Abstract. Sun-lit snow is increasingly recognized as a chemical reactor that plays an active role in uptake, transformation, and release of atmospheric trace gases. Snow is known to influence boundary layer air on a local scale, and given the large global surface coverage of snow may also be significant on regional and global scales. We present a new detailed one-dimensional snow chemistry module that has been coupled to the 1-D atmospheric boundary layer model MISTRA. The new 1-D snow module, which is dynamically coupled to the overlaying atmospheric model, includes heat transport in the snowpack, molecular diffusion, and wind pumping of gases in the interstitial air. The model includes gas phase chemical reactions both in the interstitial air and the atmosphere. Heterogeneous and multiphase chemistry on atmospheric aerosol is considered explicitly. The chemical interaction of interstitial air with snow grains is simulated assuming chemistry in a liquid-like layer (LLL) on the grain surface. The coupled model, referred to as MISTRA-SNOW, was used to investigate snow as the source of nitrogen oxides (NOx) and gas phase reactive bromine in the atmospheric boundary layer in the remote snow covered Arctic (over the Greenland ice sheet) as well as to investigate the link between halogen cycling and ozone depletion that has been observed in interstitial air. The model is validated using data taken 10 June–13 June, 2008 as part of the Greenland Summit Halogen-HOx experiment (GSHOX). The model predicts that reactions involving bromide and nitrate impurities in the surface snow can sustain atmospheric NO and BrO mixing ratios measured at Summit, Greenland during this period.

[1] OH and HO 2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO 2 was generally 1.20 of modeled HO 2 . This observed-to-modeled comparison is similar to that for TRACE-P, another midlatitude study for which the median observed-to-modeled ratio was 1.08 for OH and 1.34 for HO 2 , and to that for PEM-TB, a tropical study for which the median observed-tomodeled ratio was 1.17 for OH and 0.97 for HO 2 . HO 2 behavior above 8 km was markedly different. The observed-to-modeled HO 2 ratio increased from $1.2 at 8 km to $3 at 11 km with the observed-to-modeled ratio correlating with NO. Above 8 km, the observed-to-modeled HO 2 and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO 2 /OH, which is sensitive to cycling reactions between OH and HO 2 , increased from $1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HO x source and additional reactants that cycle HO x from OH to HO 2 . In the continental planetary boundary layer, the observed-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HO x sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HO x sources.

The adsorption of nitric acid (HNO 3 ) from a flowing gas stream is studied for a variety of wall materials to determine their suitability for use in atmospheric sampling instruments. Parts per billion level mixtures of HNO 3 in synthetic air flow through tubes of different materials such that >80% of the molecules interact with the walls. A chemical ionization mass spectrometer with a fast time response and high sensitivity detects HNO 3 that is not adsorbed on the tube walls. Less than 5% of available HNO 3 is adsorbed on Teflon fluoropolymer tubing after 1 min of HNO 3 exposure, whereas >70% is lost on walls made of stainless steel, glass, fused silica, aluminum, nylon, silica-steel, and silanecoated glass. Glass tubes exposed to HNO 3 on the order of hours passivate with HNO 3 adsorption dropping to zero. The adsorption of HNO 3 on PFA Teflon tubing (PFA) is nearly temperature-independent from 10 to 80 °C, but below -10 °C nearly all HNO 3 that interacts with PFA is reversibly adsorbed. In ambient and synthetic air, humidity increases HNO 3 adsorption. The results suggest that Teflon at temperatures above 10 °C is an optimal choice for inlet surfaces used for in situ measurements of HNO 3 in the ambient atmosphere.

[1] A chemical ionization mass spectrometer (CIMS) utilizing protonated acetone dimer ion chemistry to measure gas-phase ammonia (NH 3 ) from the NOAA WP-3D aircraft is described. The average sensitivity determined from in-flight standard addition calibrations ranged from 2.6 to 5 ion counts s À1 pptv À1 , depending on flow conditions, for 1 MHz of reagent ion signal. The instrument time response was determined to be 5 s from the 2 e-folding signal decay time after removal of a standard addition calibration. The instrumental background varied from flight to flight ranging from 0.5 to 1.3 ppbv. The variability between successive background measurements ranged from 50 pptv to 100 pptv. Total uncertainty for the 5 s data was conservatively estimated to be ±(30% + 125 pptv). Two NH 3 sources were sampled during the New England Air Quality Study-Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) 2004 campaign, one urban and one agricultural. During the 25 July flight, enhancements in NH 3 mixing ratios were coincident with enhancements in CO, NOx, and SO 2 mixing ratios downwind of New York City. The NH 3 mixing ratios in the urban outflow plume ranged from 0.4 to 1 ppbv, or enhancements of 0.2 to 0.8 ppbv above local background. During the 15 August flight, NH 3 mixing ratios were enhanced 0.3 to 0.45 ppbv above local background directly downwind of an agricultural area northeast of Atlanta, Georgia. The NH 3 CIMS instrument has shown the ability to measure sub-ppbv NH 3 levels at high time resolution from an aircraft.

[1] We present hygroscopic and cloud condensation nuclei (CCN) relevant properties of the water-soluble fraction of Mexico City aerosol collected upon filters during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign. Application of -Köhler theory to the observed CCN activity gave a fairly constant hygroscopicity parameter ( = 0.28 ± 0.06) regardless of location and organic fraction. Köhler theory analysis was used to understand this invariance by separating the molar volume and surfactant contributions to the CCN activity. Organics were found to depress surface tension (10-15%) from that of pure water. Daytime samples exhibited lower molar mass (∼200 amu) and surface tension depression than nighttime samples (∼400 amu); this is consistent with fresh hygroscopic secondary organic aerosol (SOA) condensing onto particles during peak photochemical hours, subsequently aging during nighttime periods of high relative humidity. Changes in surface tension partially compensate for shifts in average molar volume to give the constant hygroscopicity observed, which implies the amount (volume fraction) of soluble material in the parent aerosol is the key composition parameter required for CCN predictions. This finding, if applicable elsewhere, may explain why CCN predictions are often found to be insensitive to assumptions of chemical composition and provides a very simple way to parameterize organic hygroscopicity in atmospheric models (i.e., org = 0.28" WSOC ). Special care should be given, however, to surface tension depression from organic surfactants, as its nonlinear dependence with organic fraction may introduce biases in observed (and predicted) hygroscopicity. Finally, threshold droplet growth analysis suggests the water-soluble organics do not affect activation kinetics.