The Framework for 0-D Atmospheric Modeling (F0AM) is a flexible and user-friendly MATLAB-based platform for simulation of atmospheric chemistry systems. The F0AM interface incorporates front-end configuration of observational constraints and model setups, making it readily adaptable to simulation of photochemical chambers, Lagrangian plumes, and steady-state or time-evolving solar cycles. Six different chemical mechanisms and three options for calculation of photolysis frequencies are currently available. Example simulations are presented to illustrate model capabilities and, more generally, highlight some of the advantages and challenges of 0-D box modeling.

The chemical link between isoprene and formaldehyde (HCHO) is a strong, nonlinear function of NO_x (i.e., NO + NO₂). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall photochemical mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the southeast US, we quantify HCHO production across the urban–rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NO_x values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv⁻¹), while background HCHO increases by a factor of 2 (from 1.6 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D steady-state box model. Both models reproduce the NO_x dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models underestimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or underestimated hydroxyl radical concentrations. Detailed process rates from the box model simulation demonstrate a 3-fold increase in HCHO production across the range of observed NO_x values, driven by a 100 % increase in OH and a 40 % increase in branching of organic peroxy radical reactions to produce HCHO.

Isoprene is emitted mainly by terrestrial vegetation and is the dominant volatile organic compound (VOC) in Earth’s atmosphere. It plays key roles in determining the oxidizing capacity of the troposphere and the formation of organic aerosol. Daytime infrared satellite observations of isoprene reported here broadly agree with emission inventories, but we found substantial differences in the locations and magnitudes of isoprene hotspots, consistent with a recent study. The corresponding nighttime infrared observations reveal unexpected hotspots over tropical South America, the Congo basin, and Southeast Asia. We used an atmospheric chemistry model to link these nighttime isoprene measurements to low-NO _x regions with high biogenic VOC emissions; at sunrise the remaining isoprene can lead to the production of epoxydiols and subsequently to the widespread seasonal production of organic aerosol in the tropical upper troposphere.

Direct aqueous photolysis of cis-pinonic acid (PA; 2-(3-acetyl-2,2-dimethylcyclobutyl)acetic acid; CAS Registry No. 473-72-3) by 280–400 nm radiation was investigated. The photolysis resulted in Norrish type II isomerization of PA leading to 3-isopropenyl-6-oxoheptanoic acid (CAS Registry No. 4436-82-2), also known as limononic acid, as the major product, confirmed by ¹H and ¹³C NMR analysis, chemical ionization mass spectrometry, and electrospray ionization mass spectrometry. Several minor products resulting from Norrish type I splitting of PA were also detected. The molar extinction coefficients of aqueous PA were measured and used to calculate the photolysis quantum yield of aqueous PA as 0.5 ± 0.3 (effective average value over the 280–400 nm range). The gas-phase photolysis quantum yield of 0.53 ± 0.06 for PA methyl ester (PAMe; CAS Registry No. 16978-11-3) was also measured for comparison. These results indicate that photolysis of PA is not significantly suppressed by the presence of water. This fact was confirmed by photodissociation dynamics simulations of bare PA and of PAMe hydrated with one or five water molecules using on-the-fly dynamics simulations on a semiempirical potential energy surface. The calculations correctly predicted the occurrence of both Norrish type I and Norrish type II photolysis pathways, both driven by the dynamics on the lowest triplet excited state of PA and PAMe. The rate of removal of PA by direct aqueous photolysis in cloudwater and in aerosol water was calculated for a range of solar zenith angles and compared with rates of other removal processes such as gas-phase oxidation by OH, aqueous-phase oxidation by OH, and gas-phase photolysis. Although the direct photolysis mechanism was not the most significant sink for PA in cloud and fog droplets, direct photolysis can be expected to contribute to removal of PA and more soluble/less volatile biogenic oxidation products in wet particulate matter.

Mainland and maritime Southeast Asia is home to more than 655 million people, representing nearly 10 % of the global population. The dry season in this region is typically associated with intense biomass burning activity, which leads to a significant increase in surface air pollutants that are harmful to human health, including ozone (O₃). Latitude-based differences in the dry season and land use distinguish two regional biomass burning regimes: (1) burning on the peninsular mainland peaking in March and (2) burning across Indonesia peaking in September. The type and amount of material burned in each regime impact the emissions of nitrogen oxides (NO_x = NO + NO₂) and volatile organic compounds (VOCs), which combine to produce ozone. Here, we use the nested GEOS-Chem atmospheric chemistry transport model (horizontal resolution of 0.25^∘ × 0.3125^∘), in combination with satellite observations from the Ozone Monitoring Instrument (OMI) and ground-based observations from Malaysia, to investigate ozone photochemistry over Southeast Asia in 2014. Seasonal cycles of tropospheric ozone columns from OMI and GEOS-Chem peak with biomass burning emissions. Compared to OMI, the model has a mean annual bias of −11 % but tends to overestimate tropospheric ozone near areas of seasonal fire activity. We find that outside these burning areas, the underlying photochemical environment is generally NO_x-limited and dominated by anthropogenic NO_x and biogenic non-methane VOC emissions. Pyrogenic emissions of NO_x play a key role in photochemistry, shifting towards more VOC-limited ozone production and contributing about 30 % of the regional ozone formation potential during both biomass burning seasons. Using the GEOS-Chem model, we find that biomass burning activity coincides with widespread ozone exposure at levels that exceed world public health guidelines, resulting in about 260 premature deaths across Southeast Asia in March 2014 and another 160 deaths in September. Despite a positive model bias, hazardous ozone levels are confirmed by surface observations during both burning seasons.