarbons. However, the ratio approach found only minor emissions of CFCs, such as CFC-11, and the emission of CFC-114 and CFC-113 were close to zero. Emissions of other anthropogenic halocarbons, such as CCl₄, CHCl₃, CH₃Br, and CH₃Cl, were also estimated. Where possible, the emissions estimated from the measured ratios were compared with results from source inventory techniques, we found that both approaches gave emissions at similar magnitude for most of the halocarbons, except CFC-11. The comparison suggested that the ratio method may be a useful tool for assessing regional halocarbon emissions, and emission uncertainty could be further reduced by incorporating both longer-term and higher-frequency observations, as well as improving the accuracy of the CO inventory.