Chemistry transport models (CTMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth’s climate system. This study demonstrates the use of airborne in situ observations to diagnose the representation of atmospheric composition by global CTMs. Process-based diagnostics are developed which minimize the spatial and temporal sampling differences between airborne in situ measurements and CTM grid points. The developed diagnostics make use of dynamical and chemical vertical coordinates as a means of highlighting areas where focused model improvement is needed. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves a unique pathway for rapid transport of surface emissions and pollutants to the stratosphere. Two global CTM configurations are examined for their representation of the ASM upper troposphere and lower stratosphere (UTLS), using airborne observations collected over south Asia. Application of the developed diagnostics to the CTMs reveals the limitations of zonally-averaged surface boundary conditions for species with sufficiently short tropospheric lifetimes, and that species whose stratospheric loss rates are dominated by photolysis have excellent agreement compared to that observed. Overall, the diagnostics demonstrate the strength of airborne observations toward improving model predictions, and highlight the utility of highly-resolved CTMs to improve the understanding of reactive transport of anthropogenic pollutants to the stratosphere.

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s−1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

Halogen atoms affect the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Yet their sources and significances in polluted continental regions are poorly understood. Here we report the observation of unprecedented levels (averaging to 60 parts per trillion) of bromine chloride (BrCl) at a mid-latitude site in North China during winter. Widespread coal burning in rural households and a photo-assisted process were the primary source of BrCl and other bromine gases. BrCl contributed about 55% of both bromine (Br) and chlorine (Cl) atoms. The halogen atoms increased the abundance of ‘conventional’ tropospheric oxidants (OH, HO2, and RO2) by 26-73%, and enhanced oxidation of hydrocarbon by nearly a factor of two and the net ozone production by 55%. Our study reveals the significant role of reactive halogen in winter atmospheric chemistry and the deterioration of air quality in continental regions where uncontrolled coal combustion is prevalent.

In this work we describe the compilation and homogenization of an extensive dataset of aerosol iodine field observations in the period between 1963 and 2018 and we discuss the spatial and temporal dependences of total iodine in bulk aerosol by comparing the observations with CAM-Chem model simulations. Total iodine in aerosol shows a distinct latitudinal dependence, with an enhancement towards the northern hemisphere (NH) tropics and lower values towards the poles. This behavior, which has been predicted by atmospheric models to depend on the global distribution of the main oceanic iodine source (which in turn depends on the reaction of surface ozone with aqueous iodide on the sea water-air interface, generating gas-phase I2 and HOI), is confirmed here by field observations for the first time. Longitudinally, there is some indication of a wave-one profile in the Tropics, which peaks in the Atlantic and shows a minimum in the Pacific, following the wave-one longitudinal variation of tropical tropospheric ozone. New data from Antarctica show that the south polar seasonal variation of iodine in aerosol mirrors that observed previously in the Arctic, with two equinoctial maxima and the dominant maximum occurring in spring. While no clear seasonal variability is observed in NH middle latitudes, there is an indication of different seasonal cycles in the NH tropical Atlantic and Pacific. A weak positive long-term trend is observed in the tropical annual averages, which is consistent with an enhancement of the anthropogenic ozone-driven global oceanic source of iodine over the last 50 years.

We have compiled and analyzed a comprehensive dataset of field observations of iodine speciation in marine aerosol. The soluble iodine content of fine aerosol (PM1) is dominated by soluble organic iodine (SOI) (~50%) and iodide (~30%), while the coarse fraction is dominated by iodate (~50%), with non-negligible amounts of iodide (~20%). The SOI fraction shows an equatorial maximum and minima coinciding with the ocean ‘deserts’, which suggests a link between soluble iodine speciation in aerosol and ocean productivity. Among the mayor aerosol ions, organic anions and non-sea-salt sulfate show positive correlations with SOI in PM1. Alkaline cations are positively correlated to iodate and negatively correlated with SOI and iodide in coarse aerosol. These relationships suggest that under acidic conditions iodate is reduced to HOI, which reacts with organic matter to form SOI, a possible source of iodide. In less acidic sea-salt or dust-rich coarse aerosols, HOI oxidation to iodate and reaction with organic matter likely compete.