Enhanced wintertime oxidation of VOCs via sustained radical sources in the urban atmosphere.

Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO2) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants. We present a set of observations of ClNO2 and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO2 is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO2 often persists through the daylight hours at mixing ratios above 10-20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO (>65 ppt) provide a strong source of OH radicals throughout the day. The combined effects of ClNO2 and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO2 and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models.

Marine iodine emissions in a changing world.

Iodine is a critical trace element involved in many diverse and important processes in the Earth system. The importance of iodine for human health has been known for over a century, with low iodine in the diet being linked to goitre, cretinism and neonatal death. Research over the last few decades has shown that iodine has significant impacts on tropospheric photochemistry, ultimately impacting climate by reducing the radiative forcing of ozone (O3) and air quality by reducing extreme O3 concentrations in polluted regions. Iodine is naturally present in the ocean, predominantly as aqueous iodide and iodate. The rapid reaction of sea-surface iodide with O3 is believed to be the largest single source of gaseous iodine to the atmosphere. Due to increased anthropogenic O3, this release of iodine is believed to have increased dramatically over the twentieth century, by as much as a factor of 3. Uncertainties in the marine iodine distribution and global cycle are, however, major constraints in the effective prediction of how the emissions of iodine and its biogeochemical cycle may change in the future or have changed in the past. Here, we present a synthesis of recent results by our team and others which bring a fresh perspective to understanding the global iodine biogeochemical cycle. In particular, we suggest that future climate-induced oceanographic changes could result in a significant change in aqueous iodide concentrations in the surface ocean, with implications for atmospheric air quality and climate.

Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions.

The influence of organic compounds on iodine (I2) emissions from the O3 + I- reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I2 was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I2 emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I2 emissions compared to artificial solutions with equivalent [I-], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I2 or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I2 product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O3 + I- reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.