Publications

<strong class="journal-contentHeaderColor">Abstract.</strong> The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10 s (2 km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> (O<span class="inline-formula">₃</span>, CH<span class="inline-formula">₄</span>, CO, H<span class="inline-formula">₂</span>O, HCHO, H<span class="inline-formula">₂</span>O<span class="inline-formula">₂</span>, CH<span class="inline-formula">₃</span>OOH, C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span>, higher alkanes, alkenes, aromatics, NO<span class="inline-formula">_<i>x</i></span>, HNO<span class="inline-formula">₃</span>, HNO<span class="inline-formula">₄</span>, peroxyacetyl nitrate, other organic nitrates), consisting of 146 494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> photochemical tendencies from this modeling data stream for ATom 1. We find that 80 %–90 % of the total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. In other words, accurate simulation of the least reactive 50 % of the troposphere is unimportant for global budgets. Surprisingly, the probability densities of species and<span id="page13730"/> reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find excellent agreement with the loss of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> but sharp disagreement with production of O<span class="inline-formula">₃</span>. The models sharply underestimate O<span class="inline-formula">₃</span> production below 4 km in both Pacific and Atlantic basins, and this can be traced to lower NO<span class="inline-formula">_<i>x</i></span> levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation.

Atomic chlorine and bromine monoxide concentrations ([Cl] and [BrO]) and dimethyl sulfide (DMS) sea‐air fluxes are estimated from data collected during a Lagrangian flight made near Christmas Island (2°N, 157°W) during August 1996 aboard the NASA P3‐B aircraft. Intensive hydrocarbon sampling began in the surface layer (SL) one‐half hour after sunrise and continued until ∼1300 local solar time. Our empirical model includes in situ observations of hydroxyl [HO] and precise measurements of ethane (C 2 H 6 ) mixing ratios. Ethane was ∼40 pptv higher in the buffer layer (BuL) than in the SL, thus vertical exchange tended to replace any C 2 H 6 that was photochemically removed in the SL. In spite of this, SL C 2 H 6 mixing ratios decreased significantly during the flight. Using only the measured [HO] and estimated vertical mixing, our mass balance equation cannot explain all of the observed SL C 2 H 6 loss. However, when an initial [Cl] of 8.4 (±2.0) × 10 4 Cl cm −3 , decreasing to 5.7 (±2.0) × 10 4 Cl cm −3 at midday, is included, the observed and estimated C 2 H 6 values are in excellent agreement. Using our [Cl], we estimate a DMS flux a factor of 2 higher than when HO is the only oxidant considered. This flux estimate, when compared to that derived by Lenschow et al. (1999), suggests that if the differences are real, we may be missing a loss term. Best agreement occurs when an average BrO mixing ratio of 1.3 (±1.3) pptv is included in our mass balance equation.

Abstract. An extensive set of measurements was made in and around Mexico City as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) experiments in March 2006. Simulations with the Model for Ozone and Related Chemical Tracers, version 4 (MOZART-4), a global chemical transport model, have been used to provide a regional context for these observations and assist in their interpretation. These MOZART-4 simulations reproduce the aircraft observations generally well, but some differences in the modeled volatile organic compounds (VOCs) from the observations result from incorrect VOC speciation assumed for the emission inventories. The different types of CO sources represented in the model have been "tagged" to quantify the contributions of regions outside Mexico, as well as the various emissions sectors within Mexico, to the regional air quality of Mexico. This analysis indicates open fires have some, but not a dominant, impact on the atmospheric composition in the region around Mexico City, when averaged over the month. However, considerable variation in the fire contribution (2–15% of total CO) is seen during the month. The transport and photochemical aging of Mexico City emissions were studied using tags of CO emissions for each day, showing that typically the air near Mexico City was a combination of many ages. Ozone production in MOZART-4 is shown to agree well with the net production rates from box model calculations constrained by the MILAGRO aircraft measurements. Ozone production efficiency derived from the ratio of Ox to NOz is higher in MOZART-4 than in the observations for moderately polluted air. OH reactivity determined from the MOZART-4 results shows the same increase in relative importance of oxygenated VOCs downwind of Mexico City as the reactivity inferred from the observations. The amount of ozone produced by emissions from Mexico City and surrounding areas has been quantified in the model by tracking NO emissions, showing little influence beyond Mexico's borders, and also relatively minor influence from fire emissions on the monthly average tropospheric ozone column.

<strong class="journal-contentHeaderColor">Abstract.</strong> Large wildfires influence regional atmospheric composition, but chemical complexity challenges model predictions of downwind impacts. Here, we elucidate key connections within gas-phase photochemistry and assess novel chemical processes via a case study of the 2013 California Rim Fire plume. Airborne in situ observations, acquired during the NASA Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC<span class="inline-formula">⁴</span>RS) mission, illustrate the evolution of volatile organic compounds (VOCs), oxidants, and reactive nitrogen over 12 h of atmospheric aging. Measurements show rapid formation of ozone and peroxyacyl nitrates (PNs), sustained peroxide production, and prolonged enhancements in oxygenated VOCs and nitrogen oxides (NO<span class="inline-formula">_<i>x</i></span>). Observations and Lagrangian trajectories constrain a 0-D puff model that approximates plume photochemical history and provides a framework for evaluating process interactions. Simulations examine the effects of (1) previously unmeasured reactive VOCs identified in recent laboratory studies and (2) emissions and secondary production of nitrous acid (HONO). Inclusion of estimated unmeasured VOCs leads to a 250 % increase in OH reactivity and a 70 % increase in radical production via oxygenated VOC photolysis. HONO amplifies radical cycling and serves as a downwind NO<span class="inline-formula">_<i>x</i></span> source, although impacts depend on how HONO is introduced. The addition of initial HONO (representing primary emissions) or particulate nitrate photolysis amplifies ozone production, while heterogeneous conversion of NO<span class="inline-formula">₂</span> suppresses ozone formation. Analysis of radical initiation rates suggests that oxygenated VOC photolysis is a major radical source, exceeding HONO photolysis when averaged over the first 2 h of aging. Ozone production chemistry transitions from VOC sensitive to NO<span class="inline-formula">_<i>x</i></span> sensitive within the first hour of plume aging, with both peroxide and organic nitrate formation contributing significantly to radical termination. To simulate smoke plume chemistry accurately, models should simultaneously account for the full reactive VOC pool and all relevant oxidant sources.

There is a poor understanding of how the physiology of polymicrobial communities in cystic fibrosis (CF) lungs contributes to pulmonary exacerbations and lung function decline. In this study, a microbial culture system based on the principles of the Winogradsky column (WinCF system) was developed to study the physiology of CF microbes. The system used glass capillary tubes filled with artificial sputum medium to mimic a clogged airway bronchiole. Chemical indicators were added to observe microbial physiology within the tubes. Characterization of sputum samples from seven patients showed variation in pH, respiration, biofilm formation and gas production, indicating that the physiology of CF microbial communities varied among patients. Incubation of homogenized tissues from an explant CF lung mirrored responses of a Pseudomonas aeruginosa pure culture, supporting evidence that end-stage lungs are dominated by this pathogen. Longitudinal sputum samples taken through two exacerbation events in a single patient showed that a two-unit drop in pH and a 30% increase in gas production occurred in the tubes prior to exacerbation, which was reversed with antibiotic treatment. Microbial community profiles obtained through amplification and sequencing of the 16S rRNA gene showed that fermentative anaerobes became more abundant during exacerbation and were then reduced during treatment where P. aeruginosa became the dominant bacterium. Results from the WinCF experiments support the model where two functionally different CF microbial communities exist, the persistent Climax Community and the acute Attack Community. Fermentative anaerobes are hypothesized to be the core members of the Attack Community and production of acidic and gaseous products from fermentation may drive developing exacerbations. Treatment targeting the Attack Community may better resolve exacerbations and resulting lung damage.

The seasonal variation of ozone (O 3 ) in the boundary layer (BL) over the western Pacific is investigated using a chemistry‐transport model. The model results for January and April–May 2002 were evaluated by comparison with PEACE aircraft observations. In January, strong northwesterlies efficiently transported NO x from the continent, leading to an O 3 increase of approximately 5–10 ppbv over a distance of about 3000 km. In April, southwesterlies dominated due to anticyclone development over the western Pacific. Along this flow, O 3 continued to be produced by NO x emitted from East Asia. This resulted in the formation of a high‐O 3 (&gt; 50 ppbv) region extending along the coastal areas of East Asia. This seasonal change in O 3 was driven in part by a change in the net O 3 production rate due to increases in solar UV and H 2 O. Its exact response depended on the NO x values in the BL. The net O 3 production rate increased between winter and spring over the Asian continent and decreased over the remote western Pacific. Model simulations show that about 25% of the total O 3 (of 10–20 ppbv) increase over the coastal region of Northeast Asia was due to local production from NO x emissions from China, and the rest was due to changes in background levels as well as emissions from Korea, Japan, and east Siberia. Uplift of BL air over Asia, horizontal transport in the free troposphere, and subsidence were the principal mechanisms of transporting Asian O 3 to the central and eastern North Pacific.