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[1] During the field campaign of the Study of Houston Atmospheric Radical Precursors/ Surface-Induced Oxidation of Organics in the Troposphere (SHARP/SOOT) in Houston, Texas, a suite of aerosol instruments was deployed to directly measure a comprehensive set of aerosol properties, including the particle size distribution, effective density, hygroscopicity, and light extinction and scattering coefficients. Those aerosol properties are employed to quantify the mixing state and composition of ambient particles and to gain a better understanding of the formation and transformation of fine particulate matter in this region. During the measurement period, aerosols are often internally mixed, with one peak in the effective density distribution at 1.55 ± 0.07 g cm À3 , consistent with a population composed largely of sulfates and organics. Episodically, a second mode below 1.0 g cm À3 is identified in the effective density distributions, reflecting the presence of freshly emitted black carbon (BC) particles. The measured effective density demonstrates a clear diurnal cycle associated with primary emissions from transportation and photochemical aging, with a minimum during the morning rush hour, increasing from 1.4 to 1.5 g cm À3 on average over 5 h, and remaining nearly constant throughout the afternoon. The average BC concentration derived from light-absorption measurements is 0.31 ± 0.22 μg m À3 , and the average measured particle single scattering albedo is 0.94 ± 0.04. When elevated BC concentrations are observed, typically during the morning rush hours, single scattering albedo decreases, with a smallest measured value of about 0.7. Aerosol hygroscopicity measurements indicate that larger particles (e.g., 400 nm) are more hygroscopic than smaller particles (e.g., 100 nm). The measurements also reveal discernable meteorological impacts on the aerosol properties. After a frontal passage, the average particle effective density decreases, the average BC concentration increases, and the aerosol size distribution is dominated by new particle formation.

Atmospheric aerosols affect weather and global general circulation by modifying cloud and precipitation processes, but the magnitude of cloud adjustment by aerosols remains poorly quantified and represents the largest uncertainty in estimated forcing of climate change. Here we assess the effects of anthropogenic aerosols on the Pacific storm track, using a multiscale global aerosol-climate model (GCM). Simulations of two aerosol scenarios corresponding to the present day and preindustrial conditions reveal long-range transport of anthropogenic aerosols across the north Pacific and large resulting changes in the aerosol optical depth, cloud droplet number concentration, and cloud and ice water paths. Shortwave and longwave cloud radiative forcing at the top of atmosphere are changed by −2.5 and +1.3 W m −2 , respectively, by emission changes from preindustrial to present day, and an increased cloud top height indicates invigorated midlatitude cyclones. The overall increased precipitation and poleward heat transport reflect intensification of the Pacific storm track by anthropogenic aerosols. Hence, this work provides, for the first time to the authors' knowledge, a global perspective of the effects of Asian pollution outflows from GCMs. Furthermore, our results suggest that the multiscale modeling framework is essential in producing the aerosol invigoration effect of deep convective clouds on a global scale.aerosol-cloud-climate interaction | convective storms, cloud invigoration A tmospheric aerosols, formed from both natural and anthropogenic sources (1, 2), affect the Earth's energy budget directly, by scattering or absorbing solar radiation, and indirectly, by altering cloud microphysical characteristics and regulating the hydrological cycle (3-5). In particular, the aerosols have an indirect effect by serving as cloud condensation nuclei, and their interaction with the microphysics and dynamics of deep convective clouds (DCCs) may modify the cloud structure and redistribute latent heating (6), leading to an enhanced precipitation efficiency (7-12), invigorated convection strength (13-15), and intensified lightning activities (16)(17)(18). At this time, the indirect radiative forcing of anthropogenic aerosols represents the least-understood component in the forcing of climate change (19,20).Increasing levels of particulate matter (PM) pollutants from the Asian continent and their associated outflows have raised considerable concern because of their potential effects on regional climate and global atmospheric circulation (21-23). Intense emissions of anthropogenic aerosols and their long-range transport from Asia are clearly documented by satellite and in situ measurements (21,22). Zhang and colleagues (23) first suggested that Asian pollution likely accounts for a climatically increased DCC amount over the north Pacific on the basis of long-term analysis of cloud measurements from the International Satellite Cloud Climatology Project and high-resolution infrared sounder. In addition, a trend of increasing win...

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Because of the importance of HONO as a radical reservoir, consistent and accurate measurements of its concentration are needed. As part of SHARP (Study of Houston Atmospheric Radical Precursors), time series of HONO were obtained by six different measurement techniques on the roof of the Moody Tower at the University of Houston. Techniques used were long path differential optical absorption spectroscopy (DOAS), stripping coil‐visible absorption photometry (SC‐AP), long path absorption photometry (LOPAP®), mist chamber/ion chromatography (MC‐IC), quantum cascade‐tunable infrared laser differential absorption spectroscopy (QC‐TILDAS), and ion drift‐chemical ionization mass spectrometry (ID‐CIMS). Various combinations of techniques were in operation from 15 April through 31 May 2009. All instruments recorded a similar diurnal pattern of HONO concentrations with higher median and mean values during the night than during the day. Highest values were observed in the final 2 weeks of the campaign. Inlets for the MC‐IC, SC‐AP, and QC‐TILDAS were collocated and agreed most closely with each other based on several measures. Largest differences between pairs of measurements were evident during the day for concentrations < ~100 parts per trillion (ppt). Above ~ 200 ppt, concentrations from the SC‐AP, MC‐IC, and QC‐TILDAS converged to within about 20%, with slightly larger discrepancies when DOAS was considered. During the first 2 weeks, HONO measured by ID‐CIMS agreed with these techniques, but ID‐CIMS reported higher values during the afternoon and evening of the final 4 weeks, possibly from interference from unknown sources. A number of factors, including building related sources, likely affected measured concentrations.

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