Investigation and modeling of diurnal variation in suburban ambient formaldehyde concentration

Formaldehyde (HCHO) is a naturally occurring compound found in ambient air which can induce cancer and sick-building syndrome. It plays an important role in the formation of OH radicals, which are connected to the formation of various airborne chemicals. Herein, we present a simple modeling for the simulation of diurnal variations in the HCHO concentration of ambient air. This was achieved using data collected during different seasons from November 2015 to March 2017 at a suburban location in Toyama City (Japan), where non-methane hydrocarbon (NMHC) levels were low at sub carbon ppm (ppmC) order. The modeling was based on the assumption that photochemical reactions of methane were the major factor of secondary HCHO formation. The model took into account the production and decomposition of HCHO by photochemical reactions as well as its loss due to other reactions such as dry deposition. Accordingly, the model's equation contained terms for solar radiation, temperature, and methane concentration. The results predicted using the model showed good agreement with the experimental data observed on fine days, i.e., except rainy, foggy, and heavily cloudy days. The relationships between HCHO concentration and solar radiation/temperature on different days as well as the seasonal variation of HCHO concentration were also interpreted by the proposed model. This study contributes to the evaluation of the pollution levels of formaldehyde. Moreover, the model may be used to demonstrate the impact of increasing methane levels, with regard to global warming and the background levels of HCHO in the atmosphere.

Mapping hydroxyl variability throughout the global remote troposphere via synthesis of airborne and satellite formaldehyde observations

The hydroxyl radical (OH) fuels tropospheric ozone production and governs the lifetime of methane and many other gases. Existing methods to quantify global OH are limited to annual and global-to-hemispheric averages. Finer resolution is essential for isolating model deficiencies and building process-level understanding. In situ observations from the Atmospheric Tomography (ATom) mission demonstrate that remote tropospheric OH is tightly coupled to the production and loss of formaldehyde (HCHO), a major hydrocarbon oxidation product. Synthesis of this relationship with satellite-based HCHO retrievals and model-derived HCHO loss frequencies yields a map of total-column OH abundance throughout the remote troposphere (up to 70% of tropospheric mass) over the first two ATom missions (August 2016 and February 2017). This dataset offers unique insights on near-global oxidizing capacity. OH exhibits significant seasonality within individual hemispheres, but the domain mean concentration is nearly identical for both seasons (1.03 ± 0.25 × 106 cm-3), and the biseasonal average North/South Hemisphere ratio is 0.89 ± 0.06, consistent with a balance of OH sources and sinks across the remote troposphere. Regional phenomena are also highlighted, such as a 10-fold OH depression in the Tropical West Pacific and enhancements in the East Pacific and South Atlantic. This method is complementary to budget-based global OH constraints and can help elucidate the spatial and temporal variability of OH production and methane loss.

Formaldehyde in the Tropical Western Pacific: Chemical sources and sinks, convective transport, and representation in CAM-Chem and the CCMI models

Formaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HO_x. In remote marine environments, such as the Tropical Western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here, we have used observations from the CONTRAST field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 pptv near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM-Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry-Climate Model Initiative (free-running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NO_x emissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere.

Formaldehyde in the Tropical Western Pacific: Chemical sources and sinks, convective transport, and representation in CAM-Chem and the CCMI models

Formaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HOx. In remote marine environments, such as the Tropical Western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here, we have used observations from the CONTRAST field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 pptv near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM-Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry-Climate Model Initiative (free-running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NOx emissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere.

Summertime ambient formaldehyde in five U.S. metropolitan areas: Nashville, Atlanta, Houston, Philadelphia, and Tampa

First, we briefly review the atmospheric chemistry and previous intercomparison measurements for HCHO, with special reference to the diffusion scrubber Hantzsch reaction based fluorescence instrument used in the field studies reported herein. Then we discuss summertime HCHO levels in five major U.S. cities measured over 1999-2002, primarily from ground-based measurements. Land-sea breeze circulations play a major role in observed concentrations in coastal cities. Very high HCHO peak mixing ratios were observed in Houston (>47 ppb) where the overall median mixing ratio was 3.3 ppb; the corresponding values in Atlanta were approximately >18 and 7.9 ppb, respectively. The peak and median mixing ratios (9.3 and 2.3 ppb) were the lowest for Tampa, where the land-sea breeze also played an important role. In several cities, replicate HCHO measurements were made by direct spectroscopic instruments; the instruments were located kilometers from each other and addressed very different heights (e.g., 106 vs 10 m). Even under these conditions, there was remarkable qualitative and often quantitative agreement between the different instruments, when they were all sampling the same air mass within a short period of each other. Local chemistry dominates how HCHO is formed and dissipated. The high concentrations in Houston resulted from emissions near the ship channel; the same formaldehyde plume was measured at two sites and clearly ranged over tens of kilometers. Local micrometeorology is another factor. HCHO patterns measured at a high-rise site in downtown Nashville were very much in synchrony with other ground sites 12 km away until July 4 celebrations whence HCHO concentrations at the downtown site remained elevated for several days and nights. The formation and dissipation of HCHO in the different cities are discussed in terms of other concurrently measured species and meteorological vectors. The vertical profiles of HCHO in and around Tampa under several different atmospheric conditions are presented. The extensive data set represented in this paper underscores that urban HCHO measurements can now be made easily; the agreement between disparate instruments (that are independently calibrated or rely on the absolute absorption cross section) further indicates that such measurements can be done reliably and accurately for this very important atmospheric species. The data set presented here can be used as a benchmark for future measurements if the use of formaldehyde precursors such as methanol or methyl tert-butyl ether (MTBE) as oxygenated fuel additives increases in the future.