Soil microbial nitrous acid (HONO) production has been identified as a significant emission source to the atmosphere and, thus, considerably affects atmospheric oxidizing capacity and formation of secondary air pollutants. Soil HONO emissions are controlled by soil water content, soil temperature, and nitrogen content, which are affected by climatic conditions and fertilizer application rates. However, long-term global soil HONO emissions driven by climate change and increasing fertilizer use have not yet been quantified, hindering the assessment and prediction of the global impact of soil emissions on air quality and vegetation. Here, we derive the first global soil HONO emissions from natural and fertilized lands over the past four decades and evaluate the variation of their impacts on global ozone (O3) and vegetation exposure. Our results show that climate change and the enhanced fertilizer use have increased global soil HONO emissions from 11.0 Tg N in 1980 to 13.4 Tg N in 2016. The rise in fertilizer consumption has substantially intensified soil emissions in the three major grain-producing regions, India, China, and North America with growth rates of 14.9, 9.5, and 0.9 kt N yr-1 , respectively; climate change has predominantly increased soil emissions in Africa and South America with growth rates of 22.3 and 10.9 kt N yr-1 , respectively. Incorporating soil HONO emissions in a global chemistry-climate model improves the model performance in simulating global Page 2 of 22 atmospheric HONO. Simulations with the updated model showed that soil HONO emissions increased global surface O3 concentrations by 2.9% (up to 29%) and the subsequent risk of vegetation exposure to O3, especially in crop-production regions. The enhancement effect of soil HONO emissions on O3 is increased from 1980s to 2016 in regions with low anthropogenic emissions. Therefore, we speculate that with future decreases in anthropogenic emissions, the relative impact of soil HONO emissions on air quality and vegetation is expected to increase. Thus, we recommend the consideration of soil HONO emissions in strategies for mitigating global air pollution.

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.