Abstract. In March 2006 two instrumented aircraft made the first detailed field measurements of biomass burning (BB) emissions in the Northern Hemisphere tropics as part of the MILAGRO project. The aircraft were the National Center for Atmospheric Research C-130 and a University of Montana/US Forest Service Twin Otter. The initial emissions of up to 49 trace gas or particle species were measured from 20 deforestation and crop residue fires on the Yucatan peninsula. This included two trace gases useful as indicators of BB (HCN and acetonitrile) and several rarely, or never before, measured species: OH, peroxyacetic acid, propanoic acid, hydrogen peroxide, methane sulfonic acid, and sulfuric acid. Crop residue fires emitted more organic acids and ammonia than deforestation fires, but the emissions from the main fire types were otherwise fairly similar. The Yucatan firesCorrespondence to: R. J. Yokelson (bob.yokelson@umontana.edu) emitted unusually high amounts of SO 2 and particle chloride, likely due to a strong marine influence on this peninsula. As smoke from one fire aged, the ratio O 3 / CO increased to ∼15% in <∼1 h similar to the fast net production of O 3 in BB plumes observed earlier in Africa. The rapid change in O 3 occurs at a finer spatial scale than is employed in global models and is also faster than predicted by microscale models. Fast increases in PAN, H 2 O 2 , and two organic acids were also observed. The amount of secondary organic acid is larger than the amount of known precursors. Rapid secondary formation of organic and inorganic aerosol was observed with the ratio PM 2.5 / CO more than doubling in ∼1.4±0.7 h. The OH measurements revealed high initial levels (>1×10 7 molecules/cm 3 ) that were likely caused in part by high initial HONO (∼10% of NO y ). Thus, more research is needed to understand critical post emission processes for the second-largest trace gas source on Earth. It is estimated that ∼44 Tg of biomass burned in the Yucatan in the spring Published by Copernicus Publications on behalf of the European Geosciences Union.

Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO ≡ NO + NO) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEACRS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NO from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEACRS observations of NO and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO columns. Our results indicate that NEI NO emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO emissions. Upper tropospheric NO from lightning makes a large contribution to satellite observations of tropospheric NO that must be accounted for when using these data to estimate surface NO emissions. We find that only half of isoprene oxidation proceeds by the high-NO pathway to produce ozone; this fraction is only moderately sensitive to changes in NO emissions because isoprene and NO emissions are spatially segregated. GEOS-Chem with reduced NO emissions provides an unbiased simulation of ozone observations from the aircraft, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NO oxidation products. However, the model is still biased high by 8±13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

Nitrogen oxides are essential for the formation of secondary atmospheric aerosols and of atmospheric oxidants such as ozone and the hydroxyl radical, which controls the self-cleansing capacity of the atmosphere. Nitric acid, a major oxidation product of nitrogen oxides, has traditionally been considered to be a permanent sink of nitrogen oxides. However, model studies predict higher ratios of nitric acid to nitrogen oxides in the troposphere than are observed. A 'renoxification' process that recycles nitric acid into nitrogen oxides has been proposed to reconcile observations with model studies, but the mechanisms responsible for this process remain uncertain. Here we present data from an aircraft measurement campaign over the North Atlantic Ocean and find evidence for rapid recycling of nitric acid to nitrous acid and nitrogen oxides in the clean marine boundary layer via particulate nitrate photolysis. Laboratory experiments further demonstrate the photolysis of particulate nitrate collected on filters at a rate more than two orders of magnitude greater than that of gaseous nitric acid, with nitrous acid as the main product. Box model calculations based on the Master Chemical Mechanism suggest that particulate nitrate photolysis mainly sustains the observed levels of nitrous acid and nitrogen oxides at midday under typical marine boundary layer conditions. Given that oceans account for more than 70 per cent of Earth's surface, we propose that particulate nitrate photolysis could be a substantial tropospheric nitrogen oxide source. Recycling of nitrogen oxides in remote oceanic regions with minimal direct nitrogen oxide emissions could increase the formation of tropospheric oxidants and secondary atmospheric aerosols on a global scale.

Abstract. The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during the second phase of Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign, which was focused on the Asian pollution outflow over Pacific Ocean and was based in Hawaii and Alaska. The OH reactivity was measured by adding OH, generated by photolyzing water vapor with 185 nm UV light in a moveable wand, to the flow of ambient air in a flow tube and measuring the OH signal with laser induced fluorescence. As the wand was pulled back away from the OH detector, the OH signal decay was recorded; the slope of − ln(signal)/ time was the OH reactivity. The overall absolute uncertainty at the 2σ confidence levels is about 1 s −1 at low altitudes (for decay about 6 s −1 ), and 0.7 s −1 at high altitudes (for decay about 2 s −1 ). From the median vertical profile obtained in the second phase of INTEX-B, the measured OH reactivity (4.0±1.0 s −1 ) is higher than the OH reactivCorrespondence to: J. Mao (mao@fas.harvard.edu) ity calculated from assuming that OH was in steady state (3.3±0.8 s −1 ), and even higher than the OH reactivity that was calculated from the total measurements of all OH reactants (1.6±0.4 s −1 ). Model calculations show that the missing OH reactivity is consistent with the over-predicted OH and under-predicted HCHO in the boundary layer and lower troposphere. The over-predicted OH and under-predicted HCHO suggest that the missing OH sinks are most likely related to some highly reactive VOCs that have HCHO as an oxidation product.

[1] Tropospheric O 3 concentrations are functions of the chain lengths of NO x (NO x NO + NO 2 ) and HO x (HO x OH + HO 2 + RO 2 ) radical catalytic cycles. For a fixed HO x source at low NO x concentrations, kinetic models indicate the rate of O 3 production increases linearly with increases in NO x concentrations (NO x limited). At higher NO x concentrations, kinetic models predict ozone production rates decrease with increasing NO x (NO x saturated). We present observations of NO, NO 2 , O 3 , OH, HO 2 , H 2 CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (P O3 ) as a function of NO abundances and the primary HO x production rate (P HOx ). These observations provide quantitative evidence for the response of P O3 to NO x . For high P HOx (0.5 < P HOx < 0.7 ppt/s), O 3 production at this site increases linearly with NO to $500 ppt. P O3 levels out in the range 500-1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently P O3 , have a large error in the rate or product yield of the RO 2 + HO 2 reaction for the classes of RO 2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO 2 + HO 2 ! ROOH + O 2 , is reduced by a factor of 3-12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HO x reservoir. This result implies that O 3 production in or near urban areas with similar hydrocarbon reactivity and HO x production rates may be NO x saturated more often than current models suggest.