UNDERSTANDING THE LONG - TERM EFFECTS OF 1 ENVIRONMENTAL EXPOSURE ON ROOF REFLECTANCE IN 2 CALIFORNIA 3 4 Meng -

10 Cool-pigmented roofing systems could play an important role in the reduction of energy 11 consumption. Cool roofs are those that show high reflectance of sunlight and are designed to 12 retain the ability over time. Airborne particulate matter that settles on a roof can either reflect or 13 absorb incoming solar radiation, dependent on the chemical content and size of the particles 14 thereby affecting the ability of a roof to cool. These light scattering and absorption processes 15 occur within a few microns of the surface, and can affect the solar reflectance of the roof. 16 Contaminants collected from samples of roof products (i.e., coupons) exposed at seven 17 California weathering sites were analyzed for elements and carbons to characterize the chemical 18 profile of the particles soiling each roof sample and to identify those elements that degrade or 19 enhance solar reflectance. The losses in solar reflectance varied from site to site and also varied 20 at a give site based on the color of the coupon. The least drop in reflectance was observed in the 21 alpine climate of McArthur while the largest drop occurred in sites near urban development. 22 Light color samples were soiled after just one year of exposure. The darker color coupons did not 23 show the same seasonal variations in solar reflectance as observed for the lighter colors. 24 However, after an additional year of exposure the samples at all sites regained most of their solar 25 reflectance due to rain and/or wind washing. The change of reflectance appears cyclical with the 26 onset of seasons having more rainfall. Solar reflectance of the cool pigmented coupons always 27 exceeded that of the conventional pigmented coupons. Climatic soiling did not cause the cool 28 pigmented roof coupons to lose any more solar reflectance than their conventional pigmented 29 Manuscript Prepared for Submission to “Construction and Building Materials” counterparts. The effect of roof slope appears to have more of an effect on lighter color roofs 30 whose solar reflectance exceeds at least 0.5 and visually shows the accumulation of airborne 31 contaminants. The thermal emittance remained invariant with time and location and was 32 therefore not affected by climatic soiling. A thin-film deposition model was developed based on 33 first principles, which simulates light interaction with a soiled substrate. This model was used in 34 combination with the measurements to determine the solar absorption and reflectance of 35 particulate matter at each of the sites calculated using the least squares fitting routines. Principal 36 Component Analysis was used to determine the most important combinations of chemicals 37 correlated with changes in solar absorption. The linear regression model identifies an 38 approximate correlation using chromium, iron and elemental carbon concentrations. It appears 39 that chromium ranks first, iron ranks second, and elemental carbon ranks third in importance to 40 soil light absorption. 41 42 43