Aerosols impact climate, human health, and the chemistry of the atmosphere, and aerosol pH plays a major role in the physicochemical properties of the aerosol. However, there remains uncertainty as to whether aerosols are acidic, neutral, or basic. In this research, we show that the pH of freshly emitted (nascent) sea spray aerosols is significantly lower than that of sea water (approximately four pH units, with pH being a log scale value) and that smaller aerosol particles below 1 μm in diameter have pH values that are even lower. These measurements of nascent sea spray aerosol pH, performed in a unique ocean−atmosphere facility, provide convincing data to show that acidification occurs “across the interface” within minutes, when aerosols formed from ocean surface waters become airborne. We also show there is a correlation between aerosol acidity and dissolved carbon dioxide but no correlation with marine biology within the seawater. We discuss the mechanisms and contributing factors to this acidity and its implications on atmospheric chemistry.

Nitrous acid (HONO), a highly reactive trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its sources and sinks, especially in the marine boundary layer (MBL). Herein, we have investigated HONO formation from the irradiation of nitrate solutions in the presence of increasingly complex photosensitizers including marine dissolved organic matter (m-DOM), which contains chromophoric organic matter, collected from a large-scale mesocosm experiment. In particular, aqueous nitrate solutions in the presence of m-DOM, humic acid (HA), and 4-benzoylbenzoic acid (4-BBA) as well as ethylene glycol (EG) were irradiated with a solar simulator. Gas-phase HONO and NO 2 produced during the irradiation of these samples were detected using incoherent broad band cavity enhanced absorption spectroscopy (IBBCEAS). The relative amounts of HONO and NO 2 formation varied for the different samples. The addition of all of these different organic containing samples (m-DOM, HA, 4-BBA, and EG) to nitrate solutions caused an enhancement in HONO formation, with m-DOM showing the greatest total amount over a 6 h time period. Mechanisms for this enhancement are discussed as well as the strong pH dependence, with the greatest amount of HONO at a low pH. Overall, HONO formation from nitrate photolysis in the presence of m-DOM provides insights into the HONO formation pathway in the MBL and ultimately contributes to improving atmospheric models.

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO2 and gaseous HNO3 followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO2 and HNO3 are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO2 near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO2 is predicted to be the dominant HONO generation channel, where NO2 is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.