Nitrate anion (NO3ˉ) is ubiquitous in the environment and its photochemistry produces nitrous acid (HONO), a major source of tropospheric hydroxyl radical (OH). Enhanced HONO(g) emissions have been observed from NO3ˉ(aq) photolysis in field studies, although the underlying reasons for this enhancement are debated. Here, we show that the enhancement is induced by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Increased yields of superoxide radical (O2ˉ) and HONO were observed when NO3ˉ solutions (pH 6) were photolyzed in the presence of DAOM surrogates of varying solubility.An additional experiment titrated with additional DAOM showed a further increase in O2ˉ(aq) and HONO(g) simultaneously with decreased yields of gaseous nitric oxide (NO) and nitrogen dioxide (NO2). To our knowledge, this is the first time that superoxide was directly observed as an intermediate in nitrate photolysis experiments, produced through DOAM oxidation by OH(aq).Herein, we suggest that enhanced HONO(g) emissions from NO3ˉ(aq) photolysis result from the reaction of O2ˉ(aq) with NO2(aq) and NO(aq) to respectively form peroxynitrate (OONO2ˉ) and peroxynitrite (OONOˉ), which are precursors to nitrite (NO2ˉ). Overall, this points to an important role of O2ˉ in aqueous aerosol chemistry, which is currently under-appreciated.

Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above‐canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above‐canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state‐of‐science atmospheric model (GEOS‐Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below‐canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH.

Reactive nitrogen oxides (NOy; NOy= NO + NO2+ HONO) decrease air quality and impact radiative forcing, yet the factors responsible for their emission from nonpoint sources (i.e., soils) remain poorly understood. We investigated the factors that control the production of aerobic NOyin forest soils using molecular techniques, process-based assays, and inhibitor experiments. We subsequently used these data to identify hotspots for gas emissions across forests of the eastern United States. Here, we show that nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil chemical characteristics (pH and C:N), and nitrogen (N) transformation rates (net nitrification). We find that, while nitrification rates are controlled primarily by chemoautotrophic ammonia-oxidizing archaea (AOA), the production of NOyis mediated in large part by chemoautotrophic ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in forest communities, as stands dominated by arbuscular mycorrhizal (AM) trees had greater N transformation rates and NOyfluxes than stands dominated by ectomycorrhizal (ECM) trees. Given mapped distributions of AM and ECM trees from 78,000 forest inventory plots, we estimate that broadleaf forests of the Midwest and the eastern United States as well as the Mississippi River corridor may be considered hotspots of biogenic NOyemissions. Together, our results greatly improve our understanding of NOyfluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.

Volatile nitrogen oxides (N2O, NO, NO2, HONO, …) can negatively impact climate, air quality, and human health. Using soils collected from temperate forests across the eastern United States, we show microbial communities involved in nitrogen (N) cycling are structured, in large part, by the composition of overstory trees, leading to predictable N‐cycling syndromes, with consequences for emissions of volatile nitrogen oxides to air. Trees associating with arbuscular mycorrhizal (AM) fungi promote soil microbial communities with higher N‐cycle potential and activity, relative to microbial communities in soils dominated by trees associating with ectomycorrhizal (ECM) fungi. Metagenomic analysis and gene expression studies reveal a 5 and 3.5 times greater estimated N‐cycle gene and transcript copy numbers, respectively, in AM relative to ECM soil. Furthermore, we observe a 60% linear decrease in volatile reactive nitrogen gas flux (NOy ≡ NO, NO2, HONO) as ECM tree abundance increases. Compared to oxic conditions, gas flux potential of N2O and NO increase significantly under anoxic conditions for AM soil (30‐ and 120‐fold increase), but not ECM soil—likely owing to small concentrations of available substrate (NO3‐) in ECM soil. Linear mixed effects modeling shows that ECM tree abundance, microbial process rates, and geographic location are primarily responsible for variation in peak potential NOy flux. Given that nearly all tree species associate with either AM or ECM fungi, our results indicate that the consequences of tree species shifts associated with global change may have predictable consequences for soil N cycling.

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