Ozone is a pollutant of major concern because of its well recognised effects upon human health and crop yields. This study analyses in depth a new dataset for ozone from Jeddah, a coastal city in Saudi Arabia within the Middle Eastern region, for which very few ozone data are currently available, collected between March 2012 and February 2013. The measurements presented include NO, NO 2 and ozone as well as relevant meteorological variables. The data show a marked seasonal variation in ozone with highest concentrations in the summer months and lowest average concentrations in the winter. Concentrations also show a substantial difference between weekdays and weekends, with higher NO and NO 2 on weekdays, but lower concentrations of ozone. Plots of total oxidant versus NO x concentration indicate background concentrations of ozone (at zero NO x ) ranging from 38.2 ppb in January to 59 ppb in May consistent with the northern hemisphere spring maximum in ozone concentrations. The slope of total oxidant/NO x varies from 0.13 in March to 0.68 in August. The two summer months of July and August are anomalous with slopes of around double that of other months, suggesting a higher efficiency of ozone production at lower primary pollutant concentrations arising from much reduced daytime traffic.A substantial weekend/weekday difference in ozone which is higher at weekends appears to be attributable to lower daytime traffic activity and hence reduced emissions of NO x to a "NO x -saturated" atmosphere.

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Abstract. Atmospheric aerosols have significant effects on human health and the climate. A large fraction of these aerosols originates from secondary new particle formation (NPF), where atmospheric vapors form small particles that subsequently grow into larger sizes. In this study, we characterize NPF events observed at a rural background site of Hada Al Sham (21.802∘ N, 39.729∘ E), located in western Saudi Arabia, during the years 2013–2015. Our analysis shows that NPF events occur very frequently at the site, as 73 % of all the 454 classified days were NPF days. The high NPF frequency is likely explained by the typically prevailing conditions of clear skies and high solar radiation, in combination with sufficient amounts of precursor vapors for particle formation and growth. Several factors suggest that in Hada Al Sham these precursor vapors are related to the transport of anthropogenic emissions from the coastal urban and industrial areas. The median particle formation and growth rates for the NPF days were 8.7 cm−3 s−1 (J7 nm) and 7.4 nm h−1 (GR7−12 nm), respectively, both showing highest values during late summer. Interestingly, the formation and growth rates increase as a function of the condensation sink, likely reflecting the common anthropogenic sources of NPF precursor vapors and primary particles affecting the condensation sink. A total of 76 % of the NPF days showed an unusual progression, where the observed diameter of the newly formed particle mode started to decrease after the growth phase. In comparison to most long-term measurements, the NPF events in Hada Al Sham are exceptionally frequent and strong both in terms of formation and growth rates. In addition, the frequency of the decreasing mode diameter events is higher than anywhere else in the world.

We investigated the temporal variation of PM and aerosol particle number concentration (TC, D p 0.25-32 µm) during year 2012 in Jeddah city. We focused on the daily patterns, wind sector analysis, and the effect of ambient conditions. The daily median values of the PM 10 rarely exceeded 200 µg/m 3 . The PM 2.5 and PM 1 concentrations showed similar temporal variation with fraction of 39.5% and 19.3% of the PM 10 , respectively. The daily median values of the TC were generally below 500 cm -3 but showed three distinguished peaks (as high as 950 cm -3 ) occurred during late September-early November, which was during the time of the Hajj season. These peaks were also observable in the PM 2.5 and PM 1 . The PM 10 daily pattern on workdays and Thursdays were similar with a pronounced peak as high as 90 µg/m 3 during the morning time. The PM 2.5 and PM 1 daily patterns were also similar to those of the PM 10 , especially on Thursdays and workdays, but with lower concentrations. The TC daily pattern was almost the same on all days showing high concentrations (between 150 and 200 cm -3 ) during the first half of the day. The daily patterns of the TC were similar to those of the PM 1 , especially on Workdays and Fridays. The emissions from the industrial city (located in the south of Jeddah) are the main source of PM in Jeddah. During the sea breeze that occurred in the afternoon accompanied low concentrations of PM and TC concentrations. The PM concentrations showed the famous U-shape with respect to the wind speed, whereas the TC was around 150 cm -3 when the wind speed was over 3 m/s. In general, the TC had a different behaviour with respect to the other ambient conditions than that of the PM concentrations, especially PM 10 .

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