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.

Aerosol pH is a useful diagnostic of aerosol chemistry for formation of secondary aerosol and has been hypothesized to be a key factor in specific chemical reaction routes producing sulfate and nitrate. In this study, we measured hourly concentrations of water‐soluble ions in particulate matter with an aerodynamic diameter less than 2.5 μm, along with gaseous pollutants in Tianjin, China, from 4 to 31 January 2015. The following source contributions to water‐soluble ions were estimated by positive matrix factorization: secondary sulfate (13%), secondary nitrate (44%), coal (14%), vehicle (16%), and dust (13%). ISORROPIA‐II was used to investigate the complex relationships among aerosol pH, ammonia, and secondary aerosol formation. The estimated hourly aerosol pH varied from −0.3 to 7.7, with an average of 4.9 (±0.78); the median value was 4.89, and the interquartile range was 0.72. During less polluted conditions, aerosol pH ranged from less than 0 to about 7; during heavily polluted conditions, pH was close to 5 (3.9–7.9) despite large amounts of sulfate. Sufficient ammonia/ammonium was present to balance high sulfate and nitrate formation. NH₄⁺/NH₃ (g) helped stabilize pH while nonvolatile cations contributed less to decreasing aerosol acidity. High acidy (pH < 3), light pollution (total water soluble ions < 30 μg/m³), and low water content (less than 5 μg/m³) were more correlated with higher rates of sulfate formation than nitrate formation in the winter.