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Measurements of NOx (NO + NO2) and the sum of reactive nitrogen constituents, NOy, were made near the surface at Alert (82.5 deg. N), Canada during March and April 1998. In early March when solar insolation was absent or very low, NOx mixing ratios were frequently near zero. After polar sunrise whenthe sun was above the horizon for much or all of the day a diurnal variation in NOx and NOy was observed with amplitudes as large as 30-40 pptv. The source of active nitrogen is attributed to release from the snow surface by a process that is apparentlysensitized by sunlight. If the source from the snowpack is a large scale feature of the Arctic then the diurnal trends also require a competing process for removal to the surface. From the diurnal change inthe NO/NO2 ratio, mid-April mixing ratios for the sum of peroxy and halogen oxide radicals of 10 pptv were derived for periods when ozone mixing ratios were in the normal range of 30-50 ppbv. Mid-day ozone production and loss rates with the active nitrogen sourcewere estimated to be ~l-2 ppbv/day and in near balance. NOy mixing ratios which averaged only 295 +/- 66 pptv do not support alarge accumulation in the high Arctic surface layer in the winter and spring of 1998. The small abundance of NOy relative to the elevated mixing ratios of other long-lived anthropogenic constituents requires that reactive nitrogen be removed to the surface during transport to or during residence within the high Arctic. [ABSTRACT FROM AUTHOR]

Long-term measurements of light hydrocarbons (C2-C5 HC's) were performed in the course of the EUROTRAC sub-project TOR (Tropospheric Ozone Research) in the southern part of the Black Forest in southwest Germany. The measurements cover a time period of five years (January 1989-January 1994) and the air samples were analyzed onsite by an automated GC-system. Pronounced annual cycles with maxima in late winter and minima in late summer were observed in the case of the slowly reacting hydrocarbons ethane, propane and acetylene,reflecting the fact that the seasonal variation of these species is photochemically driven. For the shorter lived compounds the seasonal variations are considerably weaker, connected with a stronger scatterof the individual measurements, which is caused by different distances to their main sources for different wind vectors as well as by varying source strengths. From a detailed characterization of the hydrocarbon patterns the influence of two different sources could be distinguished. An extrapolation to photochemical age of zero and completionof our data with those from a speciated VOC inventory yields an estimated [VOC]/NOx source ratio for Schauinsland of ?5 [ppbC/ppb]. Comparable [VOC]/NOx ratios are observed in automobile exhaust gases under low speed conditions, which points to the important role of traffic under conditions, when freshly polluted air masses from a near-by city are advected to the site. From an investigation of the photochemical age of the advected air masses it becomes clear that there must exist biogenic sources of light olefins in the vicinity of the observatory during the vegetation period. For propene and the butenes we are able to estimate a lower limit of their contributions in terms of reactivity to the total reactivity (? [HC](i)7 kOH(i), i=C2-C5) of the measured hydrocarbons. For lowest pollution levels in summer (acetylene <300 ppt, about [ABSTRACT FROM AUTHOR]

A new configuration of the Community Earth System Model (CESM)/Community Atmosphere Model with full chemistry (CAM-chem) supporting the capability of horizontal mesh refinement through the use of the spectral element (SE) dynamical core is developed and called CESM/CAM-chem-SE. Horizontal mesh refinement in CESM/CAM-chem-SE is unique and novel in that pollutants such as ozone are accurately represented at human exposure relevant scales while also directly including global feedbacks. CESM/CAM-chem-SE with mesh refinement down to ∼14 km over the conterminous US (CONUS) is the beginning of the Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICAv0). Here, MUSICAv0 is evaluated and used to better understand how horizontal resolution and chemical complexity impact ozone and ozone precursors over CONUS as compared to measurements from five aircraft campaigns, which occurred in 2013. This field campaign analysis demonstrates the importance of using finer horizontal resolution to accurately simulate ozone precursors such as nitrogen oxides and carbon monoxide. In general, the impact of using more complex chemistry on ozone and other oxidation products is more pronounced when using finer horizontal resolution where a larger number of chemical regimes are resolved. Large model biases for ozone near the surface remain in the Southeast US as compared to the aircraft observations even with updated chemistry and finer horizontal resolution. This suggests a need for adding the capability of replacing sections of global emission inventories with regional inventories, increasing the vertical resolution in the planetary boundary layer, and reducing model biases in meteorological variables such as temperature and clouds., (© 2022. The Authors. Journal of Advances in Modeling Earth Systems published by Wiley Periodicals LLC on behalf of American Geophysical Union.)

Measurements of photolysis quantum yields are challenging because of the difficulties in measuring the first-generation photodissociation products, interference from other products or contaminants, sufficient photon fluxes and/or low absorption cross sections of the photolyte to make detectable amounts of products, and quantification of the photon flux. In the case of acetone (and other atmospherically relevant ketones) the uncertainty in the photolysis quantum yield creates uncertainty in the calculated OH radical and acyl peroxy nitrate production in the atmosphere. We present a new method for determining photodissociation product quantum yields by measuring acyl peroxy radicals (RC(O)O 2 ) produced in the photolysis of ketones in air using chemical ionization mass spectrometry (CIMS). We show good agreement of our CIMS method with previously published quantum yields of the acyl radical from photolysis of biacetyl and methyl ethyl ketone (MEK) at 254 nm. Additionally, we highlight the capabilities of this CIMS method through the measurement of photolysis branching ratios for MEK. We suggest future applications of CIMS (in the laboratory and field) to measure RC(O)O 2 and associated photolysis processes.

The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs 405 ) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs., Competing Interests: The authors declare no competing interest.

Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv -1 using a maximum enhancement method, with the median similar to previous observations of temperate forest fire plumes. Measured HONO to NO x enhancement ratios were generally factors of 2, or higher, at early plume ages than previous studies. Enhancement ratios scale with modified combustion efficiency and certain nitrogenous trace gases, which may be useful to estimate HONO release when HONO observations are lacking or plumes have photochemical exposures exceeding an hour as emitted HONO is rapidly photolyzed. We find that HONO photolysis is the dominant contributor to hydrogen oxide radicals (HO x = OH + HO 2 ) in early stage (<3 h) wildfire plume evolution. These results highlight the role of HONO as a major component of reactive nitrogen emissions from wildfires and the main driver of initial photochemical oxidation.

The concentrations of the hydrogen radicals OH and HO2 in the middle and upper troposphere were measured simultaneously with those of NO, O3, CO, H2O, CH4, non-methane hydrocarbons, and with the ultraviolet and visible radiation field. The data allow a direct examination of the processes that produce O3 in this region of the atmosphere. Comparison of the measured concentrations of OH and HO2 with calculations based on their production from water vapor, ozone, and methane demonstrate that these sources are insufficient to explain the observed radical concentrations in the upper troposphere. The photolysis of carbonyl and peroxide compounds transported to this region from the lower troposphere may provide the source of HOx required to sustain the measured abundances of these radical species. The mechanism by which NO affects the production of O3 is also illustrated by the measurements. In the upper tropospheric air masses sampled, the production rate for ozone (determined from the measured concentrations of HO2 and NO) is calculated to be about 1 part per billion by volume each day. This production rate is faster than previously thought and implies that anthropogenic activities that add NO to the upper troposphere, such as biomass burning and aviation, will lead to production of more O3 than expected.